Chapter 13: Antimicrobial Susceptibility Testing. All Answers

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MULTIPLE CHOICE 1. When should antimicrobial susceptibility testing be performed on a bacterial isolate from a clinical specimen? a. When the isolate is determined to be a probable cause of the p ... atient’s infection b. When the susceptibility of the isolate to particular antimicrobial agents cannot be reliably predicted c. Both a and b d. None of the above C Antimicrobial susceptibility testing should be performed on a bacterial isolate from a clinical specimen if the isolate is determined to be a probable cause of the patient’s infection and the susceptibility of the isolate to particular antimicrobial agents cannot be reliably predicted. Susceptibility tests are not performed on bacteria that are predictably susceptible to the antimicrobial agents commonly used to treat infections caused by these bacteria. REF: 275 OBJ: Level 1: Recall 2. A 12-year-old girl goes to the doctor complaining of a sore throat, fever, headache, and general malaise. The doctor does a quick strep test that is positive for group A -hemolytic Streptococcus. No antibiotic testing is needed because Streptococcus is: a. universally susceptible to penicillin. b. universally susceptible to erythromycin. c. not treated in throat cultures. d. not considered a pathogen in the throat. A Group a -hemolytic Streptococcus, for example, is not routinely tested for antimicrobial susceptibility because it is universally susceptible to penicillin, the drug of choice in treating infections caused by this bacterium. REF: 276 OBJ: Level 3: Synthesis 3. Important factors that must be considered when determining whether antimicrobial susceptibility testing is warranted include: a. the body site from which the organism was isolated. b. the presence of other bacteria and the quality of the specimen from which the organism was grown. c. the host’s status. d. all the above. D In addition to the unpredictable susceptibility of a potential pathogen, other important factors must be considered when determining whether antimicrobial susceptibility testing is warranted; these factors include the following: the body site from which the organism was isolated, the presence of other bacteria, the quality of the specimen from which the organism was grown, and the host’s status. REF: 276 OBJ: Level 1: Recall 4. Why should antimicrobial susceptibility testing of normal flora isolates or isolates likely to represent contamination or colonization not be performed? a. This may encourage a physician to treat a normal condition. b. This will expose normal flora to various antibiotics and cause resistance to develop. c. The amount of antibiotic needed to kill all of the normal flora in a site can cause toxic effects to the patient. d. Antimicrobial susceptibility testing should be performed on all bacteria isolated from the patient. A Testing of normal flora isolates or isolates likely to represent contamination or colonization should be avoided because reporting of antimicrobial susceptibility results may encourage a physician to treat a normal condition and refrain from further investigation of the true cause of the patient’s problem. REF: 276 OBJ: Level 2: Interpretation 5. Why would normal flora isolates from immunosuppressed patients undergo antimicrobial susceptibility testing? a. These are not normal flora; they are other potential pathogens that have the same colonial morphology as normal flora. b. Species usually viewed as normal flora may be responsible for an infection in an immunosuppressed patient. c. All bacteria isolated from immunosuppressed patients must have antimicrobial susceptibility tests performed. d. Cultures from immunosuppressed patients only grow pathogenic organisms that must undergo antimicrobial susceptibility testing. B The host status of the patient often influences susceptibility-testing decisions. Species usually viewed as normal flora may be responsible for an infection and at times may require testing in immunosuppressed patients. REF: 276 OBJ: Level 2: Interpretation 6. How are antibiotics chosen for inclusion into laboratory antimicrobial testing protocols? a. There are no set standards, because physicians demand which drugs they want tested for particular bugs each time an antimicrobial susceptibility test is performed. b. The Clinical and Laboratory Standards Institute (CLSI) recommends the drugs that should be tested and reported for each type of specimen. c. The institutional pharmacy and therapeutics committee sets the list of drugs that are used in their facility d. Both b and c. D CLSI sets recommended antibiotics that should be tested for each group of organisms; however, the institution’s pharmacy and therapeutics committee along with input from clinicians typically set the list based on clinical utility of various agents in an institution. REF: 276 OBJ: Level 2: Interpretation 7. Two antibiotics are contraindicated in pediatric patients. They are: a. erythromycin and Augmentin. b. cleocin and Zithromax. c. tetracycline and Ceclor. d. tetracycline and fluoroquinolones. D The patient population must be considered in the choice of antimicrobial agents to be tested. Some agents are contraindicated in pediatric patients (e.g., fluoroquinolones, which may impair cartilage development, and tetracycline, which damages developing teeth). REF: 276 OBJ: Level 1: Recall 8. Because the identity of the bacterial isolate is often unknown at the time the susceptibility testing is performed, it may include drugs that are inappropriate for reporting. What does the microbiologist do when this occurs? a. A drug should not be indiscriminately reported because results may be misleading. b. If the drug is tested, the results are reported because the microbiologist does not know what antibiotic was prescribed to the patient. c. The microbiologist reports this immediately to the microbiology supervisor because this is a wasteful practice. d. The microbiologist only reports on the drugs to which the bacteria are sensitive. A Because the identity of the bacterial isolate is often unknown at the time the susceptibility test is performed, some drugs may be tested on an isolate that may be inappropriate for reporting. In such instances, a drug should not be indiscriminately reported because results may be misleading. Some drugs may appear active against certain species in vitro but are inappropriate for clinical use. REF: 277 OBJ: Level 2: Interpretation 9. A primary tenet of antimicrobial therapy is to use: a. the least toxic agents. b. the most cost-effective agents. c. the most clinically effective agents. d. all of the above. D A primary tenet of antimicrobial therapy is to use the least toxic, most cost-effective, and most clinically effective agents and to refrain from more costly, broader spectrum agents when they are unnecessary. Physician compliance with this objective is often difficult. REF: 277 OBJ: Level 1: Recall 10. Guidance for the development of a selective-reporting or cascade-reporting protocol for antibiotics is available from the: a. Centers for Disease Control and Prevention (CDC). b. Clinical Laboratory and Standards Institute (CLSI). c. United States Army Medical Research Institute (USAMRIID). d. Food and Drug Administration (FDA). B The CLSI provides guidance for development of such a selective-reporting or cascade-reporting protocol. REF: 277 OBJ: Level 1: Recall 11. As a general guideline, it is suggested that within a particular antimicrobial class, primary (group A) agents should be reported first and secondary (group B) agents should be reported only if the: a. physician insists that a secondary agent be reported. b. hospital pharmacy no longer stocks the group A agent. c. patient cannot tolerate the primary agents. d. FDA has recalled the primary agents. C As a general guideline, it is suggested that within a particular antimicrobial class, primary (group A) agents be reported first and that secondary (group B) agents be reported only if one of the following conditions exists: the isolate is resistant to the primary agents, the patient cannot tolerate the primary agents, the infection has not responded to the primary agents, a secondary agent would be a better clinical choice for the particular infection (e.g., meningitis), or the patient has organisms isolated from another site, and a secondary agent might be useful to treat both organisms. REF: 277 OBJ: Level 2: Interpretation 12. Why are aminoglycosides not an effective treatment for meningitis? a. They do not cross the blood-brain barrier. b. They are nephrotoxic and will harm the patient’s kidneys. c. They are ototoxic and will harm the patient’s ears. d. They are not effective against Neisseria, Streptococcus, or Haemophilus. A Aminoglycosides are not effective for treating meningitis because they do not readily cross the blood-brain barrier. REF: 278 OBJ: Level 2: Interpretation 13. When would it be appropriate for an institution to routinely test group C agents? a. For urinary tract infections b. If a particular institution routinely encounters large numbers of isolates resistant to group A and group B agents c. For blood cultures d. If a particular institution routinely encounters large numbers of isolates susceptible to group A and group B agents B Agents with very broad-spectrum activity or increased potency (group C) may be tested and reported for the reasons listed for secondary agents. In addition, group C agents would be considered for routine testing if a particular institution encounters large numbers of isolates resistant to group A and group B agents. REF: 278 OBJ: Level 2: Interpretation 14. What antibiotics should be reported for urinary tract infections? a. Group A antibiotics b. Group B antibiotics c. Urinary tract active agents d. Group C antibiotics C Agents with activity only in the urinary tract should be reported only on isolates from urine, because these drugs are clinically ineffective in treating infections other than urinary tract infections. REF: 278 OBJ: Level 1: Recall 15. What is the most critical step in any susceptibility test? a. Plate streaking b. Organism isolation c. Gram stain identification d. Inoculum preparation D Inoculum preparation is one of the most critical steps in any susceptibility test. Inocula are prepared by adding cells from four to five isolated colonies of similar colony morphology to a broth medium and then allowing them to grow to the log phase. Inocula may also be prepared directly by suspending colonies in saline. REF: 278 OBJ: Level 1: Recall 16. What is the most widely used method of inoculum standardization? a. McFarland turbidity standards b. Remel inoculum standards c. Becton-Dickson standards d. Difco standards A The most widely used method of inoculum standardization involves McFarland turbidity standards. REF: 279 OBJ: Level 1: Recall 17. What McFarland standard provides turbidity comparable to that of a bacterial suspension containing approximately 1.5 108 CFU/mL? a. 1.0 b. 1.5 c. 0.5 d. 2.0 C The McFarland 0.5 standard provides turbidity comparable to that of a bacterial suspension containing approximately 1.5  108 CFU/mL. REF: 279 OBJ: Level 1: Recall 18. The minimal inhibitory concentration (MIC) is the _____ antibiotic concentration to _____ the growth of an organism. a. lowest; slow b. highest; slow c. lowest; inhibit d. highest; inhibit C Dilution antimicrobial susceptibility test methods are used to determine the MIC, or the lowest concentration of antimicrobial agent required to inhibit the growth of a bacterial isolate. REF: 279 OBJ: Level 1: Recall 19. What concentrations of antibiotics are typically tested in minimal inhibitory concentrations (MICs)? a. Serial fivefold dilution concentrations are tested. b. Serial tenfold dilution concentrations are tested. c. Serial onefold dilution concentrations are tested. d. Serial twofold dilution concentrations are tested. D Generally, serial twofold dilution concentrations are tested (expressed in µg/mL), and these approximate concentrations are attainable in vivo following standard dosages for the respective antimicrobial agent. REF: 279 OBJ: Level 1: Recall 20. Note that for some antimicrobial agents, different minimal inhibitory concentration (MIC) interpretive criteria exist for: a. organisms or organism groups. b. anatomic sites. c. age groups. d. immune status. A Note that for some antimicrobial agents, different MIC interpretive criteria exist for different organisms or organism groups. This can be caused by the infection site—some antibiotics require more antibiotic to achieve the same range in the blood as in the cerebrospinal fluid (CSF). REF: 279 OBJ: Level 1: Recall 21. What broth is recommended in the tube dilution minimal inhibitory concentration (MIC) tests? a. Trypticase soy b. Mueller-Hinton c. Columbia d. Cooked meat B Mueller-Hinton broth is the medium recommended for broth dilution MIC tests of nonfastidious bacteria. REF: 280 OBJ: Level 1: Recall 22. What does the broth microdilution minimal inhibitory concentration (MIC) test use for testing? a. Test tubes b. Plastic strips c. Multiwell microdilution trays d. Multiwell macrodilution trays C The broth macrodilution test has been miniaturized and adapted to multiwell microdilution trays for broth microdilution MIC testing. Plastic trays containing between 80 and 100 wells are filled with small volumes of twofold dilution concentrations of antimicrobial agent in broth. REF: 280 OBJ: Level 1: Recall 23. What is the approximate final concentration of organisms in each well of a microdilution tray? a. 3  105 colony-forming units (CFU)/mL b. 1  105 CFU/mL c. 4  105 CFU/mL d. 5  105 CFU/mL D An intermediate dilution of this inoculum suspension is prepared in water or saline, and a multipronged inoculator or other type of inoculating device is used to inoculate the wells to obtain a final concentration of approximately 5  105. The actual dilution factor used for preparation of the intermediate dilution depends on the volume of inoculum delivered to each well by the inoculating device and the organism being tested. REF: 281 OBJ: Level 1: Recall 24. When reading broth microdilution minimal inhibitory concentration (MIC) testing, the MIC for a particular drug is the _____ concentration showing _____ growth. a. lowest; no obvious b. highest; no obvious c. lowest; confluent d. highest; confluent A Provided that growth is adequate in the growth control well, the MIC for a particular drug is the lowest concentration showing no obvious growth. Growth may be seen as turbidity, a haze, or a pellet in the bottom of the well. REF: 281 OBJ: Level 1: Recall 25. What is the storage temperature for frozen panels? a. 0 C b. –20 to –70 C c. –75 to –100C d. 273 K B Frozen panels are stored at –20 to –70 C until used. REF: 287 OBJ: Level 1: Recall 26. What is a breakpoint panel? a. A standard minimal inhibitory concentration (MIC) panel that contains many more dilution wells for each drug tested b. An MIC panel with only three or four drugs tested c. An MIC panel with only one or a few concentrations of each drug d. An MIC panel that contains nonroutine drugs C A variation of the standard broth microdilution MIC panel is the breakpoint panel, in which only one or a few concentrations of each anitmicrobial agent are tested on a single panel. Breakpoint is the term applied to the concentration of an antimicrobial agent that coincides with a susceptible or intermediate MIC breakpoint for a particular drug. REF: 281 OBJ: Level 1: Recall 27. On a breakpoint panel, when two concentrations are tested and no growth is present in either well, the isolate is: a. susceptible. b. resistant. c. intermediate. d. none of the above. A When two concentrations are tested and no growth is present in either well, the isolate is susceptible. REF: 281 OBJ: Level 2: Interpretation 28. On a breakpoint panel, when there is growth in the low concentration but no growth in the high concentration, the isolate is: a. susceptible. b. resistant. c. intermediate. d. none of the above. C On a breakpoint panel, when there is growth in the low concentration but no growth in the high concentration, the isolate has intermediate susceptibility. REF: 281 OBJ: Level 2: Interpretation 29. On a breakpoint panel, when there is growth in both wells, the isolate is: a. susceptible. b. resistant. c. intermediate. d. none of the above. B A resistant isolate grows in both wells. REF: 281 OBJ: Level 2: Interpretation 30. In terms of MIC microdilution trays, a skipped well occurs when: a. a well on the tray is skipped with inoculum and it does not grow. b. the inoculator is turned around and the sterility well is accidentally inoculated and the growth well is accidentally skipped. c. there is growth at higher concentrations and no growth at one or more of the lower concentration wells. d. none of the above. C Skipped wells involve growth at higher concentrations and no growth at one or more of the lower concentrations. This may occur as a result of contamination, improperly inoculated wells, improper concentrations of antimicrobial agent in the wells, the presence of unusual resistance with the test isolate, or a combination of two or more of these factors. REF: 282 OBJ: Level 2: Interpretation 31. In this method of antimicrobial susceptibility testing, specific volumes of various concentrations of antimicrobial solutions are dispensed into premeasured volumes of molten and cooled agar, which is subsequently poured into standard Petri dishes. Organisms are inoculated onto the plate. What type of antimicrobial susceptibility testing is this? a. Macrodilution b. Broth dilution c. Disk diffusion d. Agar dilution D Specific volumes of various concentrations of antimicrobial solutions are dispensed into premeasured volumes of molten and cooled agar, which is subsequently poured into standard Petri dishes. Organisms are inoculated onto the plate. After overnight incubation, the minimal inhibitory concentration (MIC) is read as the lowest concentration of antimicrobial agent that inhibits the visible growth of the test bacterium. REF: 282 OBJ: Level 2: Interpretation 32. In this type of antimicrobial susceptibility test, a McFarland 0.5 standardized suspension of bacteria is swabbed over the surface of a Mueller-Hinton agar plate, and paper disks containing single concentrations of each antimicrobial agent are placed onto the inoculated surface. What is the name of this test? a. Kirby-Bauer b. MIC c. Breakpoint d. Agar dilution A The disk diffusion test, also commonly known as the Kirby-Bauer test, has been widely used in clinical laboratories since 1966, when the first standardized method was described. A McFarland 0.5 standardized suspension of bacteria is swabbed over the surface of a Mueller-Hinton agar plate, and paper disks containing single concentrations of each antimicrobial agent are placed onto the inoculated surface. After overnight incubation, the diameters of the zones produced by antimicrobial inhibition of bacterial growth are measured, and the isolate is interpreted as either susceptible, intermediate, or resistant to a particular drug, according to preset criteria. REF: 282 OBJ: Level 2: Interpretation 33. A zone of inhibition is: a. the area on the agar plate where the drug no longer diffuses into the agar. b. the area around the antibiotic disk where the bacteria cannot grow. c. the zone around the edge of the plate where the streaking does not cover the agar and bacterial growth is inhibited. d. all of the above. B The disk diffusion test depends on the formation of a gradient of antimicrobial concentrations as the antimicrobial agent diffuses radially into the agar. The drug concentration decreases at increasing distances from the disk. At a critical point, the amount of drug at a specific location in the medium is unable to inhibit the growth of the test organism, and a zone of inhibition is formed. The zones of inhibition are related to minimal inhibitory concentrations (MICs). REF: 282 OBJ: Level 1: Recall 34. What agencies develop the zone interpretive criteria? a. Centers for Disease Control and Prevention (CDC) and U.S. Army Medical Research Institute (USAMRIID) b. U.S. Food and Drug Administration (FDA) and CDC c. FDA and Clinical and Laboratory Standards Institute (CLSI) d. CLSI and CDC C The CLSI and the FDA are involved in developing zone interpretive criteria. REF: 288 OBJ: Level 1: Recall 35. In the Kirby-Bauer test, what is used to grow streptococci that do not grow adequately on unsupplemented Mueller-Hinton agar? a. Chocolate agar b. PEA agar c. Mueller-Hinton agar with 5% sheep blood d. Mueller-Hinton agar with 5% rabbit blood C Mueller-Hinton agar containing 5% sheep blood is used for testing streptococci that do not grow adequately on unsupplemented Mueller-Hinton agar. REF: 284 OBJ: Level 1: Recall 36. After the plate is incubated in the Kirby-Bauer test, what occurs? a. The plate is tested to make sure an isolated organism grew on the plate. b. The diameter of each inhibition zone for each antibiotic is measured using a ruler or caliper. c. The amount of carbon dioxide in the plate is measured to ensure confluent organism growth. d. None of the above occurs. B Provided that growth is satisfactory, the diameter of each inhibition zone is measured using a ruler or calipers. Plates are placed a few inches above a black, nonreflecting surface, and zones are examined from the back side of the plate illuminated with reflected light. REF: 284 OBJ: Level 1: Recall 37. Once zone measurements have been made, the millimeter reading for each antimicrobial agent is: a. sent to the physician to determine the appropriateness of antibiotic therapy for his patient. b. converted into minimal inhibitory concentration (MIC) units. c. interpreted as susceptible, intermediate, or resistant. d. none of the above. C Once zone measurements have been made, the millimeter reading for each antimicrobial agent is compared with that specified in the interpretive tables of the CLSI documents, and results are interpreted as either susceptible, intermediate, or resistant. REF: 285 OBJ: Level 1: Recall 38. Agar dilution and disk diffusion tests for Streptococcus pneumoniae and Streptococcus spp. use: a. Mueller-Hinton agar supplemented with X and V factors. b. Columbia base supplemented with 5% sheep blood. c. trypticase soy agar base supplemented with 5% lysed horse blood. d. Mueller-Hinton agar supplemented with 5% sheep blood. D S. pneumoniae and Streptococcus spp. require a more nutritious medium for antimicrobial susceptibility testing; they will not grow satisfactorily unsupplemented. Broth dilution tests are performed in Mueller-Hinton broth that has been supplemented with 2% to 5% lysed horse blood. Agar dilution and disk diffusion tests are performed using Mueller-Hinton agar supplemented with 5% sheep blood. REF: 285 OBJ: Level 2: Interpretation 39. A patient is admitted to the hospital with a diagnosis of endocarditis. Two separate blood cultures grow out viridans streptococci. The penicillin minimal inhibitory concentration (MIC) of the organism is 0.12 µg/mL. What antibiotic should the physician prescribe? a. Penicillin b. Erythromycin c. Clindamycin d. Gentamicin A Accurate penicillin susceptibility results are needed for viridans streptococci isolated from serious infections, such as bacteremia or endocarditis. If the isolate has a penicillin MIC of 0.12 µg/mL or less, penicillin alone is often prescribed; however, higher penicillin MICs (0.25 to 2.0 µg/mL) suggest the need for concomitant therapy with an aminoglycoside. REF: 287 OBJ: Level 3: Synthesis 40. When performing disk diffusion testing with Haemophilus influenzae and H. parainfluenzae, what type of medium is used? a. Mueller-Hinton with 5% sheep blood b. Mueller-Hinton with hematin and nicotinamide adenine dinucleotide (NAD) c. Mueller-Hinton with 5% lysed horse blood d. Mueller-Hinton B Haemophilus test medium (HTM), which consists of Mueller-Hinton medium base supplemented with X (hematin) and V (or NAD) factors, has been standardized for testing H. influenzae and H. parainfluenzae. REF: 288 OBJ: Level 1: Recall 41. Therapy of disseminated meningococcal infections and various types of gonococcal infections are: a. hard to treat and require special -lactamase tests. b. an important public health concern, so a full spectrum of antimicrobial susceptibility testing is always performed. c. generally empiric based on recommendations of Centers for Disease Control and Prevention (CDC). d. none of the above. C Neisseria gonorrhoeae and N. meningitidis are organisms of public health significance that may be isolated by clinical laboratories with varying frequencies. Therapy of disseminated meningococcal infections and various types of gonococcal infections are generally empiric based on recommendations from CDC and various professional groups. Most of the time, clinical microbiology laboratories are not required to perform antimicrobial susceptibility testing of these two species. REF: 288 OBJ: Level 2: Interpretation 42. A man goes to his doctor because he has a cloudy substance dripping from his penis. A Gram stain is performed and gram-negative intracellular diplococci are observed. What is the drug of choice to treat this infection? a. Gentamicin b. Erythromycin c. Vancomycin d. Ceftriaxone D Although for many years penicillin was the drug of choice for treating uncomplicated gonorrhea, the increased incidence of penicillin-resistant isolates has led to the use of ceftriaxone or a fluoroquinolone as first-line therapy. REF: 288 OBJ: Level 3: Synthesis 43. On what media is Kirby-Bauer testing performed for Neisseria gonorrhoeae? a. Gonococcal (GC) agar base supplemented with various nutrients b. Mueller-Hinton agar supplemented with vancomycin c. Mueller-Hinton agar supplemented with X and V factors d. Mueller-Hinton agar supplemented with biotin, and X and V factors A GC agar base is supplemented with various nutrients for testing N. gonorrhoeae. Disk diffusion tests are performed on the same agar (GC base), and all tests are incubated in an atmosphere containing 5% to 7% CO2. Clinical and Laboratory Standards Institute (CLSI) has specified interpretive criteria unique for this species. REF: 288 OBJ: Level 1: Recall 44. What type of antimicrobial susceptibility testing method does Clinical and Laboratory Standards Institute (CLSI) recommend for anaerobes? a. Microdilution b. Agar dilution c. Tube broth macrodilution d. Disk diffusion B The reference method described by the CLSI for testing anaerobic bacteria is an agar dilution method, and the recommended medium is supplemented Brucella blood agar. As previously mentioned, however, agar dilution is not practical for use in the routine clinical laboratory, and a broth microdilution method is more often used. REF: 289 OBJ: Level 1: Recall 45. How long are minimal inhibitory concentration (MIC) panels for anaerobes incubated? a. 24 Hours b. 36 Hours c. 48 Hours d. 72 Hours C Additionally, the number of organisms in the test inoculum is 0.5 log10 higher (106 colony-forming units [CFU]/mL) than that for testing aerobes, and panels are incubated anaerobically at 35 C for 48 hours. REF: 289 OBJ: Level 1: Recall 46. The Clinical and Laboratory Standards Institute (CLSI) has published methods for susceptibility testing of several agents of potential bioterrorism, including Brucella anthracis, Yersinia pestis, Burkholderia mallei, B. pseudomallei, Francisella tularensis, and Brucella spp. How does CLSI suggest clinical laboratories perform antimicrobial susceptibility testing on these isolates? a. Use the agar dilution technique. b. Use the Kirby-Bauer method. c. Use the minimal inhibitory concentration (MIC) plates. d. Do not attempt to perform antimicrobial susceptibility on any of these isolates; refer them to a public health laboratory equipped to handle them. D Primarily for use by public health laboratories capable of safely working with select agents, the CLSI has published methods for susceptibility testing of several agents of potential bioterrorism, including B. anthracis, Y. pestis, B. mallei, B. pseudomallei, F. tularensis, and Brucella spp. Clinical laboratories should not attempt to perform susceptibility tests on these species, but should refer any suspected isolates to appropriate public health laboratories. REF: 289 OBJ: Level 2: Interpretation 47. Oxacillin-resistant Staphylococcus aureus is called _____-resistant S. aureus. a. methicillin b. oxacillin c. penicillin d. penicillinase A Isolates of oxacillin-resistant S. aureus are commonly referred to as methicillin-resistant S. aureus (MRSA) for historical reasons. REF: 289 OBJ: Level 1: Recall 48. In vitro testing conditions that can be modified to enhance the expression of oxacillin resistance include all the following except: a. using Mueller-Hinton media. b. making final test readings after a full 24 hours of incubation. c. incubating tests at temperatures no greater than 35° C. d. preparing inocula using the direct inoculum suspension procedure. A In vitro testing conditions can be modified to enhance the expression of oxacillin resistance; they are as follows: preparation of inocula using the direct inoculum suspension procedure, incubation of tests at temperatures no greater than 35C, and making final test readings after a full 24 hours of incubation. REF: 289 OBJ: Level 2: Interpretation 49. Oxacillin-resistant staphylococci can appear susceptible in vitro to other -lactam agents such as: a. quinolones. b. tetracyclines. c. cephalosporins. d. aminoglycosides. C Sometimes, oxacillin-resistant staphylococci can appear susceptible in vitro to other -lactam agents, such as cephalosporins; however, these are clinically ineffective. Consequently, all oxacillin-resistant staphylococci must be reported as resistant to all -lactam agents, including cephalosporins, -lactam/-lactamase inhibitor combinations, and carbapenems, if those agents are tested, regardless of the in vitro test results. REF: 290 OBJ: Level 2: Interpretation 50. To obtain a bactericidal effect in enterococci, ampicillin or penicillin must be given in combination with: a. macrolides. b. peptidoglycans. c. cephalosporins. d. aminoglycosides. D To obtain a bactericidal effect, ampicillin or penicillin (or vancomycin in the penicillin-allergic patient or with Enterococcus faecium) must be given in combination with an aminoglycoside—usually gentamicin or sometimes streptomycin. REF: 291 OBJ: Level 1: Recall 51. Which enterococcus is/are the most common species demonstrating vancomycin resistance among clinical isolates? a. Enterococcus faecium b. E. faecalis c. Both a and b d. Neither a nor b A The incidence of vancomycin resistance in enterococci increased sharply in the 1990s and the early part of the decade of 2000s. E. faecium is the most common species demonstrating vancomycin resistance among clinical isolates, followed by E. faecalis. REF: 292 OBJ: Level 1: Recall 52. Spontaneous mutations occur in the -lactamase genes that result in extended-spectrum -lactamases (ESBL). These ESBL inactivate: a. cephalosporins. b. penicillins. c. aztreonam. d. all of the above. D Spontaneous mutations occur that may result in novel -lactamases that can inactivate extended-spectrum cephalosporins, penicillins, and aztreonam. These -lactamases are known as extended-spectrum -lactamases. REF: 292 OBJ: Level 2: Interpretation 53. When testing Escherichia coli, Klebsiella spp., and Proteus mirabilis, which drug is mostly likely to indicate the presence of an extended-spectrum -lactamases (ESBL)? a. Aztreonam b. Cefpodoxime c. Ceftriaxone d. Ceftazidime B Strategies for laboratory detection of ESBL-producing E. coli, Klebsiella spp., and P. mirabilis include testing of those drugs that are most likely to indicate the presence of an ESBL (the “indicator” drugs). These include, in decreasing order of sensitivity for detection, cefpodoxime, ceftazidime, cefotaxime, ceftriaxone, aztreonam, and use of special screening zone or minimal inhibitory concentration (MIC) breakpoints to facilitate recognition of ESBL production. REF: 292 OBJ: Level 1: Recall 54. What antibiotics are active against extended-spectrum -lactamases (ESBL)-producing strains of bacteria? a. Cephalosporins b. Penicillins c. Carbapenems d. Aztreonam C Consequently, when an ESBL-producing isolate is identified, it should be reported as clinically resistant to all cephalosporins, penicillins, and aztreonam, despite the standard in vitro test results. The carbapenems (imipenem, meropenem, ertapenem) are active against ESBL-producing strains, as are the cephamycins (cefoxitin and cefotetan). REF: 293 OBJ: Level 1: Recall 55. What principle do most current automated antimicrobial susceptibility instruments use to read growth? a. Turbidimetric detection b. Turbidity suppression c. Hydrolysis of a fluorogenic growth substrate d. Carbon dioxide concentration A Most current instruments use the principle of turbidimetric detection of bacterial growth in a broth medium by use of a photometer to examine the test wells. REF: 293 OBJ: Level 1: Recall 56. The BD Phoenix system uses what principle to measure bacterial growth in the susceptibility test wells? a. Hydrolysis of a fluorogenic growth substrate b. Turbidimetric detection c. Carbon dioxide concentration d. Redox indicator system D The Phoenix uses a redox indicator system (similar to resazurin) to measure bacterial growth in the susceptibility test well. The indicator is added to the broth at the time of organism inoculation. REF: 294 OBJ: Level 1: Recall 57. How many hours of incubation are necessary for the antimicrobial susceptibility test run on the BD Phoenix? a. 6 to 8 hours b. 8 to 12 hours c. 12 to 18 hours d. 24 hours A The Phoenix reads the susceptibility determinations following approximately 6 to 8 hours of incubation. REF: 294 OBJ: Level 1: Recall 58. What principle does the MicroScan Walkaway SI use to determine antimicrobial susceptibility? a. Redox indicator system b. Turbidimetry c. Hydrolysis of a bioluminescent substrate d. Carbon dioxide concentration B Most current instruments use the principle of turbidimetric detection of bacterial growth in a broth medium by use of a photometer to examine the test wells. REF: 293 OBJ: Level 1: Recall 59. What principle does the TREK ARIS 2X System use to determine antimicrobial susceptibility? a. Redox indicator system b. Turbidimetry c. Hydrolysis of a fluorogenic substrate d. Carbon dioxide concentration C The Sensititre Automated Incubator Reader (ARIS 2X, TREK Diagnostics, Cleveland, OH) was the first system to be marketed in the United States that used a fluorometric detection system for detecting growth endpoints of common, rapidly growing bacteria using minimal inhibitory concentration (MIC) or breakpoint formats with either a 5-hour or 18-hour incubation period. REF: 295 OBJ: Level 1: Recall 60. What automated system was originally designed for use in outer space? a. TREK ARIS b. MicroScan Walkaway SI c. BD Phoenix d. VITEK D The VITEK System (bioMérieux Vitek, Durham, NC) was originally designed for use in the United States space exploration efforts of the 1970s as an onboard test system for spacecraft exploring other planets for life. REF: 295 OBJ: Level 1: Recall 61. What principle does the VITEK series use to determine antimicrobial susceptibility? a. Turbidimetry b. Redox indicator system c. Hydrolysis of a fluorogenic substrate d. Carbon dioxide concentration A VITEK 1 hardware consists of a filling module for inoculation of the cards, an incubator/reader module that incorporates a carousel to hold the test cards, a robotic system to manipulate the cards, a photometer for turbidimetric measurement of growth once per hour, and a computer module with video display terminal and printer for viewing and printing results. REF: 295 OBJ: Level 1: Recall 62. What is the principle of the Etest? a. Turbidimetry b. Establishing an antimicrobial density gradient in agar c. Carbon dioxide concentration d. Redox indicator system B The Etest (AB Biodisk, Solna, Sweden) uses the principle of establishing an antimicrobial density gradient in an agar medium as a means of determining antimicrobial susceptibility. The Etest uses very thin plastic test strips that are impregnated on the undersurface with an antimicrobial concentration gradient and are marked on the upper surface with a concentration index or scale. REF: 296 OBJ: Level 1: Recall 63. Who sets the criteria in the table of values that relates the diameter of the zone to a category of susceptibility in disk diffusion testing? a. Centers for Disease Control and Prevention (CDC) b. College of American Pathologists (CAP) c. Clinical and Laboratory Standards Institute (CLSI) d. Joint Commission on Accreditation of Healthcare Organizations (JCAHO) C The table used for such interpretations must represent the most up-to-date criteria that have been reviewed and accepted by the CLSI. It is important to recognize that the CLSI documents are updated frequently, usually once per year. Use of old or outdated CLSI tables could represent a serious shortcoming in the reporting of patients’ results. REF: 297 OBJ: Level 1: Recall 64. The inhibition zone size and minimal inhibitory concentration (MIC) interpretive criteria published by the Clinical and Laboratory Standards Institute (CLSI) are established by careful analysis of all of the following except: a. microbiologic data. b. pharmacokinetic data. c. results of clinical studies during the phase before Food and Drug Administration (FDA) approval. d. pharmacodynamic data. D The inhibition zone size and MIC interpretive criteria published by the CLSI are established by careful analysis of three kinds of data: microbiologic data (e.g., a comparison of MICs versus zone sizes on a large number of bacterial strains, including those with known resistance mechanisms), pharmacokinetic data (e.g., serum, cerebrospinal fluid [CSF], urine, and other secretion and tissue levels of an antimicrobial agent), and results of clinical studies obtained during the phase before FDA approval and marketing of an antimicrobial agent. REF: 297 OBJ: Level 1: Recall 65. If the minimal inhibitory concentration (MIC) or zone size is interpreted as susceptible using the latest CLSI tables: a. the clinical interpretation of the result is that the patient’s infecting organism should respond to therapy with that antimicrobial agent using the dosage normally recommended for that type of infection and that species. b. the patient’s infecting organism is unlikely to be inhibited by the usually achievable concentrations of the antimicrobial agent based on the dosages normally used with that drug. c. the patient’s infecting organism is likely to require the maximum amount of antimicrobial for which the clinical response is likely to be less than with a susceptible strain. d. none of the above. A The clinical interpretation of the result is that the patient’s infecting organism should respond to therapy with that antimicrobial agent using the dosage normally recommended for that type of infection and that species. The drug should kill the organism. REF: 297 OBJ: Level 2: Interpretation 66. If the minimal inhibitory concentration (MIC) or zone size is interpreted as resistant using the latest Clinical and Laboratory Standards Institute (CLSI) tables: a. the clinical interpretation of the result is that the patient’s infecting organism should respond to therapy with that antimicrobial agent using the dosage normally recommended for that type of infection and that species. b. the patient’s infecting organism is unlikely to be inhibited by the usually achievable concentrations of the antimicrobial agent based on the dosages normally used with that drug. c. the patient’s infecting organism is likely to require the maximum amount of antimicrobial or more than can be achieved and for which the clinical response is likely to be less than with a susceptible strain. d. none of the above. B The patient’s infecting organism is unlikely to be inhibited by the usually achievable concentrations of the antimicrobial agent based on the dosages normally used with that drug. The drug will not kill the organism. REF: 297 OBJ: Level 2: Interpretation 67. If the minimal inhibitory concentration (MIC) or zone size is interpreted as intermediate using the latest Clinical and Laboratory Standards Institute (CLSI) tables: a. the clinical interpretation of the result is that the patient’s infecting organism should respond to therapy with that antimicrobial agent using the dosage normally recommended for that type of infection and that species. b. the patient’s infecting organism is unlikely to be inhibited by the usually achievable concentrations of the antimicrobial agent based on the dosages normally used with that drug. c. the patient’s infecting organism is likely to require the maximum amount of antimicrobial or more than can be achieved and for which the clinical response is likely to be less than with a susceptible strain. d. none of the above. C The patient’s infecting organism is likely to require the maximum amount of antimicrobial or more than can be achieved and for which the clinical response is likely to be less than with a susceptible strain. The blood levels it will take to kill the organism are unattainable; therefore, the organism will probably survive treatment with that antimicrobial. REF: 297 OBJ: Level 2: Interpretation 68. All the following organisms often produce -lactamase except: a. Haemophilus influenzae. b. Neisseria gonorrhoeae. c. Moraxella catarrhalis. d. Neisseria meningitidis. D Production of -lactamase is a significant mechanism contributing to resistance to some -lactams in certain organisms, such as H. influenzae, N. gonorrhoeae, M. catarrhalis, Staphylococcus spp., and some Bacteroides spp. REF: 297 OBJ: Level 1: Recall 69. What is the principle of the most commonly used test to detect -lactamase production? a. Chromogenic cephalosporin nitrocefin b. Redox reaction c. Hydrolysis of fluorogenic substrate d. Reduction of acetoin A The product consists of filter paper disks impregnated with nitrocefin. A disk is moistened with water or saline, and a loopful of organism is applied directly onto the disk. Within 10 minutes (or within 60 minutes for staphylococci), the area where the organisms were applied will turn red in the case of a -lactamase–producing organism. No color change occurs with -lactamase–negative organisms. REF: 297 OBJ: Level 1: Recall 70. A microbiology technologist is performing a -lactamase test on a staphylococci isolated from a hospitalized patient. The test was negative, so the technologist reported out in vitro antimicrobial susceptibility test results for -lactam antibiotics. After 2 days, the patient was not improving and the physician changed the patient’s antibiotic to an aminoglycoside. The patient showed improvement immediately. Why was the -lactamase test on the staphylococci negative, when the organism produced -lactamase? a. The test was a false negative—the organism really did produce -lactamase, but it did not react with the test. b. Staphylococci must be exposed to the -lactam agent before it will produce a positive test result. c. The test was a false positive—the organism reacted with the test, but it did not produce a -lactamase. d. The staphylococci on the plate were more than 24 hours old and false-negative test results can occur when an “old” organism is used. B Production of -lactamase in staphylococci is inducible, and exposure to an inducing agent (a -lactam agent) is often required to obtain enough concentrations of the enzyme for detection with conventional -lactamase tests. REF: 298 OBJ: Level 3: Synthesis 71. Quality control of antimicrobial susceptibility tests involves: a. performing antimicrobial susceptibility tests on organisms using the Clinical and Laboratory Standards Institute (CLSI) charts to interpret the results. b. using organisms grown from patient cultures to test the reactivity of the antimicrobial drugs. c. testing standard reference strains that have defined antimicrobial susceptibility (or resistance) to the drugs tested. d. none of the above. C Quality control of antimicrobial susceptibility tests involves testing standard reference strains that have defined antimicrobial susceptibility (or resistance) to the drugs tested. It is important to use quality control strains that represent the types of patient isolates tested in the respective laboratory. Additionally, the quality control strains should represent varying degrees of susceptibility (or resistance). REF: 299 OBJ: Level 1: Recall 72. How should quality control testing occur? a. One specified microbiologist should use extreme care when doing quality control testing, using special pipettes, special agar plates, and special incubators. b. The procedure should be followed to a T, and if any mistake is made, the test should be discarded and started again. c. The microbiology supervisor should perform all quality control testing. d. Quality control testing should be included in all the patients tested and treated like a patient specimen. D The procedures followed in testing quality control reference strains must be identical to that used for testing patient isolates. REF: 299 OBJ: Level 2: Interpretation 73. What is a mechanism for ensuring the testing personnel are proficient in their tasks? a. Supervisory review of reported results b. Self-assessment checklists c. Periodic competency testing through proficiency surveys d. All of the above D A less obvious component of a quality control program involves the inclusion of mechanisms to ensure that personnel performing testing are proficient in the tasks. Self-assessment checklists and supervisory review of reported results are examples of such mechanisms, as well as periodic competency testing through proficiency surveys. REF: 300 OBJ: Level 1: Recall 74. What is the most widely used supplementary quality control measure for antimicrobial susceptibility testing? a. -Lactamase testing b. Use of antibiograms to verify results generated on patient isolates c. Correlation of clinical outcomes with laboratory findings d. None of the above B The most widely used supplemental quality control measure is the use of antibiograms to verify results generated on patient isolates. An antibiogram is the overall antimicrobial susceptibility profile of a bacterial isolate to a battery of antimicrobial agents. Certain species have “typical” antibiograms, which can be used to verify the identification and the susceptibility results generated on the isolate. REF: 300 OBJ: Level 1: Recall 75. All of the following steps are taken to verify the results from an atypical antibiogram except: a. reexamining the disk diffusion plate, or minimal inhibitory concentration (MIC) tray, to ensure that results were properly interpreted. b. checking earlier reports to see whether the particular patient previously had an isolate with an atypical antibiogram. c. calling around to other hospitals to see if they are encountering the same organism with that same antibiogram. d. repeating the test, if necessary. C When atypical antibiograms are encountered, the results must be verified. Verification procedures include the following: reexamining the disk diffusion plate, MIC tray, and other components to ensure that results were properly interpreted and that the material were not overtly defective (e.g., empty well in tray); checking earlier reports to see whether the particular patient previously had an isolate with an atypical antibiogram (that was verified); and repeating the test, if necessary (sometimes it is necessary to repeat the identification tests and the antimicrobial susceptibility tests to verify the atypical results; sometimes testing with an alternative method is useful). REF: 300 OBJ: Level 2: Interpretation 76. What is generated by analysis of individual susceptibility results on isolates from a particular institution in a defined period and represents the percentage of isolates of a given species that is susceptible to the antimicrobial agents commonly tested against the species? a. Historical antibiogram b. Laboratory workload report c. Infection control committee report d. Cumulative antibiogram D Cumulative antibiograms are generated by analysis of individual susceptibility results on isolates from a particular institution in a defined period, which represents the percentage of isolates of a given species that is susceptible to the antimicrobial agents commonly tested against the species. Cumulative antibiograms are generally compiled annually for the purpose of guiding physicians in empiric therapy decisions. REF: 301 OBJ: Level 1: Recall 77. Advantages of the commercial microdilution susceptibility test methods include all the following except: a. more accurate results than manual methods. b. automated panel readers. c. the quantitative nature of a minimal inhibitory concentration (MIC). d. computerized data management systems. A Advantages of the MIC method include its quantitative nature: that is, a MIC rather than a strict category result, the fact that MICs may be determined with some organisms for which the disk test may not be standardized, and the attraction of automated panel readers. In addition, the computerized data management systems that accompany some of the instruments may be helpful to some laboratories for storage and calculation of cumulative antibiograms and other susceptibility statistics. REF: 302 OBJ: Level 1: Recall 78. A drug will generally inhibit multiplication of the bacteria so that the patient’s immune defense mechanisms are no longer overwhelmed when: a. all the debris and necrotic cells are removed. b. the drug concentration exceeds the MIC for the microbe. c. the patient is not infected with a gram-negative rod. d. two synergistic drugs are used to kill the bacteria. B If a concentration of antimicrobial agent that exceeds the MIC is attained at the infection site, the drug generally inhibits multiplication of bacteria so that the patient’s immune defense mechanisms are no longer overwhelmed. The immune defense mechanisms (e.g., phagocytic cells, antibody) work in concert with the antimicrobial agents to eradicate infecting bacteria; for this reason, inhibitory concentrations of the drug at the infection site are generally sufficient for treating most infections. REF: 306 OBJ: Level 2: Interpretation 79. The minimum bactericidal concentration (MBC) test differs from the minimal inhibitory concentration (MIC) test because the MIC test gives the concentration needed at the site of infection to inhibit bacterial multiplications, whereas the MBC gives: a. the highest concentration of antimicrobial that will kill an organism. b. the amount of antimicrobial that must be achieved at the infection site to inhibit the organism. c. the amount of antimicrobial that must be achieved at the infection site to kill the organism. d. none of the above. C In an immunosuppressed patient, however, and in patients with serious infections such as endocarditis and osteomyelitis, the immune defense mechanisms are suboptimal. Inhibitory concentration of the drug may not be sufficient, and obtaining bactericidal concentrations of antimicrobial agents at the infection site is important to achieve a cure. REF: 306 OBJ: Level 3: Synthesis 80. Goals of combination antimicrobial therapy include all the following except: a. obtain broad-spectrum coverage. b. enhance antibacterial activity through synergistic interactions. c. minimize resistance development. d. ensure that at least one antimicrobial will obtain the minimum bactericidal concentration (MBC) at the infection site. D Goals of combination therapy are to obtain broad-spectrum coverage, enhance antibacterial activity through synergistic interactions, and minimize resistance development. REF: 308 OBJ: Level 1: Recall 81. A combination of antimicrobials is said to show synergism if the antibacterial activity is _____ the single agents. a. significantly greater than that of b. significantly less than that of c. sustained for a much prolonged time compared with d. sustained for a much shorter time compared with A A combination is said to show synergism if its antibacterial activity is significantly greater than that of the single agents; that is, when the minimal inhibitory concentration (MIC) for each drug in the combination is less than or equal to one fourth of the single-agent MICs. REF: 308 OBJ: Level 2: Interpretation 82. When the activity of the combination of antimicrobial agents is less than that of the single agents, we say the agents are: a. synergistic. b. antagonistic. c. indifferent. d. uncomplicated. B Conversely, antagonism is defined as the activity of the combination less than (and minimal inhibitory concentrations [MICs] are greater than) that of the single agents. In indifference, the activity of the combination is equal to that of the single agents. REF: 308 OBJ: Level 1: Recall 83. All of the following techniques are used to measure the amount of antimicrobial agent in the serum except: a. biologic assays. b. immunoassays. c. chromatographic assays. d. bioluminescence assays. D Bioassays are sometimes used today when the focus is on the amount of biologically active drug present rather than the amount of “chemical” present. RIA, FIA, FPIA, and EIA have all been used to measure antimicrobial agents in serum and other body fluids. Various chromatographic methods, including gas-liquid, thin-layer, and paper chromatography, have been used on occasion for antimicrobial agent assays. REF: 310 OBJ: Level 1: Recall [Show More]

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