Deck 13: Antimicrobial Susceptibility Testing

Question 1

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

A) the physician insists that a secondary agent be reported.
B) the hospital pharmacy no longer stocks the group A agent.
C) the patient cannot tolerate the primary agents.
D) the FDA has recalled the primary agents.

Answer 1

C

Question 2

Why would normal flora isolating 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.

Answer 2

B

Question 3

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 patient's infection
B) When the susceptibility of the isolate to particular antimicrobial agents cannot be reliably predicted
C) When the organism is part of the normal microbiota
D) Both A and B

Answer 3

C

Question 4

What concentrations of antibiotics are typically tested in MICs?

A) Serial fivefold dilution concentrations are tested.
B) Serial 10-fold dilution concentrations are tested.
C) Serial onefold dilution concentrations are tested.
D) Serial twofold dilution concentrations are tested.

Question 5

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

Question 6

Note that for some antimicrobial agents, different MIC interpretive criteria exist for

A) organisms or organism groups.
B) anatomic sites.
C) age groups.
D) immune status.

Question 7

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).

Question 8

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.

Question 9

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.

Question 10

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

Question 11

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 $\beta$ -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.

Question 12

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

Question 13

What McFarland standard provides turbidity comparable to that of a bacterial suspension containing approximately 1.5 $\times$ 10⁸ colony-forming units (CFU)/mL?

A) 1.0
B) 1.5
C) 0.5
D) 2.0

Question 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

Question 15

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

Question 16

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.

Question 17

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.

Question 18

The minimal inhibitory concentration (MIC) is

A) the lowest antibiotic concentration to slow the growth of an organism.
B) the highest antibiotic concentration to slow the growth of an organism.
C) the lowest antibiotic concentration to inhibit the growth of an organism.
D) the highest antibiotic concentration to inhibit the growth of an organism.

Question 19

What is the most critical step in any susceptibility test?

A) Plate streaking
B) Organism isolation
C) Gram stain identification
D) Inoculum preparation

Question 20

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.

Question 21

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

Question 22

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.

Question 23

A patient is admitted to the hospital with a diagnosis of endocarditis. Two separate blood cultures grow out of viridans streptococci. The penicillin MIC of the organism is 0.12 µg/mL. What antibiotic should the physician prescribe?

A) Penicillin
B) Erythromycin
C) Clindamycin
D) Gentamicin

Question 24

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

Question 25

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

Question 26

When reading broth microdilution MIC testing, the MIC for a particular drug is

A) the lowest concentration showing no obvious growth.
B) the highest concentration showing no obvious growth.
C) the lowest concentration showing confluent growth.
D) the highest concentration showing confluent growth.

Question 27

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 MIC units.
C) interpreted as susceptible, intermediate, or resistant.
D) None of the above

Question 28

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

Question 29

What is the approximate final concentration of organisms in each well of a microdilution tray?

A) 3 $\times$ 10⁵ CFU/mL
B) 1 $\times$ 10⁵ CFU/mL
C) 4 $\times$ 10⁵ CFU/mL
D) 5 $\times$ 10⁵ CFU/mL

Question 30

In terms of MIC microdilution trays, a skipped well occurs

A) when a well on the tray is skipped with inoculum and it does not grow.
B) when the inoculator is turned around and the sterility well is accidentally inoculated and the growth well is accidentally skipped.
C) when there is growth at higher concentrations and no growth at one or more of the lower concentration wells.
D) None of the above

Question 31

What agencies develop the zone interpretive criteria?

A) CDC and USAMRIID
B) U.S. FDA and CDC
C) FDA and CLSI
D) CLSI and CDC

Question 32

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

Question 33

The broth microdilution MIC test uses these for the testing.

A) Test tubes
B) Plastic strips
C) Multiwell microdilution trays
D) Multiwell macrodilution trays

Question 34

What broth is recommended in the tube dilution MIC tests?

A) Trypticase soy
B) Mueller-Hinton
C) Columbia
D) Cooked meat

Question 35

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

Question 36

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

Question 37

What is the storage temperature for frozen panels?

A) 0° C
B) -20° X to -70° C
C) -75° X to -100° C
D) 273 K

Question 38

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

Question 39

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.

Question 40

What is a breakpoint panel?

A) A standard 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.

Question 41

What antibiotics are active against ESBL-producing strains of bacteria?

A) Cephalosporins
B) Penicillins
C) Carbapenems
D) Aztreonam

Question 42

Spontaneous mutations occur in the $\beta$ -lactamase genes that result in extended-spectrum $\beta$ -lactamases (ESBL). These ESBL inactivate

A) cephalosporins.
B) penicillins.
C) aztreonam.
D) All of the above

Question 43

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

Question 44

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

Question 45

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

Question 46

A man goes to his doctor because he has a cloudy substance dripping from his penis. A Gram stain is performed and gonorrhea is isolated. What is the drug of choice to treat this infection?

A) Gentamicin
B) Erythromycin
C) Vancomycin
D) Ceftriaxone

Question 47

To obtain a bactericidal effect in enterococci, ampicillin or penicillin must be given in combination with

A) macrolides.
B) peptidoglycans.
C) cephalosporins.
D) aminoglycosides.

Question 48

What type of antimicrobial susceptibility testing method does CLSI recommend for anaerobes?

A) Microdilution
B) Agar dilution
C) Tube broth macrodilution
D) Disk diffusion

Question 49

The 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 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.

Question 50

On what media is Kirby-Bauer testing performed for Neisseria gonorrhoeae?

A) 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

Question 51

Oxacillin-resistant staphylococci can appear susceptible in vitro to other $\beta$ -lactam agents such as

A) quinolones.
B) tetracyclines.
C) cephalosporins.
D) aminoglycosides.

Question 52

What automated system was originally designed for use in outer space?

A) TREK ARIS
B) MicroScan Walkaway SI
C) BD Phoenix
D) VITEK

Question 53

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

Question 54

Therapy of disseminated meningococcal infections and various types of gonococcal (GC) infections are

A) hard to treat and require special $\beta$ -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 CDC.
D) None of the above

Question 55

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

Question 56

Oxacillin-resistant Staphylococcus aureus is called

A) methicillin-resistant S. aureus (MRSA).
B) oxacillin-resistant S. aureus.
C) penicillin-resistant S. aureus.
D) penicillinase-resistant S. aureus.

Question 57

In vitro testing conditions that can be modified to enhance the expression of oxacillin resistance include all the following, except

A) use Mueller-Hinton media.
B) making final test readings after a full 24 hours of incubation.
C) incubation of tests at temperatures no greater than 35 $\degree$ C.
D) preparation of inocula using the direct inoculum suspension procedure.

Question 58

When testing Escherichia coli, Klebsiella spp., and Proteus mirabilis, which drug is mostly likely to indicate the presence of an ESBL?

A) Aztreonam
B) Cefpodoxime
C) Ceftriaxone
D) Ceftazidime

Question 59

The BD Phoenix system uses this 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

Question 60

How long are MIC panels for anaerobes incubated?

A) 24 hours
B) 36 hours
C) 48 hours
D) 72 hours

Question 61

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.

Question 62

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

Question 63

The inhibition zone size and MIC interpretive criteria published by the 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 FDA approval.
D) pharmacodynamic data.

Question 64

All the following organisms often produce $\beta$ -lactamase, except

A) Haemophilus influenzae.
B) Neisseria gonorrhoeae.
C) Moraxella catarrhalis.
D) Neisseria meningitidis.

Question 65

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

Question 66

The minimum bactericidal concentration (MBC) test differs from the 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

Question 67

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 MBC at the infection site.

Question 68

What is the most widely used supplementary quality control measure for antimicrobial susceptibility testing?

A) $\beta$ -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

Question 69

A microbiology technologist is performing a $\beta$ -lactamase test on a staphylococcus isolated from a hospitalized patient. The test was negative, so the technologist reported out in vitro antimicrobial susceptibility test results for $\beta$ -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 $\beta$ -lactamase test on the staphylococci negative, when the organism produced $\beta$ -lactamase?

A) The test was a false negative-the organism really did produce $\beta$ -lactamase, but it did not react with the test.
B) Staphylococci must be exposed to the $\beta$ -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 $\beta$ -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.

Question 70

If the 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

Question 71

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 MIC.
D) computerized data management systems.

Question 72

If the MIC or zone size is interpreted as intermediate 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 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

Question 73

All of the following steps are taken to verify the results from an atypical antibiogram, except

A) reexamining the disk diffusion plate, or 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.

Question 74

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

Question 75

If the MIC or zone size is interpreted as resistant 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 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

Question 76

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.

Question 77

What is the principle of the most commonly used test to detect $\beta$ -lactamase production?

A) Chromogenic cephalosporin nitrocefin
B) Redox reaction
C) Hydrolysis of fluorogenic substrate
D) Reduction of acetoin

Question 78

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) CDC
B) College of American Pathologists (CAP)
C) CLSI
D) Joint Commission on Accreditation of Healthcare Organizations (JCAHO)

Question 79

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

Question 80

Quality control of antimicrobial susceptibility tests involves

A) performing antimicrobial susceptibility tests on organisms using the 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