Deck 11: Laboratory Methods and Strategies for Antimicrobial Susceptibility Testing

Question 1

A bacterial suspension of 1.5× 10⁸ colony-forming units (CFUs)/mL is comparable with which McFarland standard?

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

Answer 1

A
Accurate and reliable antimicrobial susceptibility testing results require standardized conditions.Properly prepared inocula are key to any antimicrobial susceptibility testing method.Two important requirements for correct inoculum preparation are use of a pure culture and use of a standard-sized inoculum.Pure inocula are obtained by selecting four or five colonies of the same morphology,inoculating them into broth medium or 0.9% saline solution to achieve a turbid suspension.A standard inoculum size is accomplished by comparing the turbidity of the organism suspension with a turbidity standard.The 0.5 McFarland standard,which is commercially available,provides an optical density comparable to the density of a bacterial suspension of 1.5× 10⁸ CFU/mL.If the bacterial suspension does not match the standard's turbidity,the suspension must be further diluted or supplemented with more organisms as needed.

Question 2

The primary goal of antimicrobial susceptibility testing is to:

A)Determine whether the pathogen expresses resistance to the potential therapeutic antimicrobial agents.
B)Determine the spectrum of an organism's intrinsic resistance.
C)Determine whether the administered antibiotic is effective at killing the pathogen in vivo.
D)Identify toxins secreted by bacterial pathogens isolated from a clinical specimen.

Answer 2

A
The primary goal of antimicrobial susceptibility testing is to determine whether the bacterial cause of concern is capable of expressing resistance to the antimicrobial agents that are potential choices for therapy.Because intrinsic resistance is usually well known for most organisms,testing to establish this resistance is not usually necessary and organism identification is sufficient.In essence,for any clinically important organism whose antimicrobial susceptibility profile is unpredictable,antimicrobial susceptibility tests are designed to determine the extent of an organism's acquired resistance to potential choices of antimicrobial agents.

Question 3

Chromogenic cephalosporins are used to test for:

A)degradative/hydrolytic action of b-lactamase inhibitors.
B)intrinsic resistance to b-lactam drugs by increased efflux of the drug.
C)production of b-lactamase by the infecting organism.
D)intrinsic resistance to b-lactam drugs by altered antibiotic targets.

Answer 3

C
Various assays are available to detect b-lactamases,but the most useful one for clinical laboratories is the chromogenic cephalosporinase test.b-lactamases exert their effect by opening the b-lactam ring.With the use of a chromogenic cephalosporin as the substrate,this process results in a colored product.

Question 4

Up-to-date tables that list potential antimicrobial agents to include in batteries for testing against particular organisms or organism groups are published by the:

A)Centers for Disease Control and Prevention (CDC).
B)U.S.Food and Drug Administration (FDA).
C)Manual of Standard Operating Procedures (MSOP).
D)Clinical and Laboratory Standards Institute (CLSI).

Question 5

Staphylococcus aureus was isolated from a patient's surgical wound.In a disk diffusion assay,the isolate was found to be resistant to cefoxitin (30 µg disk).With which antibiotic should this patient's infection be treated?

A)Oxacillin
B)Methicillin
C)Vancomycin
D)Cephalosporin

Question 6

The minimum inhibitory concentration (MIC)result recorded in antimicrobial testing can be defined as the:

A)lowest concentration of antimicrobial agent that should be used for optimal therapy.
B)highest concentration of antimicrobial agent that should be used for optimal therapy.
C)lowest antimicrobial agent concentration that completely inhibits visible bacterial growth.
D)highest antimicrobial agent concentration that completely inhibits visible bacterial growth.

Question 7

The concentrations in a broth dilution test that separate the categories of susceptible,intermediate,and resistant are called:

A)border concentrations.
B)tolerance limits.
C)breakpoints.
D)reference values.

Question 8

In the microdilution broth dilution method,the interpretive criteria for the susceptible,intermediate,or resistant categories are based on the:

A)quantitative differences (i.e. ,CFU/mL)in the turbidity in the test well.
B)correlation of the MIC with serum-achievable levels of the common doses of the antimicrobial agent and therapeutic outcomes based on clinical studies.
C)either the presence or absence of zones of inhibition.
D)FDA's recommendations based on a national database of participating health care agencies providing antibiograms for review and publication as a summative report.

Question 9

Daptomycin requires free calcium for its antibacterial action,is inhibited by pulmonary surfactant,and has activity against gram-positive bacteria.Gram-positive bacterial isolates can be tested for daptomycin susceptibility.For which type of infection should daptomycin susceptibility NOT be reported?

A)Bacteremia
B)Meningitis
C)Pneumonia
D)Skin/Soft tissue infection

Question 10

Staphylococcal resistance to oxacillin is used to determine and report resistance to:

A)aminoglycosides.
B)cephalosporins.
C)vancomycin.
D)amikacin.

Question 11

Detection of the haze produced by some resistant staphylococci and enterococci against methicillin and vancomycin can best be accomplished by:

A)using fluorescent microscopy.
B)holding plate in front of reader and using transmitted light.
C)holding plate in front of reader and using reflected light.
D)placing plate on black background with a light directly above it.

Question 12

As a result of the organism going through several doubling generations of growth before inhibition,a haze of bacterial growth occurs on the agar around the disk.This phenomenon,which should be ignored,can be observed when testing which class of antibiotics?

A)Aminoglycosides
B)Sulfonamides
C)Penicillins
D)Cephalosporins

Question 13

The antibiotic nitrofurantoin is effective against both gram-positive and gram-negative bacteria.This antibiotic is effective against which type of specimen?

A)Blood
B)Cerebrospinal fluid (CSF)
C)Sputum
D)Urine

Question 14

Bactericidal tests should be performed:

A)for severe and life-threatening infections.
B)for every clinically significant organism isolated from a clinical specimen.
C)for evaluation of antimicrobial agents usually considered bacteriostatic.
D)for bacteria isolated from wounds,whether or not they are clinically significant.

Question 15

Enterococcal resistance to ampicillin predicts resistance to:

A)vancomycin.
B)aminoglycosides.
C)cephalosporins.
D)linezolid.

Question 16

An important requirement for appropriate inoculum preparation in susceptibility testing includes the use of a(n):

A)pure culture.
B)single isolated colony.
C)variety of colony morphotypes.
D)inoculum equivalent to the bacterial burden at the site of infection.

Question 17

In the disk diffusion method of antimicrobial susceptibility testing,what is the standard medium used for testing most bacterial organisms?

A)Mueller-Hinton broth
B)Mueller-Hinton agar
C)Brain-heart infusion agar
D)Brucella broth with 5% lysed horse blood

Question 18

The minimal bactericidal concentration (MBC)test:

A)utilizes disks impregnated with series of concentrations (i.e. ,twofold dilutions)of each antimicrobial agent being tested against the bacterial isolate.
B)will have an endpoint that is less than or equal to the MIC for each specific antimicrobial tested.
C)is the antimicrobial concentration that yields a 99.9% reduction in CFU/mL,compared to concentration of the original bacterial inoculum.
D)measures the ability of an antimicrobial to kill all bacteria in the endpoint MIC tube/well by planting an aliquot from it and incubating it overnight.

Question 19

The serum bactericidal test (SBT)involves:

A)serial dilutions of antimicrobials in cation-adjusted Mueller-Hinton broth.
B)99.9% reduction in CFU/mL,compared with the organism concentration in the original inoculum.
C)using the patient's serum to detect its bacteriostatic and bactericidal activity.
D)subculturing the tubes of a completed broth dilution test showing no visible growth.

Question 20

Enterococcal high-level gentamicin resistance predicts resistance to:

A)vancomycin.
B)aminoglycosides.
C)cephalosporins.
D)levofloxacin.

Question 21

For clinical decision-making carbapenemase production should be detected using:

A)MALDI-TOF mass spectrometry.
B)Modified Hodge test or Carba NP test.
C)molecular methods for specific carbapenemases.
D)routine AST methods using current CLSI interpretive criteria.

Question 22

Which antimicrobial susceptibility testing method uses a series of plates with different antimicrobial concentrations inoculated with multiple bacterial isolates per plate?

A)Agar dilution
B)Automated broth dilution
C)Disk Diffusion
D)E-test

Question 23

A sputum culture from a cystic fibrosis patient grew Stenotrophomonas maltophila.A broth microdilution panel was inoculated.Antimicrobials tested included: ceftazidime,imipenem,ticarcillin-clavulanate,levofloxacin,and trimethoprim-sulfamethoxazole.After 20-hour incubation at 35°C in ambient air,no growth was seen in any well of the microtiter plate.Which of the following statements is true?

A)The isolate is susceptible to all antimicrobials tested.
B)The microdilution plate was incubated under incorrect conditions.
C)The test results are invalid and retesting would be recommended.
D)A Cefinase test for b-lactamase activity should be done as an adjunct to MIC results.

Question 24

A newly defined interpretive category termed "Susceptible-Dose Dependent" (SDD)implies susceptibility of an isolate is dependent on dosing regimen.Which of the following statements is correct?

A)SDD is included within the susceptible category definition.
B)SDD is assigned to therapeutic agents that are approved under investigational use only.
C)The infecting organism may be effectively treated by using higher doses or more frequent doses or both of the drugs.
D)Drugs in this category have narrow therapeutic indices and their use at nonconventional ways must be monitored closely.

Question 25

Antimicrobial susceptibility testing should be performed when

A)a single bacterial species grows from any body site.
B)an isolate has a predictable susceptibility profile to therapeutic drugs of choice.
C)reliable standardized methods for testing a clinically significant isolate exist.
D)administering intravenous or oral antibiotics to treat an infection.

Question 26

A D-test is set up on a S.aureus isolate.There was no zone around the Erythromycin disk and the zone of inhibition around the clindamycin disk adjacent to the erythromycin disk was interpreted as susceptible.Thus,the resistance mechanism for the erythromycin is due to:

A)altered target.
B)increased efflux.
C)decreased porin channels.
D)degradative enzymes.

Question 27

Antimicrobial susceptibility testing should be performed when:

A)a single bacterial species grows from any body site.
B)an isolate has a predictable susceptibility profile to therapeutic drugs of choice.
C)reliable standardized methods for testing a clinically significant isolate exist.
D)administering intravenous or oral antibiotics to treat an infection.

Question 28

Advantages of automated susceptibility tests (e.g. ,Vitek 2 and Microscan Walkaway)include but are not limited to:

A)automated preparation of inoculum.
B)resistance detection by molecular methods (e.g. ,real-time PCR).
C)advanced (e.g. ,expert)software that recognizes and reports resistance patterns.
D)testing of most aerobic and commonly isolated anaerobic bacterial species.

Question 29

A staphylococcal isolate that gives a negative result using the Cefinase disk and tests resistant to penicillin by disk diffusion means that:

A)the isolate may contain the mecA gene.
B)the bacteria produce the b-lactamase enzyme.
C)the isolate is also resistant to ampicillin,carbenicillin,ticarcillin,and imipenem.
D)the isolate is also resistant to oxacillin,cefoxitin,and ticarcillin-clavulanate.

Question 30

Data review process for AST profiles:

A)requires susceptibility testing results only.
B)is the responsibility of a single individual in the laboratory.
C)can be done on "as needed" basis,no less frequently than weekly.
D)allows for troubleshooting of unusual profiles and results to be held until resolved.

Question 31

What action should be taken for any isolate found to be nonsusceptible (NS)to a particular antimicrobial agent?

A)No additional action is needed,as these results can occur with any organism.
B)Organism's identification and susceptibility test results should be confirmed.
C)The patient report should indicate the isolate is resistant to the antimicrobial.
D)Patient's physician should be contacted and advised that effectiveness of this antimicrobial against the isolate cannot be judged by in vitro testing.

Question 32

Standardization of antimicrobial susceptibility testing optimizes bacterial growth so inhibition of growth can be attributed to the antimicrobial agents tested and results are accurate,reproducible,and consistent regardless of the microbiology laboratory performing the test.Which of the following incubation conditions are used for common (i.e. ,nonfastidious)bacterial isolates using manual techniques?

A)16 to 20 hours,ambient temperature,5% to 7% CO₂
B)16 to 20 hours,35°C,ambient air
C)20 to 24 hours,ambient temperature,5% to 7% CO₂
D)20 to 24 hours,35°C,ambient air

Question 33

MIC endpoints can be reported results obtained by the following conventional AST methods EXCEPT:

A)agar dilution.
B)automated broth dilution.
C)disk Diffusion.
D)E-test.

Question 34

Assaying bacterial isolates for resistance genes by molecular methods:

A)can detect unknown resistance genes not yet defined.
B)cannot detect phenotypic resistance due to a combination of mechanisms.
C)means the resistance gene is expressed if found to be present.
D)is currently part of the routine clinical microbiology workflow for all isolates.

Question 35

S.pneumoniae was isolated from a spinal fluid sample from an inpatient.An oxacillin disk screening test was performed and the zone of inhibition measured 19 mm.Which action is most appropriate?

A)Call patient's physician and report the isolate as oxacillin-resistant;no further work-up required.
B)Repeat testing to confirm the identification of the isolate and qualitative susceptibility profile.
C)Report the isolate as penicillin-resistant;inform infection control and patient's physician.
D)Do E-test and report quantitative MIC result and interpretive category using meningitis MIC criteria.

Question 36

Quality control results for disk diffusion AST using appropriate ATCC strains of bacteria showed zone size diameters decreasing over time for a few antimicrobials.Which of the following statements is a possible explanation?

A)Antimicrobial disks were stored in a frost-free freezer.
B)Concentration of the bacterial inoculum was 5× 105 CFU/mL.
C)Depth of Mueller-Hinton agar plates measured 3 mm.
D)Plates were incubated less than 16 hours in ambient air.

Question 37

AST was performed using Mueller-Hinton agar with 5% sheep blood,35°C in 5% CO₂ from 24 hours according to standard protocols,what is the most likely identity of the organism being tested?

A)S.aureus
B)Streptococcus pneumoniae
C)Neisseria gonorrhoeae
D)Moraxella catarrhalis