Deck 12: Laboratory Methods and Strategies for Antimicrobial Susceptibility Testing

Question 1

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

Answer 1

B
Proteus spp.may swarm over the surface of the plate and complicate clear interpretation of the zone boundaries.In these cases, the swarming haze is ignored and zones are measured at the point where growth is obviously inhibited.Similarly, hazes of bacterial growth may be observed when testing sulfonamides and trimethoprim as a result of the organism population going through several doubling generations before inhibition; the resulting haze of growth should be ignored for disk interpretation with these agents.

Question 2

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

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

Answer 2

A
The interpretive categories of MIC-based testing are defined according to the attainable concentrations of the antibiotic in the patient's bloodstream.For the antibiotic to be effective, the concentration of the drug in serum must be inhibitory to the bacteria.

Question 3

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

A) amount of turbidity in the test well.
B) correlations of the MIC with serum achievable levels for the antimicrobial agent, particular resistance mechanisms, and successful therapeutic outcomes.
C) size of the zone of inhibition.
D) reciprocal value of the MIC.

Answer 3

B
The interpretive categories of MIC-based testing are defined according to the attainable concentrations of the antibiotic in the patient's bloodstream.For the antibiotic to be effective, the concentration of the drug in the serum must be inhibitory to the bacteria.

Question 4

The serum bactericidal test (SBT)involves:

A) subculturing the clear tubes of a completed broth dilution test.
B) 99.9% reduction in CFUs/mL, compared with the organism concentration in the original inoculum.
C) using patient serum to detect bacteriostatic and bactericidal activity and the antibacterial impact of factors other than the antibiotics.
D) microscopically interpreting results using microtiter plates.

Question 5

Bactericidal tests should be performed:

A) for every clinically significant organism isolated from a clinical specimen.
B) to evaluate antimicrobial agents usually considered to be bacteriostatic.
C) for severe and life-threatening infections.
D) on all gram-positive and gram-negative organisms, whether or not they are clinically significant.

Question 6

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

A) pure culture.
B) inoculum that matches the severity of the infection.
C) single isolated colony.
D) variety of colony types.

Question 7

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) Urine
D) Sputum

Question 8

Enterococcal high-level gentamicin resistance predicts resistance to:

A) vancomycin.
B) aminoglycoside.
C) cephalosporin.
D) amikacin.

Question 9

Daptomycin, whose mode of action is to inhibit cell membrane function, is ineffective against which organism?

A) Staphylococcus aureus
B) Enterococcus
C) Escherichia coli
D) Streptococcus spp.

Question 10

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

A) using reflected light.
B) using transmitted light.
C) placing the medium under the light, rather than in front of it.
D) microscopy.

Question 11

Enterococcal resistance to ampicillin predicts resistance to:

A) vancomycin.
B) aminoglycoside.
C) cephalosporin.
D) amikacin.

Question 12

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

A) MICs.
B) tolerance limits.
C) breakpoints.
D) border concentrations.

Question 13

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

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

Question 14

The minimal bactericidal concentration (MBC)test involves:

A) testing the organism against gram-positive antibiotics.
B) testing the organism against gram-negative antibiotics.
C) 99.9% reduction in CFUs/mL, compared with the organism concentration in the original inoculum.
D) observing the first tube where growth is inhibited.

Question 15

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

A) vancomycin.
B) aminoglycoside.
C) cephalosporin.
D) amikacin.

Question 16

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) National Committee for Clinical Laboratory Standards (NCCLS).
C) Manual of Standard Operating Procedures (MSOP).
D) Clinical and Laboratory Standards Institute (CLSI).

Question 17

Chromogenic cephalosporins are used to test for:

A) action of beta-lactamase inhibitors.
B) intrinsic resistance to beta-lactam drugs by decreased intracellular uptake of the drug.
C) production of beta-lactamase by the infecting organism.
D) intrinsic resistance to beta-lactam drugs by altered antibiotic targets.

Question 18

Growth on the oxacillin agar screen means that a patient should be treated with which antibiotic?

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

Question 19

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

A) maximum concentration of antimicrobial agent that should be used for optimal therapy.
B) minimum 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 20

The primary goal of antimicrobial susceptibility testing is to:

A) determine whether the pathogen is capable of expressing resistance to the potential choices of antimicrobial agents.
B) determine the extent of an organism's intrinsic resistance.
C) determine in vivo whether the administered antibiotic is killing the pathogen.
D) identify toxins associated with bacterial pathogens.