Exam 1 - (CH 13) Antimicrobial Susceptibility Testing Textbook Review Questions

Question 1

Which of the following urine cultures suggests contamination?
A. One colony type with 10 to the 3rd colony-forming unit (CFU)/mL
B. Two colony types with 10 to the 2nd CFU/mL
C. Three colony types with 10 to the 5th CFU/mL
D. One colony type with 10 to the 6th CFU/mL

Answer 1

Three colony types with 10 to the 5th CFU/mL

Question 2

Generally, a laboratory will define a battery of ____ antimicrobial agents for routine testing against the Enterobacteriaceae.
A. 1 to 2
B. 3 to 5
C. 10 to 15
D. 20 to 25

Answer 2

C. 10 to 15

Question 3

Which of the following is recommended when preparing inocula for antibiotic susceptibility testing?
A. A single isolated colony diluted to a McFarland 0.5 standard turbidity
B. Four to five colonies diluted to a McFarland 0.5 standard turbidity
C. The suspension must be allowed to incubate for a minimum of 30 minutes before use
D. Two to three isolated colonies diluted to a McFarland 3.0 standard turbidity

Answer 3

B. Four to five colonies diluted to a McFarland 0.5 standard turbidity

Question 4

Once the minimal inhibitory concentration (MIC) is determined using the broth dilution method, the organism’s antibiotic susceptibility is interpreted as follows:
A. Sensitive, indeterminant, insensitive
B. Nonsusceptible, susceptible, intermediate, or resistant
C. Positive, inconclusive, negative
D. Susceptible or resistant

Answer 4

B. Nonsusceptible, susceptible, intermediate, or resistant

Question 5

This broth is the recommended medium for broth dilution MIC tests of nonfastidious bacteria.
A. Mueller-Hinton
B. MacConkey
C. Thioglycollate
D. YM broth

Answer 5

A. Mueller-Hinton

Question 6

The end point for the sulfonamides and trimethoprim is an ____ reduction of growth.
A. 30%
B. 50%
C. 65%
D. 80%

Answer 6

D. 80%

Question 7

When obvious colonies are within a clear zone on the agar surrounding a disk, which of the following actions are most appropriate?
A. This is typical of contamination and should be reported as sensitive.
B. Retest the original isolate and report as resistant if the results are the same.
C. This is indicative of a mixed culture and should be immediately reported out as resistant.
D. These colonies represent a minority resistant subpopulation but are not considered clinically significant.

Answer 7

B. Retest the original isolate and report as resistant if the results are the same.

Question 8

The standard disk diffusion procedure is typically performed using
A. Mueller-Hinton agar with 5% sheep blood incubated at 5% CO2 for 20 to 24 hours.
B. Mueller-Hinton agar with 10% sheep blood incubated at 10% CO2 for 10 to 12 hours.
C. Malt extract agar with 5% brain–heart infusion incubated at 5% CO2 for 24 to 36 hours.
D. Kirby-Bauer agar with 5% sheep blood incubated at 5% CO2 for 20 to 24 hours.

Answer 8

A. Mueller-Hinton agar with 5% sheep blood incubated at 5% CO2 for 20 to 24 hours.

Question 9

Penicillin susceptibility in Streptococcus pneumoniae should be performed using a disk impregnated with this antibiotic.
A. Penicillin
B. Amoxicillin
C. Oxacillin
D. Erythromycin

Answer 9

C. Oxacillin

Question 10

This test is helpful in identifying heteroresistant vancomycin-intermediate resistant Staphylococcus aureus because the test uses a higher concentration of organisms than the other tests.
A. Etest
B. Broth microdilution test
C. Vancomycin agar screen
D. Vancomycin disk diffusion

Answer 10

A. Etest

Question 11

Which test is used to detect inducible clindamycin resistance in staphylococci?
A. Matrix-assisted laser desorption ionization–time-of-flight (MALDI-TOF)
B. Etest
C. D-zone test
D. Sensititre

Answer 11

C. D-zone test

Question 12

Why would it be inappropriate to perform antimicrobial susceptibility tests on viridans streptococci isolated from a throat culture?

Answer 12

Viridans streptococci are normal microbiota in the throat. Reporting antimicrobial susceptibility test results often suggests to the physician that the isolated organism is clinically significant, and antimicrobial therapy should be considered. Reporting antimicrobial susceptibility results on viridans streptococci from the throat may lead to inappropriate use of antimicrobial agents and may also prevent the physician from finding the correct answer to the patient’s problem.

Question 13

The turbidity of a McFarland 0.5 standard corresponds to approximately __________ bacteria per milliliter.

Answer 13

1.5 × 10 to the 8th

Question 14

The ___________ disk cannot be used to screen for penicillin susceptibility in Streptococcus pneumoniae from sputum.

Answer 14

Oxacillin

Question 15

Which method does the CLSI suggest is most reliable for detecting oxacillin resistance in staphylococci?

a. Oxacillin disk diffusion test
b. Cefoxitin disk diffusion test
c. Cefoxitin MIC test
d. Penicillin MIC test e. Oxacillin agar screen

Answer 15

b. Cefoxitin disk diffusion test

Question 16

What does the mecA gene code for in staphylococci?

a. β-Lactamase and penicillin resistance
b. ESBLs and cephalosporin resistance
c. Penicillin-binding protein 2a and oxacillin resistance
d. Altered penicillin-binding protein and vancomycin resistance
e. β-Lactamase and oxacillin resistance

Answer 16

c. Penicillin-binding protein 2a and oxacillin resistance

Question 17

With which of the following profiles should S. aureus or β-hemolytic streptococci be subjected to D-zone testing?

a. Oxacillin-resistant
b. Erythromycin-resistant and clindamycin-susceptible
c. Erythromycin-resistant and clindamycin-resistant
d. Penicillin-resistant e. Oxacillin- and penicillin-resistant

Answer 17

b. Erythromycin-resistant and clindamycin-susceptible

Question 18

An ampicillin-susceptible Enterococcus faecalis from a blood culture has high-level resistance to the aminoglycoside gentamicin. Which of the following statements is true?

a. Ampicillin and gentamicin will be synergistic.
b. Ampicillin and gentamicin will not be synergistic.
c. Penicillin and gentamicin will be synergistic.
d. Cefazolin and gentamicin will be synergistic. e. Ampicillin and tobramycin will be synergistic.

Answer 18

b. Ampicillin and gentamicin will not be synergistic.

Question 19

ESBL-producing isolates should be considered resistant to which of the following agents?

a. Cephalosporins, penicillins, and aztreonam
b. Cephalosporins, penicillins, and aminoglycosides
c. Cephalosporins, penicillins, and β-lactamase inhibitors
d. Penicillins and aminoglycosides
e. Cefotaxime– clavulanic acid and ceftazidime clavulanic acid

Answer 19

a. Cephalosporins, penicillins, and aztreonam

Question 20

Which of the following organisms is commonly tested for β-lactamase production?

a. Neisseria meningitidis
b. Klebsiella pneumoniae
c. Streptococcus pneumoniae
d. Haemophilus influenzae
e. Escherichia coli

Answer 20

d. Haemophilus influenzae

Question 21

Which of the following is true about quality control (QC) testing?

a. A laboratory must perform QC every day that patient isolates are tested.
b. A laboratory can perform QC weekly if it performs fewer than 10 tests daily.
c. A laboratory can perform QC weekly once accurate performance of 20 to 30 days of daily quality control has been documented.
d. A laboratory does not have to perform QC if an automated test antimicrobial susceptibility test system is used.
e. Testing materials can be used beyond their expiration date if they look satisfactory.

Answer 21

c. A laboratory can perform QC weekly once accurate performance of 20 to 30 days of daily quality control has been documented.

Question 22

True or False

Ampicillin susceptibility in P. aeruginosa is unusual.

Answer 22

True

Question 23

True or False

Many S. aureus strains are vancomycin-resistant.

Answer 23

False

Question 24

True or False

Penicillin is the drug of choice for treating gonorrhea.

Answer 24

False

Question 25

True or False

S. maltophilia strains are usually susceptible to trimethoprim-sulfamethoxazole.

Answer 25

True

Question 26

True or False

E. coli strains are always resistant to all aminoglycosides.

Answer 26

False

Question 27

Which of the following statements is true?

a. An organism that is reported as “nonsusceptible” to an antimicrobial is definitely resistant to the antimicrobial.
b. The term nonsusceptible is used when the interpretive categories of “intermediate” and “resistant” do not apply.
c. The organization that oversees preparing technical documents for international use is the European Medicines Agency.

Answer 27

The term nonsusceptible is used when the interpretive categories of “intermediate” and “resistant” do not apply.

Question 28

Why is a bactericidal drug regimen necessary for treating patients with bacterial endocarditis?

Answer 28

Endocarditis is a serious infection and is located at a body site in which immune defense mechanisms are not abundant. Therefore it is essential for the antimicrobial agent to kill the bacteria to effect a cure.

Question 29

The MBC end point is the lowest concentration of antimicrobial agent that kills ______ of the test bacteria.

Answer 29

99.9%

Question 30

It is important to test bacteria in what phase of growth when performing tests to assess bactericidal activity?

Answer 30

Logarithmic

Question 31

Antimicrobial agents categorized as having bactericidal activity always kill 100% of the test bacteria when the bacteria are exposed to concentrations of the agent greater than or equal to the MIC. True or false?

Answer 31

False

Question 32

Which of the following definitions best defines synergism?

a. The activity of the drug combination is greater than that of the individual agents.
b. The activity of the drug combination is less than that of the individual agents.
c. The activity of the drug combination is equal to that of the individual agents.
d. The test organism is susceptible to both drugs in the combination.
e. The test organism is resistant to both drugs in the combination.

Answer 32

a. The activity of the drug combination is greater than that of the individual agents.

Question 33

What is another name for the serum bactericidal test?

Answer 33

The Schlichter test

Question 34

Which of the following factors contribute to results in the serum bactericidal test?

a. Antibody, opsonins, complement
b. Opsonins
c. Antibody
d. Complement
e. None of the above

Answer 34

a. Antibody, opsonins, complement

Question 35

When are serum specimens obtained for serum bactericidal and antimicrobial assays?
a. Trough, 1 hour before dose; peak, 1 hour after intravenous (IV) dose
b. Trough, 1 hour after dose; peak, 3 hours before intramuscular (IM) dose
c. Trough, 30 minutes before dose; peak, 60 minutes after IM dose
d. Trough, 30 minutes before dose; peak, 12 hours after IV dose e. Trough, 1 hour before dose; peak, 8 hours after oral dose

Answer 35

c. Trough, 30 minutes before dose; peak, 60 minutes after IM dose

Question 36

Use of molecular assays to detect antimicrobial resistance genes is limited because:

a. Genes are not responsible for most types of antimicrobial resistance.
b. Genes may be present but may not be expressed; therefore the presence of the gene does not always correlate with resistance.
c. Researchers have been unable to identify genes for antimicrobial resistance.
d. Large numbers of genes are responsible for all clinically important resistance.
e. Testing is too expensive.

Answer 36

Genes may be present but may not be expressed; therefore the presence of the gene does not always correlate with resistance.

Question 37

Which of the following classes of antimicrobial agents poses the greatest toxicity risks and therefore is frequently monitored using antimicrobial assays?

a. Penicillins
b. Cephalosporins
c. Sulfonamides
d. Aminoglycosides
e. Tetracyclines

Answer 37

d. Aminoglycosides

Question 38

Points to Remember

Answer 38

■ Antimicrobial susceptibility testing is performed only on bacteria likely to be causing an infection.
■ The Clinical and Laboratory Standards Institute (CLSI) determines standards for antimicrobial susceptibility testing and reporting in clinical laboratories.
■ The U.S. Food and Drug Administration (FDA) has the regulatory authority for setting susceptibility test interpretive criteria and quality control parameters.
■ Antimicrobial susceptibility testing protocols describing what, when, and how to test and report should be developed with input from clinicians, pharmacists, and others who have clinical experience with the antimicrobial therapy practices of the particular institution.
■ Selective reporting basically refers to reporting broader-spectrum agents only in select situations, such as when the patient’s isolate is resistant to narrower-spectrum agents.
■ Several variables must be controlled when performing any type of antimicrobial susceptibility test; inoculum standardization is one of the most important of these. Testing too few or too many bacteria can yield erroneous results.
■ The disk diffusion, Etest, broth microdilution minimal inhibitory concentration (MIC) test, and automated MIC tests are the most common methods currently used for antimicrobial susceptibility testing in clinical laboratories.
■ The disk diffusion, or Kirby-Bauer, test is a qualitative method; results are reported as susceptible, intermediate, or resistant.
■ The MIC test is a semiquantitative method and the concentration (µg/ mL) of a drug required to inhibit the growth of bacterial isolate is reported, together with a susceptible, intermediate, or resistant interpretation (e.g., ampicillin MIC = 8 µg/ mL; susceptible).

Question 39

Points to Remember

Answer 39

■ Routine antimicrobial susceptibility test methods can be modified for testing fastidious bacteria that require supplemental nutrients, modified incubation conditions, or both.
■ An oxacillin disk (1 µg) can be used to screen for penicillin susceptibility in S. pneumoniae.
■ β-Hemolytic streptococci remain universally susceptible to penicillin, the drug of choice for treating infections caused by these organisms, and susceptibility testing is usually not required.
■ β-Lactamase testing is performed on H. influenzae to determine whether the isolate is resistant to ampicillin and amoxicillin.
■ The third-generation cephalosporins ceftriaxone and cefotaxime are often used to treat meningococcal meningitis because N. meningitidis is generally susceptible to these agents and they have good penetration into cerebrospinal fluid.
■ The CLSI reference method described for testing anaerobic bacteria is agar dilution; however, a broth microdilution method can be used for less fastidious species of anaerobes, such as Bacteroides spp.
■ The gene responsible for oxacillin resistance in staphylococci is mecA.
■ The cefoxitin disk performs better than the oxacillin disk in detecting oxacillin-resistant staphylococci by the disk diffusion method.
■ Oxacillin-resistant staphylococci should be considered resistant to all β-lactam agents, despite any susceptible result for these in vitro.
■ The D-zone test is used to detect inducible clindamycin resistance in staphylococci and β Hemolytic streptococci.
■ The vancomycin broth microdilution MIC test or vancomycin agar screen is recommended to detect vancomycin-intermediate S. aureus (VISA) or vancomycin-resistant S. aureus (VRSA); some test methods are unable to detect these strains.
■ Special tests for high-level aminoglycoside resistance and routine tests with cell wall active agents such as penicillin are performed on enterococci to determine whether combination therapy would be effective in treating serious enterococcal infections, such as endocarditis.
■ Vancomycin-resistant enterococci (VRE) are important nosocomial pathogens; however, low-level vancomycin resistance is intrinsic in E. gallinarum and E. casseliflavus. These differ from true VRE for infection control purposes.
■ Extended-spectrum β-lactamase (ESBL)-producing bacteria should be considered resistant to all cephalosporins, penicillins, and aztreonam, despite a susceptible result for these in vitro.
■ Most automated instruments for antimicrobial susceptibility testing are based on turbidimetric detection of bacterial growth or detection of hydrolysis of a fluorogenic growth substrate.
■ Quality control of antimicrobial susceptibility tests is performed with standard reference strains that have defined antimicrobial susceptibility (or resistance) to the drugs tested.

Question 40

Points to Remember

Answer 40

■ Certain species have so-called typical antibiograms, which can be used to verify the identification and susceptibility results generated on the isolate.
■ Cumulative antibiograms represent the percentage of isolates of a given species susceptible to the antimicrobial agents commonly tested against the species.
■ “Nonsusceptible” does not necessarily mean that the organism is resistant to the antimicrobial.
■ Minimum bactericidal concentration (MBC) testing or serum bactericidal testing may be useful in select situations, such as for patients who are immunosuppressed with serious infections, or in whom infection is at a site at which immune mechanisms are not optimal.
■ MBC testing is performed after completion of a broth dilution MIC test, and the end point is 99.9% killing of the test bacteria.
■ Although there are CLSI guidelines for MBC and serum bactericidal tests, these tests are not as standardized as disk diffusion or MIC tests.
■ Synergism testing is performed using a time-kill or checkerboard assay in select situations, and there is no CLSI guideline or standardized method for synergism testing.
■ Serum bactericidal testing requires the use of the patient’s serum obtained at appropriate times surrounding dosing of the antimicrobial agent( s) and the bacterium causing the patient’s infection.
■ Some clinical laboratories use a mecA assay to determine oxacillin susceptibility or resistance in staphylococci.
■ When obtaining serum samples for serum bactericidal or antimicrobial assay, trough and peak specimens are usually obtained.