Microbiology Quiz: Kirby Bauer Disk Diffusion
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Kirby Bauer Disk DiffusionQuestion 1 of 20

While interpreting a Kirby-Bauer test on a Pseudomonas aeruginosa isolate, a technologist observes a large, clear zone of inhibition around the piperacillin-tazobactam disk. However, five distinct colonies are growing within this zone. The quality control test for the run was acceptable. What is the most appropriate interpretation?

The organism is susceptible, as the vast majority of the bacterial population was clearly inhibited.
The test is invalid due to likely contamination; subculture a colony and repeat the test.
The organism should be reported as resistant to piperacillin-tazobactam.
Measure the zone of inhibition by ignoring the internal colonies and report based on that measurement.
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Microbiology Quiz

Microbiology Quiz: Kirby Bauer Disk Diffusion

Practice Kirby Bauer Disk Diffusion in Microbiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Kirby Bauer Disk Diffusion, giving you a quick way to practice the rules, question types, and explanations that matter most for Microbiology.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

While interpreting a Kirby-Bauer test on a Pseudomonas aeruginosa isolate, a technologist observes a large, clear zone of inhibition around the piperacillin-tazobactam disk. However, five distinct colonies are growing within this zone. The quality control test for the run was acceptable. What is the most appropriate interpretation?

  1. The organism is susceptible, as the vast majority of the bacterial population was clearly inhibited.
  2. The test is invalid due to likely contamination; subculture a colony and repeat the test.
  3. The organism should be reported as resistant to piperacillin-tazobactam. (correct answer)
  4. Measure the zone of inhibition by ignoring the internal colonies and report based on that measurement.
Explanation: Correct Answer: C. The growth of significant colonies within a clear zone of inhibition indicates the presence of a resistant subpopulation. Even if the majority of the bacteria are susceptible, these resistant variants can be selected for during therapy, leading to clinical failure. Standard procedure is to consider the organism resistant to that antibiotic. While checking for purity is good practice, the primary interpretation based on this observation is resistance.

Question 2

A Kirby-Bauer plate testing an Enterococcus faecalis isolate against vancomycin is accidentally incubated for 36 hours instead of the standard 24 hours. The measured zone is 16 mm. The CLSI breakpoints for vancomycin are S ≥ 17 mm, I = 15-16 mm, R ≤ 14 mm. How should this result be managed?

  1. Report the result as Intermediate, as the measurement of 16 mm falls within the intermediate range.
  2. Report as Susceptible, because the prolonged incubation would artificially shrink the zone size.
  3. The test is invalid due to the deviation from standard incubation time and must be repeated. (correct answer)
  4. Report as Resistant, as any growth after 24 hours suggests the presence of a resistant subpopulation.
Explanation: Correct Answer: C. The Kirby-Bauer test is highly standardized, and incubation time is a critical parameter. Prolonged incubation allows the bacterial lawn to continue to grow slowly as the antibiotic degrades, leading to smaller, inaccurate zone sizes. Because the test was not performed according to the mandated protocol, the results are invalid and cannot be interpreted or 'corrected'. The test must be repeated.

Question 3

A routine quality control test using E. coli ATCC 25922 shows a gentamicin zone of 12 mm (acceptable range: 19-26 mm). The ampicillin zone for the same QC test is 18 mm (acceptable range: 16-22 mm). What is the most appropriate immediate action for the technologist to take?

  1. Report patient results for all antibiotics except gentamicin, as the ampicillin QC is in range.
  2. Repeat the quality control test using a new vial of gentamicin disks from a different lot number.
  3. Withhold all patient results from the batch, check the inoculum density, and repeat the entire test panel. (correct answer)
  4. Recalibrate the incubator temperature and re-incubate the QC plate for an additional 4 hours.
Explanation: Correct Answer: C. When a quality control result falls outside the acceptable range, all patient results obtained with that test run are considered invalid and must not be reported. A common technical error causing falsely small zones for multiple antibiotics (especially aminoglycosides like gentamicin) is an overly dense inoculum (too high McFarland standard). Therefore, the most appropriate first step is to check the inoculum preparation and repeat the entire test for both the QC strain and all patient isolates from that batch.

Question 4

A clinical laboratory needs to perform Kirby-Bauer susceptibility testing on an isolate of Haemophilus influenzae. Which of the following represents a critical procedural modification compared to the standard protocol for testing non-fastidious organisms like E. coli?

  1. The test must be incubated in an anaerobic environment to promote growth.
  2. The inoculum suspension must be prepared to a 1.0 McFarland standard instead of 0.5.
  3. The test must be performed on Haemophilus Test Medium (HTM) and incubated in 5% CO₂. (correct answer)
  4. Zone diameters must be measured after 12 hours of incubation due to the organism's rapid growth.
Explanation: Correct Answer: C. Haemophilus influenzae is a fastidious bacterium that requires specific growth factors (X-factor and V-factor) not present in standard Mueller-Hinton agar. Therefore, Kirby-Bauer testing for this organism must be performed on a specialized medium called Haemophilus Test Medium (HTM). Additionally, it requires an enriched CO₂ atmosphere (5%) for optimal growth, another deviation from the standard procedure.

Question 5

A Kirby-Bauer plate testing an Enterococcus faecalis isolate against vancomycin is accidentally incubated for 36 hours instead of the standard 24 hours. The measured zone is 16 mm. The CLSI breakpoints for vancomycin are S ≥ 17 mm, I = 15-16 mm, R ≤ 14 mm. How should this result be managed?

  1. Report the result as Intermediate, as the measurement of 16 mm falls within the intermediate range.
  2. Report as Susceptible, because the prolonged incubation would artificially shrink the zone size.
  3. The test is invalid due to the deviation from standard incubation time and must be repeated. (correct answer)
  4. Report as Resistant, as any growth after 24 hours suggests the presence of a resistant subpopulation.
Explanation: Correct Answer: C. The Kirby-Bauer test is highly standardized, and incubation time is a critical parameter. Prolonged incubation allows the bacterial lawn to continue to grow slowly as the antibiotic degrades, leading to smaller, inaccurate zone sizes. Because the test was not performed according to the mandated protocol, the results are invalid and cannot be interpreted or 'corrected'. The test must be repeated.

Question 6

A Kirby-Bauer test is performed on a clinical isolate of Proteus mirabilis. After incubation, a thin, transparent film of swarming growth is observed across the entire plate, extending into the zones of inhibition. How should the technologist measure the zone diameters?

  1. The test is invalid and must be repeated on an agar medium, such as CLED, that inhibits swarming.
  2. Measure the diameter to the absolute edge of the swarming growth, which will result in smaller zone sizes.
  3. Disregard the thin film of swarming growth and measure the diameter of the area where the heavy, confluent lawn is inhibited. (correct answer)
  4. Report all tested antibiotics as resistant, as the ability to swarm into the zone indicates clinical resistance.
Explanation: Correct Answer: C. According to CLSI guidelines for reading Kirby-Bauer plates with swarming organisms like Proteus spp., the thin veil of swarming growth should be ignored. The measurement must be taken from the edge of the heavy, confluent growth that represents the true lawn. Measuring to the edge of the swarm would lead to falsely resistant results.

Question 7

A laboratory inadvertently uses a set of cefepime disks that expired two months prior but were stored under proper conditions. For an organism that is truly susceptible to cefepime, what is the most probable outcome of using these expired disks in a Kirby-Bauer assay?

  1. Falsely susceptible results due to the concentration of the antibiotic over time.
  2. Falsely resistant or intermediate results due to decreased antibiotic potency. (correct answer)
  3. No significant change in zone size, as proper storage negates the effect of the expiration date.
  4. A 'double zone' phenomenon where two distinct rings of inhibition are formed.
Explanation: Correct Answer: B. Antibiotics impregnated in paper disks degrade over time, leading to a loss of potency, even with proper storage. A disk with lower-than-expected potency will not diffuse as effectively or will not inhibit growth to the same extent as a fresh disk. This results in a smaller zone of inhibition, which can cause a susceptible organism to be miscategorized as intermediate or resistant.

Question 8

A technologist measures a zone of inhibition for meropenem against an E. coli isolate. The zone edge falls exactly between the 22 mm and 23 mm marks on the ruler. The CLSI breakpoints for meropenem are S ≥ 23 mm, I = 20-22 mm, R ≤ 19 mm. What is the most appropriate action according to standard laboratory practice?

  1. Report the zone as 22.5 mm and classify the result as Susceptible based on rounding up.
  2. Report the zone as 23 mm, giving the benefit of the doubt to achieve a susceptible result.
  3. Repeat the test to obtain a more precise measurement before finalizing the report.
  4. Report the zone as 22 mm and classify the result as Intermediate. (correct answer)
Explanation: Correct Answer: D. A standard convention in microbiology for reading Kirby-Bauer zones is to be conservative to avoid reporting false susceptibility. When a zone edge falls between two markings on a ruler or caliper, the reading should be rounded down to the lower whole millimeter. Therefore, a measurement between 22 and 23 mm should be recorded as 22 mm. Based on the provided breakpoints, a 22 mm zone falls into the Intermediate category.

Question 9

When performing a Kirby-Bauer test on a Streptococcus pneumoniae isolate on Mueller-Hinton agar with 5% sheep blood, a zone of alpha-hemolysis is observed around the penicillin disk that is larger than the zone of growth inhibition. What is the correct procedure for measuring the zone diameter?

  1. Measure the diameter of the zone of hemolysis, as this represents the full extent of antibiotic effect.
  2. The test is unreadable due to the interfering hemolysis and must be repeated with a different method.
  3. Calculate and report the average of the hemolysis zone diameter and the growth inhibition zone diameter.
  4. Remove the lid and view the plate with transmitted light to visualize the true edge of growth inhibition. (correct answer)
Explanation: When testing Streptococcus pneumoniae for antibiotic susceptibility, you'll encounter a unique challenge because this organism requires enriched media (Mueller-Hinton with 5% sheep blood) due to its fastidious growth requirements. The alpha-hemolytic activity of S. pneumoniae can create zones of hemolysis around antibiotic disks that extend beyond the actual zone of growth inhibition, potentially confusing your measurements. The correct approach is option D: remove the lid and view the plate with transmitted light to visualize the true edge of growth inhibition. This technique allows light to pass through the agar from below, making it easier to distinguish between areas where bacterial growth has been inhibited (which will appear clearer) versus areas where hemolysis has occurred but bacterial growth may still be present. The zone of growth inhibition—not hemolysis—is what determines antibiotic susceptibility. Option A is incorrect because measuring the hemolysis zone would give falsely large zone diameters, leading to incorrect susceptibility interpretations. Option B is wrong because this situation doesn't make the test unreadable—it just requires proper visualization technique. The Clinical and Laboratory Standards Institute (CLSI) specifically addresses this scenario. Option C is incorrect because averaging two different biological phenomena (growth inhibition vs. hemolysis) has no scientific basis and isn't a recognized methodology. Remember: for hemolytic organisms on blood agar, always use transmitted light to find the true growth inhibition edge. This principle applies beyond just S. pneumoniae to other hemolytic bacteria requiring blood-enriched media for susceptibility testing.

Question 10

An inoculum for a Kirby-Bauer test is accidentally prepared from a broth containing an equal mixture of a susceptible strain of E. coli and a beta-lactamase-producing, resistant strain of E. coli. What is the most likely appearance of the zone of inhibition around an ampicillin disk?

  1. A standard, clear zone of inhibition, as the susceptible strain's growth will be inhibited normally.
  2. A complete absence of a zone of inhibition, as the resistant strain will grow confluently.
  3. A significantly smaller, hazy zone containing many small colonies of the resistant strain. (correct answer)
  4. A large, clear zone of inhibition with a sharply defined inner ring of resistant colonies.
Explanation: Correct Answer: C. In a mixed population, the susceptible organisms will be inhibited by the antibiotic, attempting to form a zone. However, the resistant organisms will be able to grow within this area. The resulting appearance is not a clear zone but rather a hazy, poorly defined zone that is significantly smaller than what would be seen with a pure susceptible culture. It is often described as having growth 'up to the disk' or containing many colonies within the zone.

Question 11

A technologist uses standard Mueller-Hinton II Agar for a Kirby-Bauer test on a Pseudomonas aeruginosa isolate, forgetting that cation adjustment (with Ca²⁺ and Mg²⁺) is required for this organism. Quality control with E. coli ATCC 25922 is performed on the same medium and is acceptable. For the aminoglycoside tobramycin, what is the most likely consequence of this error?

  1. A falsely susceptible result due to insufficient divalent cation concentration in the medium. (correct answer)
  2. A falsely resistant result because the absence of cations inhibits the antibiotic's activity.
  3. No significant effect is expected, as the acceptability of the E. coli QC validates the medium.
  4. Falsely resistant results for all beta-lactam antibiotics, but not for tobramycin.
Explanation: Correct Answer: A. The activity of aminoglycosides (like tobramycin) against Pseudomonas aeruginosa is significantly affected by the concentration of divalent cations. Insufficient levels of Ca²⁺ and Mg²⁺ in the agar lead to increased activity of the aminoglycoside, resulting in larger zones of inhibition. This can cause a truly resistant or intermediate isolate to be reported as susceptible. E. coli QC is less sensitive to this variation and may not detect the problem.

Question 12

A vancomycin disk diffusion test for a Staphylococcus aureus isolate from a bacteremia patient yields a zone of 18 mm (Susceptible ≥ 15 mm). However, the zone edge is indistinct, and a light sprinkling of tiny colonies is visible within the outer 2-3 mm of the zone. How should this finding be primarily interpreted?

  1. The isolate is susceptible to vancomycin, as the overall zone diameter meets the susceptible criterion.
  2. The finding is suspicious for heteroresistant vancomycin-intermediate S. aureus (hVISA) and warrants further investigation. (correct answer)
  3. The test is invalid due to a mixed inoculum from the patient's blood culture and must be repeated after subculture.
  4. The isolate is vancomycin-resistant (VRSA), and the result should be confirmed by a molecular method.
Explanation: Correct Answer: B. The presence of a subpopulation of colonies that can grow closer to the vancomycin disk than the main population is a classic indicator of heteroresistance (hVISA). These isolates can lead to clinical failure despite appearing susceptible by standard methods. This finding is highly significant and should prompt confirmatory testing, such as determining the MIC by Etest or broth microdilution. While contamination is a possibility, the specific pattern with vancomycin strongly suggests hVISA.

Question 13

A Klebsiella pneumoniae isolate from a urine sample has a zone of 16 mm for cefazolin (S ≥ 18, I = 15-17, R ≤ 14) and a zone of 25 mm for nitrofurantoin (S ≥ 17, I = 15-16, R ≤ 14). The patient has normal renal function. Which statement is the most accurate clinical interpretation?

  1. The 'Intermediate' result for cefazolin means it will not be effective and must be avoided for treatment.
  2. Cefazolin may be effective because it concentrates in urine, but nitrofurantoin is a more reliable option. (correct answer)
  3. Both antibiotics are equally effective options for treating a urinary tract infection with this isolate.
  4. Only cefazolin should be considered, as it is a systemic agent while nitrofurantoin is not.
Explanation: Correct Answer: B. The 'Intermediate' category implies that the antibiotic may be clinically effective in body sites where it is physiologically concentrated, such as the urinary tract for many beta-lactams like cefazolin, or when a higher dosage can be used. The 'Susceptible' category implies a high likelihood of success with standard dosing. Since nitrofurantoin is fully susceptible and is also highly concentrated in the urine, it represents a more reliable and definitive treatment choice compared to the intermediate cefazolin.

Question 14

When performing a Kirby-Bauer test on a Streptococcus pneumoniae isolate on Mueller-Hinton agar with 5% sheep blood, a zone of alpha-hemolysis is observed around the penicillin disk that is larger than the zone of growth inhibition. What is the correct procedure for measuring the zone diameter?

  1. Measure the diameter of the zone of hemolysis, as this represents the full extent of antibiotic effect.
  2. The test is unreadable due to the interfering hemolysis and must be repeated with a different method.
  3. Calculate and report the average of the hemolysis zone diameter and the growth inhibition zone diameter.
  4. Remove the lid and view the plate with transmitted light to visualize the true edge of growth inhibition. (correct answer)
Explanation: When testing Streptococcus pneumoniae for antibiotic susceptibility, you'll encounter a unique challenge because this organism requires enriched media (Mueller-Hinton with 5% sheep blood) due to its fastidious growth requirements. The alpha-hemolytic activity of S. pneumoniae can create zones of hemolysis around antibiotic disks that extend beyond the actual zone of growth inhibition, potentially confusing your measurements. The correct approach is option D: remove the lid and view the plate with transmitted light to visualize the true edge of growth inhibition. This technique allows light to pass through the agar from below, making it easier to distinguish between areas where bacterial growth has been inhibited (which will appear clearer) versus areas where hemolysis has occurred but bacterial growth may still be present. The zone of growth inhibition—not hemolysis—is what determines antibiotic susceptibility. Option A is incorrect because measuring the hemolysis zone would give falsely large zone diameters, leading to incorrect susceptibility interpretations. Option B is wrong because this situation doesn't make the test unreadable—it just requires proper visualization technique. The Clinical and Laboratory Standards Institute (CLSI) specifically addresses this scenario. Option C is incorrect because averaging two different biological phenomena (growth inhibition vs. hemolysis) has no scientific basis and isn't a recognized methodology. Remember: for hemolytic organisms on blood agar, always use transmitted light to find the true growth inhibition edge. This principle applies beyond just S. pneumoniae to other hemolytic bacteria requiring blood-enriched media for susceptibility testing.

Question 15

A Kirby-Bauer test is performed on a clinical isolate of Proteus mirabilis. After incubation, a thin, transparent film of swarming growth is observed across the entire plate, extending into the zones of inhibition. How should the technologist measure the zone diameters?

  1. The test is invalid and must be repeated on an agar medium, such as CLED, that inhibits swarming.
  2. Measure the diameter to the absolute edge of the swarming growth, which will result in smaller zone sizes.
  3. Disregard the thin film of swarming growth and measure the diameter of the area where the heavy, confluent lawn is inhibited. (correct answer)
  4. Report all tested antibiotics as resistant, as the ability to swarm into the zone indicates clinical resistance.
Explanation: Correct Answer: C. According to CLSI guidelines for reading Kirby-Bauer plates with swarming organisms like Proteus spp., the thin veil of swarming growth should be ignored. The measurement must be taken from the edge of the heavy, confluent growth that represents the true lawn. Measuring to the edge of the swarm would lead to falsely resistant results.

Question 16

A laboratory inadvertently uses a set of cefepime disks that expired two months prior but were stored under proper conditions. For an organism that is truly susceptible to cefepime, what is the most probable outcome of using these expired disks in a Kirby-Bauer assay?

  1. Falsely susceptible results due to the concentration of the antibiotic over time.
  2. Falsely resistant or intermediate results due to decreased antibiotic potency. (correct answer)
  3. No significant change in zone size, as proper storage negates the effect of the expiration date.
  4. A 'double zone' phenomenon where two distinct rings of inhibition are formed.
Explanation: Correct Answer: B. Antibiotics impregnated in paper disks degrade over time, leading to a loss of potency, even with proper storage. A disk with lower-than-expected potency will not diffuse as effectively or will not inhibit growth to the same extent as a fresh disk. This results in a smaller zone of inhibition, which can cause a susceptible organism to be miscategorized as intermediate or resistant.

Question 17

A clinical laboratory needs to perform Kirby-Bauer susceptibility testing on an isolate of Haemophilus influenzae. Which of the following represents a critical procedural modification compared to the standard protocol for testing non-fastidious organisms like E. coli?

  1. The test must be incubated in an anaerobic environment to promote growth.
  2. The inoculum suspension must be prepared to a 1.0 McFarland standard instead of 0.5.
  3. The test must be performed on Haemophilus Test Medium (HTM) and incubated in 5% CO₂. (correct answer)
  4. Zone diameters must be measured after 12 hours of incubation due to the organism's rapid growth.
Explanation: Correct Answer: C. Haemophilus influenzae is a fastidious bacterium that requires specific growth factors (X-factor and V-factor) not present in standard Mueller-Hinton agar. Therefore, Kirby-Bauer testing for this organism must be performed on a specialized medium called Haemophilus Test Medium (HTM). Additionally, it requires an enriched CO₂ atmosphere (5%) for optimal growth, another deviation from the standard procedure.

Question 18

An inoculum for a Kirby-Bauer test is accidentally prepared from a broth containing an equal mixture of a susceptible strain of E. coli and a beta-lactamase-producing, resistant strain of E. coli. What is the most likely appearance of the zone of inhibition around an ampicillin disk?

  1. A standard, clear zone of inhibition, as the susceptible strain's growth will be inhibited normally.
  2. A complete absence of a zone of inhibition, as the resistant strain will grow confluently.
  3. A significantly smaller, hazy zone containing many small colonies of the resistant strain. (correct answer)
  4. A large, clear zone of inhibition with a sharply defined inner ring of resistant colonies.
Explanation: Correct Answer: C. In a mixed population, the susceptible organisms will be inhibited by the antibiotic, attempting to form a zone. However, the resistant organisms will be able to grow within this area. The resulting appearance is not a clear zone but rather a hazy, poorly defined zone that is significantly smaller than what would be seen with a pure susceptible culture. It is often described as having growth 'up to the disk' or containing many colonies within the zone.

Question 19

An E. coli isolate is tested for ESBL production. The zone diameter for ceftazidime is 16 mm. The zone diameter for the combination disk of ceftazidime/clavulanic acid is 23 mm. According to CLSI guidelines, a ≥ 5 mm increase in zone diameter for the combination disk is a positive test. What is the correct conclusion?

  1. The organism is not an ESBL producer because the ceftazidime zone is in the susceptible range.
  2. The test is inconclusive because only one cephalosporin was tested with clavulanic acid.
  3. The clavulanic acid is inhibitory, but this does not confirm the presence of an ESBL enzyme.
  4. The organism is confirmed to be an ESBL producer and relevant beta-lactam results should be edited. (correct answer)
Explanation: ESBL (Extended-Spectrum Beta-Lactamase) detection is crucial in clinical microbiology because these enzymes confer resistance to most beta-lactam antibiotics, dramatically limiting treatment options. The standard CLSI phenotypic confirmatory test relies on clavulanic acid's ability to inhibit ESBL enzymes. Let's analyze the data: The ceftazidime zone alone is 16 mm, but with clavulanic acid added, it increases to 23 mm. This 7 mm increase exceeds the CLSI threshold of ≥5 mm, confirming ESBL production. When clavulanic acid inhibits the ESBL enzyme, the cephalosporin can effectively kill the bacteria, creating a larger zone of inhibition. Looking at the wrong answers: Choice A misunderstands that zone size alone doesn't determine ESBL status—the comparison between zones with and without clavulanic acid is what matters. Choice B incorrectly suggests you need multiple cephalosporins; while testing multiple drugs increases confidence, one positive result meeting CLSI criteria is sufficient for confirmation. Choice C acknowledges clavulanic acid's inhibitory effect but wrongly claims this doesn't confirm ESBL presence—this is exactly what the test is designed to detect. The correct answer is D. The ≥5 mm zone diameter increase confirms ESBL production, and according to CLSI guidelines, all beta-lactam susceptibility results should be edited to "resistant" regardless of initial zone sizes, since ESBLs can hydrolyze these antibiotics in vivo. Study tip: Remember that ESBL confirmation depends on the zone diameter difference, not absolute zone sizes. A ≥5 mm increase with clavulanic acid = positive ESBL test.

Question 20

A routine quality control test using E. coli ATCC 25922 shows a gentamicin zone of 12 mm (acceptable range: 19-26 mm). The ampicillin zone for the same QC test is 18 mm (acceptable range: 16-22 mm). What is the most appropriate immediate action for the technologist to take?

  1. Report patient results for all antibiotics except gentamicin, as the ampicillin QC is in range.
  2. Repeat the quality control test using a new vial of gentamicin disks from a different lot number.
  3. Withhold all patient results from the batch, check the inoculum density, and repeat the entire test panel. (correct answer)
  4. Recalibrate the incubator temperature and re-incubate the QC plate for an additional 4 hours.
Explanation: Correct Answer: C. When a quality control result falls outside the acceptable range, all patient results obtained with that test run are considered invalid and must not be reported. A common technical error causing falsely small zones for multiple antibiotics (especially aminoglycosides like gentamicin) is an overly dense inoculum (too high McFarland standard). Therefore, the most appropriate first step is to check the inoculum preparation and repeat the entire test for both the QC strain and all patient isolates from that batch.