Microbiology Quiz: Susceptibility Test Factors
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Susceptibility Test FactorsQuestion 1 of 20

A technologist prepares an inoculum for a Kirby-Bauer test that is visibly lighter than the 0.5 McFarland standard. The test is performed on an Escherichia coli isolate. If this error is not corrected, what is the most probable outcome for the zone sizes of ampicillin and gentamicin?

Falsely small zones for both agents, potentially leading to a major error in reporting.
Falsely large zones for both agents, potentially leading to a very major error in reporting.
Falsely small zone for ampicillin but a falsely large zone for gentamicin.
The results will be unreliable but without a predictable directional shift in zone sizes.
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Microbiology Quiz

Microbiology Quiz: Susceptibility Test Factors

Practice Susceptibility Test Factors 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 Susceptibility Test Factors, 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

A technologist prepares an inoculum for a Kirby-Bauer test that is visibly lighter than the 0.5 McFarland standard. The test is performed on an Escherichia coli isolate. If this error is not corrected, what is the most probable outcome for the zone sizes of ampicillin and gentamicin?

  1. Falsely small zones for both agents, potentially leading to a major error in reporting.
  2. Falsely large zones for both agents, potentially leading to a very major error in reporting. (correct answer)
  3. Falsely small zone for ampicillin but a falsely large zone for gentamicin.
  4. The results will be unreliable but without a predictable directional shift in zone sizes.
Explanation: An inoculum density that is too low (lighter than 0.5 McFarland) results in less bacterial growth to overcome the antibiotic diffusing from the disk. This leads to falsely large zones of inhibition for all antimicrobials tested. Reporting a resistant organism as susceptible based on this error is termed a 'very major error'. Conversely, a too-heavy inoculum would cause falsely small zones, potentially leading to a 'major error' (reporting a susceptible organism as resistant).

Question 2

A broth microdilution panel for an Enterococcus faecalis isolate shows an MIC of >32 µg/mL for trimethoprim-sulfamethoxazole (SXT). However, the disk diffusion test on the same isolate yields a large zone of inhibition (25 mm), suggesting susceptibility. Which factor most likely explains this discrepancy?

  1. The broth used for the MIC panel contained excessive thymidine, antagonizing the action of SXT. (correct answer)
  2. The disk diffusion plate was incubated in 5% CO₂, which potentiated the action of SXT.
  3. The inoculum for the disk diffusion test was too light, leading to a falsely susceptible result.
  4. Enterococci exhibit trailing endpoints with SXT that are difficult to read by automated systems.
Explanation: Standard susceptibility testing media like Mueller-Hinton are prepared with low levels of thymidine because this nucleoside allows bacteria to bypass the folate synthesis pathway inhibited by SXT. If the Mueller-Hinton broth used for the MIC test has excessive thymidine, it will antagonize the drug's activity, leading to a falsely resistant result. Disk diffusion agar is typically better controlled for this factor. CO₂ incubation and trailing endpoints can be issues, but the most direct cause for a high MIC in broth versus susceptibility on agar for SXT is thymidine content.

Question 3

An Etest is used to determine the ampicillin MIC for an Enterococcus faecalis isolate. A clear ellipse of inhibition forms. The bottom edge of the ellipse intersects the strip at the 2 µg/mL mark. The top edge of the ellipse intersects the strip exactly halfway between the 3 µg/mL and 4 µg/mL marks. What MIC value should be recorded?

  1. 2 µg/mL
  2. 3 µg/mL
  3. 3.5 µg/mL
  4. 4 µg/mL (correct answer)
Explanation: When reading an Etest, the MIC is the value where the edge of the inhibition ellipse intersects the strip. If the intersection falls between two markings on the scale, the MIC should always be rounded up to the next highest value provided on the strip. In this case, the intersection is between 3 and 4 µg/mL, so the recorded MIC must be rounded up to 4 µg/mL.

Question 4

A disk diffusion test on a methicillin-susceptible Staphylococcus aureus isolate shows resistance to erythromycin (12 mm zone) and susceptibility to clindamycin (22 mm zone). The disks are placed 20 mm apart. The clindamycin zone of inhibition is noticeably flattened on the side adjacent to the erythromycin disk, forming a 'D' shape.

Based on the phenomenon described in the passage, how should the susceptibility to clindamycin be reported for this isolate?

  1. Susceptible, because the measured zone diameter is within the susceptible range.
  2. Intermediate, with a recommendation to confirm with a broth microdilution MIC.
  3. Resistant, because the 'D' shape indicates inducible resistance. (correct answer)
  4. Invalid, because the disk placement was too close, causing interference.
Explanation: The 'D-zone' test result indicates inducible resistance to clindamycin. The erythromycin diffuses and induces the expression of an erm gene, which confers resistance to both macrolides (erythromycin) and lincosamides (clindamycin). Even though the clindamycin zone diameter measures as susceptible, the presence of the D-shape is definitive evidence of this resistance mechanism. Therefore, the organism must be reported as resistant to clindamycin to avoid clinical failure.

Question 5

A batch of Mueller-Hinton agar is inadvertently prepared with a pH of 7.8. If this agar is used for Kirby-Bauer testing, which of the following results is most likely to be observed with the QC strain E. coli ATCC 25922?

  1. Falsely small zone for erythromycin and falsely large zone for tetracycline.
  2. Falsely large zone for erythromycin and falsely small zone for tetracycline. (correct answer)
  3. Falsely small zones for both erythromycin and tetracycline.
  4. Falsely large zones for both erythromycin and tetracycline.
Explanation: The activity of certain antimicrobial classes is pH-dependent. Macrolides (e.g., erythromycin) and aminoglycosides are more active at a higher (alkaline) pH, which would result in falsely large zones of inhibition. Conversely, tetracyclines are more active at a lower (acidic) pH. Therefore, testing on agar with a pH of 7.8 would result in a falsely large zone for erythromycin and a falsely small zone for tetracycline.

Question 6

An automated susceptibility testing system reports an MIC of ≤ 4 µg/mL for imipenem against an isolate identified as Stenotrophomonas maltophilia, with an interpretation of 'Susceptible'. What is the correct course of action for the reporting technologist?

  1. Report the result as susceptible, as it was generated by an FDA-cleared instrument.
  2. Verify the result by performing a disk diffusion test for imipenem.
  3. Request a new specimen, as the result suggests a misidentification of the organism.
  4. Edit the report to show imipenem as 'Resistant' based on the organism's known intrinsic resistance. (correct answer)
Explanation: When you encounter antimicrobial susceptibility results, you must consider both the automated system's output and the organism's intrinsic resistance patterns. Some bacteria have natural, chromosomally-encoded resistance mechanisms that make certain antibiotics ineffective regardless of what an MIC value suggests. Stenotrophomonas maltophilia is notorious for its intrinsic resistance to carbapenems, including imipenem. This organism produces chromosomal metallo-β-lactamases (L1 and L2) that hydrolyze carbapenems, making them clinically ineffective even when automated systems report low MIC values. The reported MIC of ≤4 µg/mL is likely a false susceptible result due to technical limitations of the testing system with this particular organism-antibiotic combination. Answer D is correct because you should override the automated result and report imipenem as resistant based on established microbiological knowledge. This prevents potentially dangerous therapeutic decisions. Answer A is wrong because FDA clearance doesn't guarantee accuracy for every organism-antibiotic combination, and automated systems have known limitations with certain resistance mechanisms. Answer B is incorrect because disk diffusion testing would likely give the same misleading result—the issue isn't with the testing method but with the organism's intrinsic resistance. Answer C is wrong because the organism identification is likely correct; S. maltophilia can appear in clinical specimens, and the unexpected susceptibility pattern doesn't automatically indicate misidentification. Remember: intrinsic resistance patterns override automated susceptibility results. Always cross-reference unusual susceptibility patterns with established resistance profiles for the identified organism, especially for organisms known to carry chromosomal resistance genes.

Question 7

When measuring the zone of inhibition for trimethoprim-sulfamethoxazole (SXT) against a lawn of Proteus mirabilis, a technologist observes a distinct zone of inhibited growth measuring 20 mm in diameter. However, a thin, translucent film of swarming bacteria covers the entire plate, including the area within the zone. How should this result be recorded?

  1. The result is invalid due to swarming and the test must be repeated.
  2. The organism is resistant, as any growth within the zone indicates resistance.
  3. The measurement should only be taken from the back of the plate, viewing through the agar to minimize the swarm effect.
  4. The zone diameter should be measured by ignoring the thin swarming growth and measuring the edge of heavy growth. (correct answer)
Explanation: When you encounter swarming bacteria like Proteus mirabilis in disk diffusion testing, you need to understand that swarming is a natural motility phenomenon where bacteria spread in thin films across the agar surface, even into zones where their growth is actually inhibited by the antibiotic. The correct approach is to measure the zone by ignoring the thin swarming layer and focusing on the edge of heavy, confluent growth (Answer D). The 20 mm measurement represents genuine antimicrobial activity—the antibiotic is preventing normal bacterial multiplication, even though the thin swarm can still move through that area. This is the standard CLSI (Clinical and Laboratory Standards Institute) recommended method for handling swarming organisms. Answer A is incorrect because swarming doesn't invalidate the test; there are established protocols specifically for this situation. Simply repeating the test won't eliminate the swarming behavior, as it's an inherent characteristic of Proteus species. Answer B misinterprets what the swarming represents. The thin film isn't true resistant growth—it's motile bacteria moving across a surface where they cannot establish heavy growth due to antimicrobial pressure. Answer C suggests an impractical approach that isn't part of standard methodology. Viewing from the back doesn't address the fundamental issue and could lead to measurement errors. Remember this key principle: with swarming organisms, distinguish between true growth (thick, confluent) and swarming motility (thin, translucent film). The zone measurement should reflect where the antibiotic prevents normal bacterial multiplication, not bacterial movement.

Question 8

A Kirby-Bauer susceptibility test is mistakenly placed in an anaerobic incubator for 18 hours. The test isolate is a facultative anaerobe, Escherichia coli. What would be the expected, yet erroneous, results for gentamicin and trimethoprim-sulfamethoxazole (SXT)?

  1. Falsely resistant for gentamicin and falsely resistant for SXT. (correct answer)
  2. Falsely susceptible for gentamicin and falsely resistant for SXT.
  3. Falsely resistant for gentamicin and unaffected for SXT.
  4. Falsely susceptible for both gentamicin and SXT due to slower anaerobic growth.
Explanation: When you encounter questions about antibiotic susceptibility testing under altered conditions, think about how oxygen availability affects both bacterial metabolism and antibiotic mechanisms of action. The Kirby-Bauer test requires aerobic conditions because oxygen is essential for the uptake and activity of many antibiotics. Although E. coli is a facultative anaerobe that can survive without oxygen, the anaerobic environment fundamentally changes how antibiotics work. Gentamicin, an aminoglycoside, requires active oxygen-dependent transport systems to enter bacterial cells. Under anaerobic conditions, these transport mechanisms are significantly impaired, preventing adequate drug uptake. This creates falsely resistant results because the bacteria appear unaffected by concentrations that would normally be inhibitory. Similarly, trimethoprim-sulfamethoxazole's bactericidal activity depends on aerobic metabolic processes. While SXT can still interfere with folate synthesis anaerobically, its killing efficiency is greatly reduced, also producing falsely resistant results. Answer B incorrectly suggests gentamicin would appear falsely susceptible, but impaired uptake makes bacteria seem more resistant, not less. Answer C wrongly assumes SXT would be unaffected by oxygen levels, when in fact its bactericidal activity requires aerobic conditions. Answer D makes the opposite error, suggesting both drugs would appear more effective due to slower growth, when the real issue is reduced antibiotic efficacy under anaerobic conditions. Remember this key principle: most antibiotics require aerobic conditions for optimal activity, so anaerobic testing conditions typically produce falsely resistant results, not falsely susceptible ones.

Question 9

A urine culture yields >10⁵ CFU/mL Escherichia coli and >10⁵ CFU/mL Enterococcus faecalis. An inoculum for disk diffusion is inadvertently made from the mixed culture and tested against ampicillin. The E. coli isolate possesses a beta-lactamase gene, while the Enterococcus is intrinsically susceptible to ampicillin. What is the most likely appearance of the ampicillin zone of inhibition?

  1. A large, clear zone of inhibition consistent with the susceptible Enterococcus.
  2. A clear zone of inhibition containing numerous small, resistant colonies of E. coli.
  3. A very small or absent zone of inhibition, with heavy growth of both organisms up to the disk. (correct answer)
  4. Two distinct, overlapping zones of inhibition reflecting the different susceptibilities.
Explanation: The beta-lactamase produced by the resistant E. coli will diffuse into the surrounding agar and degrade the ampicillin. This enzymatic inactivation of the antibiotic creates a protective effect for the otherwise susceptible Enterococcus faecalis, allowing it to grow near the disk. The result is a falsely resistant phenotype for the mixed population, manifesting as little to no zone of inhibition.

Question 10

A clinical laboratory's quality control for disk diffusion testing of Pseudomonas aeruginosa ATCC 27853 consistently shows tobramycin zone diameters that are 5-7 mm larger than the upper limit of the acceptable range. Results for other antimicrobials, including piperacillin-tazobactam, are within range. An investigation into the procedure reveals no errors in inoculum preparation, incubation conditions, or disk storage. Which of the following is the most likely cause of the discrepant tobramycin results?

  1. The Mueller-Hinton agar has an excessively high concentration of thymidine.
  2. The Mueller-Hinton agar has an insufficient concentration of divalent cations (Ca²⁺ and Mg²⁺). (correct answer)
  3. The pH of the Mueller-Hinton agar is too low (e.g., 6.9).
  4. The agar plates were poured to a depth of 3 mm instead of the standard 4 mm.
Explanation: The activity of aminoglycosides, such as tobramycin, against P. aeruginosa is highly dependent on the concentration of divalent cations (Ca²⁺ and Mg²⁺) in the Mueller-Hinton agar. Insufficient cation levels result in falsely large zones of inhibition (falsely susceptible results). A high thymidine content affects sulfonamides and trimethoprim, not aminoglycosides. A low pH antagonizes aminoglycoside activity, leading to smaller zones. Thin agar would lead to larger zones for all drugs, not just tobramycin.

Question 11

A new container of carbapenem disks is removed from a -20°C freezer and immediately opened on the bench to load a dispenser. This practice continues daily. After a month, QC results show that imipenem zone sizes against P. aeruginosa ATCC 27853 have trended down and are now out of range. What is the most probable cause of this loss of disk potency?

  1. Moisture from room air condensed onto the cold disks, causing antibiotic hydrolysis. (correct answer)
  2. The disks degraded due to repeated freeze-thaw cycles from daily use.
  3. Exposure to fluorescent lighting on the benchtop caused photodegradation of the imipenem.
  4. The -20°C storage temperature is insufficient to maintain the stability of carbapenems.
Explanation: When you encounter antibiotic susceptibility testing problems, focus on how storage and handling conditions affect drug stability. Beta-lactam antibiotics like carbapenems are particularly sensitive to moisture because their beta-lactam ring is susceptible to hydrolysis. The scenario describes a critical error: opening a frozen container immediately after removing it from storage. When cold disks are exposed to warm, humid room air, water vapor rapidly condenses on their surface. This moisture causes hydrolytic breakdown of imipenem's beta-lactam ring, progressively reducing the antibiotic concentration in the disks over the month of repeated exposure. Lower antibiotic levels result in smaller inhibition zones against the quality control strain. Answer A correctly identifies moisture condensation and subsequent hydrolysis as the cause. Answer B is incorrect because the disks aren't being refrozen—they're stored at room temperature after opening, so no freeze-thaw cycles occur. Answer C misses the mark because while some antibiotics are photosensitive, carbapenems are not significantly affected by fluorescent lighting, and the timeline suggests moisture exposure rather than photodegradation. Answer D is wrong because -20°C is actually appropriate for carbapenem storage—the problem isn't the storage temperature but the handling procedure. The key principle: always allow frozen antibiotic disks to equilibrate to room temperature before opening the container. This prevents condensation and preserves disk potency. Remember that beta-lactam antibiotics are moisture-sensitive, making proper handling procedures critical for accurate susceptibility testing results.

Question 12

In a broth microdilution panel for Pseudomonas aeruginosa, the wells for piperacillin-tazobactam show the following results: growth in 8/4 µg/mL, no growth in 16/4 µg/mL, clear growth in 32/4 µg/mL, and no growth in 64/4 µg/mL and 128/4 µg/mL. How should this result be handled?

  1. The MIC should be reported as 8/4 µg/mL, ignoring the anomalous growth at 32/4 µg/mL.
  2. The result is interpreted as a 'skipped well'; the test must be repeated to obtain a valid MIC. (correct answer)
  3. The MIC should be reported as 16/4 µg/mL, which is the first well showing no growth.
  4. The organism should be checked for purity, as mixed culture could cause this pattern.
Explanation: This pattern demonstrates a 'skipped well,' where growth occurs at a higher concentration of an antimicrobial after a well with no growth. This indicates a technical error, such as contamination of a single well, an error in dispensing the antimicrobial, or improper inoculation. According to CLSI guidelines, results with skipped wells are considered invalid, and the test should be repeated to ensure accuracy before reporting.

Question 13

A technologist inoculates a batch of Mueller-Hinton plates for Kirby-Bauer testing and applies the antibiotic disks. Due to an emergency, the plates are left on the laboratory bench at 25°C for 90 minutes before being placed into the 35°C incubator for the standard 18-hour incubation. What is the most likely effect of this delay on the final zone diameters?

  1. Zone diameters will be falsely smaller due to bacterial overgrowth during the delay.
  2. Zone diameters will be falsely larger because the antibiotics diffuse before logarithmic bacterial growth begins. (correct answer)
  3. Zone diameters will be unaffected as long as the total incubation time at 35°C is correct.
  4. The zone edges will be indistinct and hazy, making accurate measurement impossible.
Explanation: The Kirby-Bauer test relies on a critical timing relationship between the rate of antibiotic diffusion and the rate of bacterial growth. Allowing the plates to sit at room temperature before incubation gives the antibiotics a 'head start' to diffuse through the agar while bacterial growth is minimal. When the plates are finally incubated, the bacteria encounter a pre-established antibiotic gradient, resulting in larger zones of inhibition and a potentially falsely susceptible result.

Question 14

A clinical isolate of Streptococcus pneumoniae is tested for susceptibility using disk diffusion on Mueller-Hinton agar with 5% sheep blood. The plate is incubated in a 5% CO₂ atmosphere to promote growth. The results show susceptibility to penicillin but resistance to erythromycin, with a very small zone. What is the most likely impact of the incubation atmosphere on the erythromycin result?

  1. The CO₂ atmosphere accurately reflects in vivo conditions and provides a reliable result.
  2. The CO₂ atmosphere has no significant effect on the activity of macrolide antibiotics.
  3. The CO₂ creates an acidic surface pH, which antagonizes erythromycin activity, causing a falsely resistant result. (correct answer)
  4. The CO₂ creates an alkaline surface pH, which potentiates erythromycin activity, causing a falsely susceptible result.
Explanation: Incubation in a CO₂-enriched atmosphere causes the formation of carbonic acid on the agar surface, lowering the pH. Macrolide antibiotics, such as erythromycin, have decreased activity at a lower (acidic) pH. This can lead to smaller zones of inhibition and potentially a falsely resistant result. Therefore, the observed resistance might be an artifact of the test conditions required for the organism's growth.

Question 15

A chromogenic cephalosporin (cefinase) disk test is used to detect beta-lactamase production in an isolate of Moraxella catarrhalis. After applying the inoculum, the technologist observes the disk for 5 minutes. No color change occurs, and the result is reported as negative. Why is this conclusion potentially incorrect?

  1. The cefinase test is not a valid method for detecting beta-lactamase in Moraxella.
  2. The test requires incubation in a CO₂-enriched atmosphere to be accurate.
  3. A positive result for Moraxella can take up to one hour to develop and should be re-examined. (correct answer)
  4. The inoculum must be taken from a blood agar plate, not a chocolate agar plate.
Explanation: While many common beta-lactamases (e.g., from Staphylococcus) produce a rapid positive result in the cefinase test, the enzymes produced by organisms like Moraxella catarrhalis and some Haemophilus influenzae strains can be slower to react. CLSI guidelines recommend that a negative test at 5 minutes should be allowed to incubate for up to 60 minutes before being finalized as negative. Reporting a negative result prematurely can lead to misclassifying the organism as susceptible to ampicillin.

Question 16

A laboratory's quality control supervisor notes that over a period of several weeks, all disk diffusion zone sizes for both E. coli ATCC 25922 and S. aureus ATCC 25923 are consistently trending towards the lower end of their acceptable ranges, with some results falling just below the limit. Inoculum density, disk potency, and incubation parameters have been verified as correct. Which of the following is the most probable cause for this systematic shift?

  1. The Mueller-Hinton agar plates are being poured to a depth of 5-6 mm. (correct answer)
  2. The antibiotic disks are being allowed to equilibrate to room temperature for too long before application.
  3. The pH of the Mueller-Hinton agar is slightly alkaline (e.g., 7.6).
  4. The incubator is consistently running at 33°C instead of 35°C.
Explanation: The depth of the agar in a Kirby-Bauer plate directly influences the diffusion of the antibiotic. Thicker agar (greater than the standard 4 mm) slows the rate of antibiotic diffusion in three dimensions, resulting in smaller zones of inhibition for all drugs. This systematic error affects all tested organisms and antimicrobials. A slightly alkaline pH or lower incubation temperature would tend to produce larger, not smaller, zones for many drug-bug combinations. Extended equilibration of disks has no significant effect.

Question 17

When reading a broth microdilution panel for an E. coli isolate against trimethoprim-sulfamethoxazole (SXT), a technologist observes heavy growth in the control well, followed by hazy, partial inhibition across all SXT concentrations. No well shows complete inhibition of growth. What is the correct procedure for determining the MIC in this situation?

  1. The test is invalid and should be repeated with a different lot of SXT.
  2. The MIC should be read as the lowest concentration that inhibits approximately 80% of growth compared to the control. (correct answer)
  3. The MIC should be recorded as the highest concentration tested, indicating resistance.
  4. The presence of any growth, however faint, means the MIC is greater than the highest concentration tested.
Explanation: This phenomenon is known as a 'trailing endpoint,' which is common with bacteriostatic agents like sulfonamides and trimethoprim. For these drugs, the endpoint (MIC) is not complete inhibition of growth. Instead, CLSI guidelines specify that the MIC should be read as the lowest concentration that causes an 80% or greater reduction in growth (or the well with a small button or faint haze) compared to the growth control well.

Question 18

A technologist performs a disk diffusion test on a clinical isolate of Haemophilus influenzae. A 0.5 McFarland suspension is prepared and swabbed onto a standard Mueller-Hinton agar plate. After 24 hours of incubation at 35°C in 5% CO₂, only a few scattered colonies are visible on the plate, and there is no confluent lawn of growth. What is the most likely reason for this test failure?

  1. The inoculum density was too low for this organism.
  2. The incubation time was insufficient for this slow-growing species.
  3. The Mueller-Hinton agar was not supplemented with required growth factors. (correct answer)
  4. The 5% CO₂ atmosphere inhibited the growth of Haemophilus influenzae.
Explanation: Haemophilus influenzae is a fastidious organism that requires specific growth factors not present in standard Mueller-Hinton agar: X factor (hemin) and V factor (NAD). For susceptibility testing, a special medium called Haemophilus Test Medium (HTM), which is Mueller-Hinton supplemented with these factors, must be used. The lack of these factors is the reason for the poor growth observed.

Question 19

A technologist performs daily QC for disk diffusion. The vancomycin zone for S. aureus ATCC 25923 measures 15 mm (acceptable range: 17-21 mm). The cefoxitin zone for the same plate is 26 mm (acceptable range: 23-29 mm). Which of the following is the most logical first step to resolve the out-of-range vancomycin result?

  1. Repeat the entire QC panel using a new Mueller-Hinton agar lot.
  2. Prepare a new 0.5 McFarland suspension from the stock culture and re-test all agents.
  3. Check the expiration date and storage of the vancomycin disks and test with a new disk. (correct answer)
  4. Recalibrate the incubator to ensure the temperature is exactly 35.0°C.
Explanation: When a single antimicrobial agent is out of control while others are in range, the most probable cause is an issue with that specific agent. The first step should be to investigate the antibiotic disk itself, including its lot number, expiration date, and storage conditions. Problems with inoculum, agar, or incubation would likely affect multiple antibiotics, not just vancomycin.

Question 20

A laboratory's quality control supervisor notes that over a period of several weeks, all disk diffusion zone sizes for both E. coli ATCC 25922 and S. aureus ATCC 25923 are consistently trending towards the lower end of their acceptable ranges, with some results falling just below the limit. Inoculum density, disk potency, and incubation parameters have been verified as correct. Which of the following is the most probable cause for this systematic shift?

  1. The Mueller-Hinton agar plates are being poured to a depth of 5-6 mm. (correct answer)
  2. The antibiotic disks are being allowed to equilibrate to room temperature for too long before application.
  3. The pH of the Mueller-Hinton agar is slightly alkaline (e.g., 7.6).
  4. The incubator is consistently running at 33°C instead of 35°C.
Explanation: The depth of the agar in a Kirby-Bauer plate directly influences the diffusion of the antibiotic. Thicker agar (greater than the standard 4 mm) slows the rate of antibiotic diffusion in three dimensions, resulting in smaller zones of inhibition for all drugs. This systematic error affects all tested organisms and antimicrobials. A slightly alkaline pH or lower incubation temperature would tend to produce larger, not smaller, zones for many drug-bug combinations. Extended equilibration of disks has no significant effect.