Bacteriology: antimicrobial susceptibility testing

18 Kerala PSC Lab Technician Gr II practice questions on Bacteriology: antimicrobial susceptibility testing, part of Microbiology. The first 18 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   Stock containers of antimicrobial discs are allowed to reach room temperature before they are opened. This is done mainly to:

   1. ASpeed diffusion of the drug into agar
   2. BPrevent moisture condensing on the discs
   3. CHelp the discs stick to the agar
   4. DActivate the dried antibiotic in the disc

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   Answer: B · Prevent moisture condensing on the discs

   A cold container opened in room air collects condensed moisture, and damp discs lose potency, beta-lactams most of all. Stock discs are kept frozen (at or below -14 °C by CLSI) or refrigerated in a sealed container with a desiccant. A small working supply is kept at 2-8 °C and used within about a week. Poorly stored discs give falsely small zones, which the daily QC strains reveal.

   • A: Diffusion depends on the agar and incubation, not on warming the container.
   • C: Discs stay on the agar because they are pressed gently onto it.
   • D: The antibiotic is already active; warming does not activate it.

2. Question 2 · Hard

   Doubling dilutions of ampicillin from 32 µg/mL (tube 1) to 0.25 µg/mL (tube 8) are inoculated with an isolate. Tubes 1-5 are clear and tubes 6-8 turbid. Subcultures from tubes 1-3 give no growth, but tubes 4 and 5 grow. What are the MIC and MBC?

   1. AMIC 1 µg/mL; MBC 4 µg/mL
   2. BMIC 2 µg/mL; MBC 8 µg/mL
   3. CMIC 2 µg/mL; MBC 4 µg/mL
   4. DMIC 8 µg/mL; MBC 2 µg/mL

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   Answer: B · MIC 2 µg/mL; MBC 8 µg/mL

   The tubes hold 32, 16, 8, 4, 2, 1, 0.5 and 0.25 µg/mL. The MIC is the lowest concentration with no visible growth: tube 5, 2 µg/mL. The MBC is the lowest concentration whose subculture shows no growth (99.9% killing of the inoculum): tube 3, 8 µg/mL. Here the MBC is four times the MIC, which is usual for a bactericidal drug; a ratio of 32 or more suggests tolerance.

   • A: 1 µg/mL is tube 6, the first turbid tube, not the last clear one.
   • C: Tube 4 (4 µg/mL) grew on subculture, so it did not kill the organism.
   • D: This swaps the two values; the MBC can never be lower than the MIC.

3. Question 3 · Easy

   How is the zone of inhibition recorded in the Kirby-Bauer disc diffusion method?

   1. AAs the gap between two adjacent discs
   2. BAs the area of the clear zone in mm²
   3. CAs the diameter in mm, including the disc
   4. DAs the width from the disc edge in mm

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   Answer: C · As the diameter in mm, including the disc

   Kirby-Bauer zones are measured across the full diameter, including the 6-mm disc, to the nearest millimetre with a ruler or calipers. The diameter is compared with standard interpretive tables (CLSI or EUCAST) to report susceptible, intermediate or resistant. Reading from the back of the plate against a dark background gives a sharp edge for most organisms. Discs are placed at least 24 mm apart, centre to centre, so that neighbouring zones do not overlap.

   • A: Discs are spaced so that zones do not overlap; the gap between them is not measured.
   • B: Zone area is not used; the interpretive tables are based on the diameter.
   • D: Measuring from the disc edge (zone width) belongs to the Stokes comparative method.

4. Question 4 · Medium

   In the Stokes method of disc diffusion, susceptibility is judged by:

   1. AComparing the zone diameter with a breakpoint table
   2. BFinding the lowest drug concentration without growth
   3. CReading where the growth ellipse meets the strip
   4. DComparing the zone with a control strain on the same plate

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   Answer: D · Comparing the zone with a control strain on the same plate

   In the Stokes method the test organism and a sensitive control strain are inoculated on the same plate, with the discs placed between them. The zone widths of test and control, from disc edge to zone edge, are compared directly. Because both share the same medium, discs and incubation, variations in technique affect them equally. The test is reported sensitive when its zone is equal to, wider than, or not more than 3 mm narrower than the control zone.

   • A: Breakpoint tables are used in the Kirby-Bauer method, which has no control on the plate.
   • B: The lowest concentration without visible growth is the MIC from a dilution test.
   • C: Reading an ellipse on a strip is the gradient diffusion (E-test) method.

5. Question 5 · Easy

   To what uniform depth is Mueller-Hinton agar poured for the Kirby-Bauer disc diffusion test?

   1. A2 mm
   2. B4 mm
   3. C10 mm
   4. D6 mm

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   Answer: B · 4 mm

   CLSI specifies Mueller-Hinton agar about 4 mm deep, which is roughly 25 mL in a 90 mm plate. Zone size depends on how far the drug diffuses before growth becomes visible, so the depth must be standard. Thin agar lets the drug spread further and gives falsely large zones, while deep agar gives falsely small zones. The interpretive zone charts are valid only for agar of the standard depth.

   • A: Agar 2 mm deep is too thin and gives falsely large zones (false susceptibility).
   • C: Agar 10 mm deep would shrink the zones badly and misclassify susceptible strains.
   • D: Agar 6 mm deep is too thick and gives falsely small zones (false resistance).

6. Question 6 · Medium

   Mueller-Hinton agar for disc diffusion must be low in thymidine. Excess thymidine in the medium causes:

   1. AFalse resistance to aminoglycosides
   2. BFalse susceptibility to penicillin
   3. CFalse susceptibility to tetracycline
   4. DFalse resistance to co-trimoxazole

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7. Question 7 · Medium

   To prepare 30 µg discs in-house by adding 10 µL of antibiotic solution to each blank filter-paper disc, the solution must contain:

   1. A30 mg/mL
   2. B3 µg/mL
   3. C3 mg/mL
   4. D0.3 mg/mL

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8. Question 8 · Hard

   Daily QC of disc diffusion with E. coli ATCC 25922 gives these zones (acceptable range in brackets): ampicillin 12 mm (15-22), gentamicin 16 mm (19-26), ciprofloxacin 25 mm (29-38), tetracycline 15 mm (18-25). The discs are from different lots. The most likely cause is:

   1. AAgar poured thinner than 4 mm
   2. BInoculum heavier than 0.5 McFarland
   3. CLoss of potency of the ampicillin discs
   4. DInoculum lighter than 0.5 McFarland

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9. Question 9 · Medium

   On an E-test (gradient strip) plate, the MIC is read:

   1. AAt the top end of the strip
   2. BAt the widest point of the ellipse
   3. CWhere colonies first touch the plate rim
   4. DWhere the ellipse edge meets the strip

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10. Question 10 · Hard

    A Staphylococcus aureus is resistant to erythromycin and appears sensitive to clindamycin. With an erythromycin disc placed 15-26 mm from the clindamycin disc, the clindamycin zone is flattened on the side next to erythromycin. What should be reported?

    1. AErythromycin sensitive
    2. BClindamycin sensitive
    3. CRepeat with a heavier inoculum
    4. DClindamycin resistant

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11. Question 11 · Easy

    Which disc separates Streptococcus pneumoniae from viridans streptococci?

    1. APolymyxin B
    2. BBacitracin
    3. CNovobiocin
    4. DOptochin

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12. Question 12 · Easy

    A beta-hemolytic streptococcus from a sore throat shows a zone of inhibition around a 0.04-unit bacitracin disc. This presumptively identifies:

    1. AEnterococcus faecalis
    2. BStreptococcus agalactiae
    3. CStreptococcus pneumoniae
    4. DStreptococcus pyogenes

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13. Question 13 · Easy

    How should the stock supply of antimicrobial discs be stored?

    1. AFrozen at -20°C with a desiccant
    2. BIn a 37°C dry incubator
    3. CRefrigerated at 4°C in a humid chamber
    4. DAt room temperature in the dark

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14. Question 14 · Medium

    A Staphylococcus aureus from a wound gives a cefoxitin (30 µg) disc zone of 16 mm. By CLSI (resistant ≤21 mm, susceptible ≥22 mm), it should be reported as:

    1. AInducible clindamycin resistant
    2. BMRSA, resistant to beta-lactams
    3. CMSSA, sensitive to cloxacillin
    4. DVancomycin-intermediate S. aureus

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15. Question 15 · Medium

    After the discs are applied, a Kirby-Bauer plate is left on the bench for 2 hours before incubation. The likely effect is:

    1. ANo change, as long as incubation is 18 hours
    2. BFalsely large zones of inhibition
    3. CFalsely small zones of inhibition
    4. DMixed growth inside the zones

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16. Question 16 · Medium

    In disc diffusion, an inoculum much heavier than the 0.5 McFarland standard gives:

    1. AZones with a double edge
    2. BUnchanged zones but slow growth
    3. CSmaller zones and false resistance
    4. DLarger zones and false susceptibility

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17. Question 17 · Hard

    A Klebsiella pneumoniae from urine is tested by the combination disc method. Ceftazidime 30 µg: 13 mm; ceftazidime-clavulanate 30/10 µg: 21 mm. Cefotaxime 30 µg: 15 mm; cefotaxime-clavulanate: 18 mm. The isolate is best reported as:

    1. AA carbapenemase producer
    2. BNot an ESBL producer
    3. CAn ESBL producer
    4. DAn AmpC producer

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18. Question 18 · Easy

    A bacterial suspension matched to the 0.5 McFarland standard contains about:

    1. A1.5 x 10^8 CFU/mL
    2. B3 x 10^9 CFU/mL
    3. C5 x 10^5 CFU/mL
    4. D1.5 x 10^6 CFU/mL

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