Microbiology interview questions (with answers)
20 original interview questions with answers for pharma microbiology freshers and candidates with up to 3 years of experience: sterility test, MLT, endotoxin, cleanrooms, media fill and more. Tap an option to check your answer, or open "Show answer".
1. Which two culture media are traditionally used in the compendial sterility test?
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Answer: B — Fluid Thioglycollate Medium is mainly intended for anaerobic bacteria (it also detects aerobes), and Soybean-Casein Digest Medium supports aerobic bacteria and fungi. The agar media in the other options are used for enumeration or selective tests, not for the sterility test.
2. For how long are the inoculated sterility test media normally incubated and observed?
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Answer: C — The compendial sterility test requires incubation and periodic observation of the media for 14 days. Shorter periods are used for non-sterile enumeration tests, not for sterility.
3. What is the purpose of the method suitability (bacteriostasis and fungistasis) test before routine sterility testing of a product?
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Answer: A — If the product itself stops microbes growing, a real contaminant could be missed and the test would give a false pass. Method suitability confirms that the chosen method (rinsing, dilution, neutralisation) allows low numbers of challenge organisms to grow.
4. When a product is filterable, why is membrane filtration usually preferred over direct inoculation for the sterility test?
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Answer: D — The membrane retains any microorganisms while the product and its preservatives or antibiotics are washed through. Incubation and growth promotion are still required, and direct inoculation can also detect fungi.
5. Which chapter covers the test for the absence of specified microorganisms (for example E. coli and Salmonella) in non-sterile products?
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Answer: B — USP <62> is the qualitative test for specified microorganisms. USP <61> is the quantitative enumeration test, <71> is sterility and <85> is bacterial endotoxins.
6. In the microbial enumeration test, what do TAMC and TYMC stand for?
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Answer: A — TAMC is the total aerobic microbial count (bacteria and fungi, counted on casein soya bean digest agar) and TYMC is the total combined yeasts and moulds count (counted on Sabouraud dextrose agar). The results are reported per gram or per mL of product.
7. What does the Bacterial Endotoxins Test (BET) detect?
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Answer: C — LAL reagent, prepared from horseshoe crab amoebocytes, reacts with bacterial endotoxin. BET does not count viable organisms, so a product can be sterile and still fail for endotoxin.
8. In the gel-clot method of the BET, what is read as a positive result?
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Answer: D — Endotoxin triggers the clotting cascade in the LAL reagent and a firm gel forms. If no firm gel is present on inversion, the tube is negative.
9. Under EU GMP Annex 1 (2022), which grade is used for high-risk operations such as the aseptic filling zone?
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Answer: A — Grade A is the zone for high-risk operations, for example the filling zone, stopper bowls and open primary packaging, normally protected by unidirectional airflow, RABS or an isolator. Grades C and D are used for less critical steps.
10. In a conventional aseptic filling line (not an isolator), what is the usual role of the Grade B area?
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Answer: B — Grade B is the background cleanroom surrounding the Grade A zone for aseptic preparation and filling. Because it is next to the critical zone, its monitoring and gowning standards are strict.
11. Which airborne particle sizes are counted when classifying a cleanroom under Annex 1 (2022)?
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Answer: C — Classification measures total particles of 0.5 micron and larger and 5 micron and larger, at rest and in simulated operation. For Grades A and B the 5 micron limit for classification is "not specified" in Table 1 unless the contamination control strategy or historical trends call for it.
12. What is the expected microbial result in a Grade A zone during environmental monitoring?
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Answer: D — The maximum action limit for viable contamination in Grade A is "no growth", and any growth in Grade A should result in an investigation. Organisms found in Grade A and B areas should be identified to species level and their impact on each implicated batch evaluated.
13. Which sampling method is typically used for monitoring microbial contamination of flat equipment and room surfaces in a cleanroom?
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Answer: A — Contact plates are pressed onto the surface to pick up viable organisms. Settle plates monitor microbes falling from air, and particle counters count non-viable particles, not viable organisms.
14. What is the purpose of a media fill (aseptic process simulation)?
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Answer: C — Sterile culture medium is processed through the whole aseptic operation, including interventions, and the filled units are incubated. Growth in any unit points to a weakness in the aseptic process or environment.
15. During initial qualification of an aseptic process, how many consecutive satisfactory media fills are expected?
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Answer: B — Initial validation needs at least three consecutive satisfactory simulation tests covering all working shifts, then periodic APS normally twice a year for each process, filling line and shift, and repeat APS after significant changes that affect sterility assurance.
16. What is the purpose of the growth promotion test on each batch of culture medium?
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Answer: D — Growth promotion (and, for selective media, inhibitory and indicative properties) shows that a batch of medium can recover microorganisms. Without it, a negative test result could simply reflect poor medium.
17. Why are alert and action levels set for the microbial count of purified water monitoring?
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Answer: A — Alert levels are early-warning triggers based on trend data, and action levels demand investigation, so the action level is set higher than the alert level. The actual numbers must come from the pharmacopoeia, validation data or the site's own limits; an in-house level should be labelled as such.
18. Why is a neutraliser used when validating a disinfectant by surface or suspension testing?
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Answer: C — Without a neutraliser, residual disinfectant carried into the recovery medium would keep killing organisms and make the disinfectant look better than it is. The neutraliser must itself be shown to be non-toxic to the test organisms (example in-house validation practice).
19. What does the F0 value represent in steam sterilisation?
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Answer: B — F0 converts the temperature-time history of a cycle into the equivalent minutes at 121 °C (taking z as 10 °C). It lets different cycles be compared for lethality.
20. Which biological indicator is normally used to challenge a moist-heat (steam) sterilisation cycle?
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Answer: D — Spores of G. stearothermophilus are highly resistant to moist heat, so killing them gives a wide margin of assurance. Vegetative cells such as E. coli are far too easy to kill to be useful as indicators.