Frequently asked questions for microbiology interview

Q 1 : What is the microbial limit of raw water, purified water and water for injection?
Ans: Raw water limit is 500 cfu/mL, Purified water limit is 100 cfu/mL and water for injection limit is 10 cfu/100 mL.

Q 2: What is the TOC limit of purified water and water for injection?
Ans: Not more than 500 ppb.

Q 3: What is the inoculum size used for growth promotion test?
Ans: Not more than 100 cfu

Q 4: What is the inoculum size used for growth inhibition test?
Ans: More than 100 cfu

Q 5: What is biological indicator?
Ans: A biological indicator is well characterized preparation of specific microorganism with a known resistance to a specific sterilization process.

Q 6: How many biological indicators are used for total viable spore count?
Ans: At least four

Q 7: Which microorganism is used for the validation of steam heat sterilizer?
Ans: Geobacillus stearothermophillus

Q 8: What is the acceptance criteria for disinfectant efficacy validation?
Ans: Not less than 3 log reduction in vegetative cells and not less than two log reduction in spore cells.

Q 9: What is endotoxin?
Ans: Endotoxin is a lipopolysaccharide or LPS. LPS consists of the lipid A portion containing fatty acids and disaccharide phosphates, core polysaccharides and the O-antigen. Endotoxin is a type of pyrogen and is a component of the exterior cell wall of Gram-negative bacteria, like E. coli.

Q10: Why we expose the settle plate for four hours?
Ans: Settle plate method is one on the technique of environment monitoring to check the contamination level of the air. Microorganisms are not freely movable as they are attached with particles. We monitor the particles of 0.5-5.0 micron. The settlement of particles depends upon the size of the particle. The heavier the particle the more easily it settles down and the lighter the particle the more difficult for it to settle down. For these range of particles, it generally takes 4 hours for their settlement. The another factor is the dryness of the plate. If we expose the plates for more time, chances of its dryness become higher. However, validation study is required to prove that plates are not dried during or after completion of incubation time. By considering all these factors plates are exposed for 4 hours.

Q 11: Why 0.5-5.0 micron particles measured by non viable particle counter?
Ans: The product contamination is mainly caused by this range of particles because this size of particles are not easily settle down and become threat to the product.






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