Frequently asked questions for microbiology interview

Hello everyone,

My today's post is quite different, interesting and very helpful for all industrial microbiologists. Here, I am posting few frequently asked questions as knowledge booster for interview preparation of microbiology/quality assurance. The level of questions will be such that it will helpful for fresher as well as experienced microbiologist also.

Q 1: What is the acceptance criteria for growth promotion test?

Ans: ±factor of two (for example if we have inoculum size of 50 cfu then it could be 25-100 cfu)

Q 2: How many pathogens are tested as per USP?

Ans: Total seven pathogens (Escherichia coli, Salmonella, Staphylococcus aureus, Pseudomonas aeruginosa, Clostridia, Candida albicans and test for bile tolerant gram negative bacteria).

Q 3: Why Salmonella (pathogen) testing requires 10.0 g sample? 

Ans: Due to non uniform distribution of salmonella in sample more sample is required for identification.

Q 4: Why MacConkey broth is incubated at 42-44°C and MacConkey agar further incubated at 30-35°C?

Ans: Click on the link

Q 5: How many passage of cultures are acceptable?

Ans: Not more than five passage from the original culture.

Q 6: What is the difference between disinfectants, antiseptic and sanitizers?

Ans: Clink on the link

Q 7: Why 70% IPA is more effective then 90%,95% or 100%?

Ans: 70% IPA is more effective than other percentage of IPA because IPA destroy the bacterial cell by denaturing the protein content present in their cell wall. There is balance in penetration of IPA into interior of the bacterial cell and denaturation of proteins. If percentage of IPA will be high then protein quickly denatures and it form a protective covering outside the cell wall and IPA not able to penetrate into interior of the cell wall to completely kill the bacterial cell. However, if concentration remains low then it is not able to denature the protein content present in the cell wall of bacterial. That's why 70% IPA is more effective than 90%, 95% or 100% concentration.

Q 8: Which chemical is used for fogging and what is its concentration?

Ans: Hydrogen peroxide (H2O2) with silver ions. The combination of these two ingredients gives a synergistic broad spectrum of activity on all kinds of viruses, bacteria, fungi, yeast, mold, protozoa and algae. Generally 20% v/v concentration is used for fogging.

Q 9: What is the role of gram's iodine in gram's staining?

Ans: Gram's iodine is used as a mordant to form crystal violet-iodine complex so that the dye can not be removed easily.

Thanks!!!