ICAR AICE-JRF/SRF General Microbiology: practice questions

1 · History and scope of microbiology · medium

Viruses were first recognised when tobacco mosaic disease was shown to be caused by an agent that:

1. Was killed by boiling but not by filtering
2. Grew on agar but only in the dark
3. Passed through bacteria-proof filters
4. Was visible as rods under oil immersion

2 · Prokaryotic and eukaryotic cell structure; spores and vegetative cells · hard

Gram-negative bacteria are often more resistant than Gram-positive bacteria to quaternary ammonium compounds and some dyes mainly because they have:

1. Teichoic acids that neutralise cationic chemicals at the surface
2. Endospores in every cell during exponential growth
3. An outer membrane that acts as a permeability barrier
4. A thicker peptidoglycan wall that traps the agents

3 · Bacteria, yeasts, moulds and actinomycetes: morphology and classification · hard

Two bacterial isolates have an average nucleotide identity of their genomes of 99%. Under current genome-based taxonomy they are:

1. Members of different species in one genus
2. Members of the same species
3. Members of different genera in one family
4. Members of different domains of life

4 · Viruses and bacteriophages · hard

Norovirus and hepatitis A virus are harder to inactivate with alcohol-based hand rubs than enveloped viruses because they:

1. Grow in foods and reach very high numbers before washing
2. Have no lipid envelope for alcohol to disrupt
3. Carry DNA genomes that are protected by thick peptidoglycan
4. Form spores that resist alcohol and drying for many months

5 · Microbial nutrition and culture media · hard

A medium contains an antibiotic that loses activity when heated. The correct way to prepare it is to:

1. Autoclave the base, cool to 50 °C, add filter-sterilised antibiotic
2. Boil the antibiotic for 1 minute and add it to the hot medium
3. Autoclave the base and antibiotic together at 121 °C for 30 minutes
4. Add the antibiotic unsterilised because it kills all contaminants

6 · Growth curve, generation time and growth kinetics · hard

What happens in a chemostat if the dilution rate is set above the organism's maximum specific growth rate?

1. The culture moves into a new steady state at a higher cell density
2. Substrate in the vessel falls to zero and cells sporulate
3. Growth rate rises above μmax to match the higher dilution rate
4. Cells wash out faster than they grow, and the culture is lost

7 · Microbial genetics: mutation, plasmids, transformation, transduction and conjugation · hard

In an Hfr × F⁻ cross, the recipient usually remains F⁻ because:

1. Hfr cells lack a sex pilus and so cannot make stable contact with recipients
2. Hfr cells transfer only plasmid DNA and never chromosomal genes
3. The rest of F enters last, and whole-chromosome transfer is rare
4. The recipient destroys the F plasmid with restriction enzymes

8 · Microbial genetics: mutation, plasmids, transformation, transduction and conjugation · medium

The Ames test screens chemicals for mutagenicity by counting:

1. His⁻ Salmonella revertants that grow without histidine
2. Yeast cells that lose the ability to ferment sucrose
3. Mouse liver cells that form tumours after injection
4. Colonies of E. coli that become resistant to phage T1 after exposure

9 · Aseptic technique and laboratory biosafety · hard

A culture tube of Listeria breaks on the bench. The correct first sequence of actions is:

1. Switch on the bench UV lamp and leave the room for ten minutes
2. Wipe it up at once with a dry paper towel and discard in the bin
3. Pour 70% ethanol and light it to burn off the culture
4. Cover with absorbent, flood with disinfectant, then wait