GATE Food Microbiology: Spoilage, Pathogens & Detection: practice questions

1 · Intrinsic and extrinsic factors controlling microbial growth · medium

A food has a water activity of 0.85. At what relative humidity of the surrounding air will it neither gain nor lose moisture?

1. 15%
2. 8.5%
3. 85%
4. 0.85%

2 · Bacterial food-borne pathogens: Salmonella, Listeria, E. coli (STEC), Campylobacter, Shigella, Vibrio and Yersinia · medium

Which characteristic helps distinguish E. coli O157:H7 from most other E. coli on selective plating media?

1. It grows only at 25 °C
2. It does not ferment sorbitol within 24 hours
3. It is oxidase positive
4. It does not ferment glucose or produce gas in any medium

3 · Spore-formers and toxin producers: Clostridium botulinum, C. perfringens, Bacillus cereus and Staphylococcus aureus · medium

Non-proteolytic (Group II) Clostridium botulinum is a particular hazard in chilled vacuum-packed foods because it:

1. Has spores more heat resistant than proteolytic strains
2. Grows at water activities down to about 0.85 in dried products
3. Grows and forms toxin at temperatures as low as about 3 °C
4. Grows at a pH as low as 3.5 in acidified products

4 · Spore-formers and toxin producers: Clostridium botulinum, C. perfringens, Bacillus cereus and Staphylococcus aureus · hard

D121 for proteolytic C. botulinum spores is taken as 0.21 min. What minimum F0 does a 12D botulinum cook require?

1. 1.75 min
2. 12.0 min
3. 2.52 min
4. 0.21 min

5 · Conventional methods: plate count, MPN, selective media and confirmation · medium

Why is a non-selective pre-enrichment step used before selective enrichment in Salmonella detection?

1. To remove fat and other inhibitors that would otherwise block the serological confirmation tests
2. To kill competing Gram-positive bacteria before plating
3. To let sublethally injured cells recover before they meet selective agents
4. To count the Salmonella present before enrichment begins

6 · Molecular methods: PCR, qPCR and whole-genome sequencing · hard

Which reagent is used in viability PCR to reduce amplification of DNA from dead cells?

1. Propidium monoazide
2. Proteinase K
3. Uracil-N-glycosylase
4. Ethidium bromide

7 · Biofilms in food plants · medium

Cells in a biofilm are much more resistant to sanitisers than the same cells in suspension mainly because:

1. The matrix limits penetration, deep cells grow slowly and persister cells survive
2. Biofilm cells convert to spores that sanitisers cannot penetrate
3. Biofilm cells acquire resistance genes directly from the sanitiser molecules that reach them
4. Sanitisers are designed to kill only free-floating planktonic cells

8 · Microbiological criteria and sampling plans · easy

In an ICMSF two-class sampling plan, the symbol c means:

1. The number of sample units tested from the lot
2. The limit separating acceptable from unacceptable counts
3. The upper limit separating marginal from unacceptable counts
4. The maximum number of sample units allowed to exceed m

9 · Spoilage of canned, bakery and beverage products · easy

Swollen cans are described in increasing order of severity as:

1. Hard swell, soft swell, springer, flipper
2. Soft swell, flipper, hard swell, springer
3. Flipper, springer, soft swell, hard swell
4. Springer, flipper, hard swell, soft swell

10 · Predictive microbiology: primary and secondary models · medium

Which of the following is a primary growth model?

1. Gamma concept model
2. Ratkowsky square-root model
3. Cardinal parameter model
4. Baranyi and Roberts model