National Technical Qualification General Microbiology: practice questions

1 · Microscopy: light, phase-contrast, fluorescence and electron · hard

The direct epifluorescent filter technique (DEFT) for raw milk counts:

1. Gas bubbles produced in Durham tubes after inoculating lactose broth
2. Electrons scattered by cells on a filter coated with gold in a vacuum
3. Fluorochrome-stained cells retained on a membrane
4. Colonies that grow on a membrane placed on agar for 48 hours at 30 °C

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

The main reason bacterial endospores resist heat far better than vegetative cells is:

1. A dehydrated core rich in calcium dipicolinate
2. High levels of catalase and superoxide dismutase in the coat
3. Active repair of heat damage by metabolism inside the spore
4. A thick outer membrane of lipopolysaccharide around the spore

3 · Viruses and bacteriophages · medium

A phage suspension diluted 10⁻⁶ gives 85 plaques when 0.1 mL is plated with host bacteria. The titre is:

1. 8.5 × 10⁹ PFU/mL
2. 8.5 × 10⁵ PFU/mL
3. 8.5 × 10⁷ PFU/mL
4. 8.5 × 10⁸ PFU/mL

4 · Microbial nutrition and culture media · medium

On MacConkey agar, a lactose-fermenting Gram-negative rod gives pink-red colonies. Which components respectively make the medium selective and differential?

1. Neutral red selects; peptone with sodium chloride differentiates
2. Lactose selects; bile salts with crystal violet differentiate by colour
3. Bile salts/crystal violet select; lactose/neutral red differentiate
4. Agar selects; bile salts with lactose differentiate by colony size

5 · Microbial nutrition and culture media · medium

To detect low numbers of Salmonella among many competitors in a food, the first culture step is usually:

1. Selective plating on MacConkey agar after 4 hours
2. Non-selective pre-enrichment in buffered peptone water
3. Membrane filtration of the undiluted food homogenate
4. Direct plating of the homogenate on blood agar without any broth

6 · Measurement of growth: serial dilution, plate count, MPN and turbidity · medium

Which counting method counts both live and dead cells?

1. Pour plate count on plate count agar
2. Most probable number in lactose broth
3. Spread plate count on selective agar after 48 hours
4. Direct microscopic count in a counting chamber

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

A Lactococcus lactis starter strain suddenly becomes a slow acid producer in milk after repeated subculture. The most likely genetic cause is:

1. Integration of an F plasmid into its chromosome
2. Loss of the plasmid for lactose use or proteinase
3. Mutation of its 16S rRNA gene to a different species
4. Gain of an R plasmid carrying resistance to penicillin

8 · Aseptic technique and laboratory biosafety · easy

An inoculating loop is correctly sterilised by:

1. Dipping in 70% ethanol and using it while wet
2. Rinsing in sterile water between samples
3. Wiping with a dry tissue between each transfer
4. Flaming until red hot, then cooling it briefly

9 · Aseptic technique and laboratory biosafety · easy

Most food-borne bacterial pathogens such as Salmonella and Listeria monocytogenes are handled in the laboratory as:

1. Risk group 1 organisms
2. Risk group 4 organisms
3. Risk group 2 organisms
4. Organisms with no risk classification

10 · Aseptic technique and laboratory biosafety · medium

Cultures and contaminated plates from a food microbiology laboratory should be disposed of by:

1. Leaving open under UV light overnight
2. Pouring into the drain with tap water
3. Autoclaving them before any disposal
4. Placing in general waste in sealed bags