Question 1

Which of the following best describes a defined culture medium?

Accepted Answer

A medium whose exact chemical composition is known.

Question 2

A microbiologist wants to isolate Staphylococcus aureus from a mixed sample. Which medium would be most appropriate and why?

Accepted Answer

Mannitol Salt Agar, because it is both selective (high NaCl) and differential (phenol red indicator for mannitol fermentation).

Question 3

Which method is most suitable for quantifying viable cells in a water sample with low microbial concentration?

Accepted Answer

Membrane filtration followed by plating on appropriate medium.

Question 4

A researcher measures optical density (OD) at 540 nm to estimate cell concentration. What must be done to convert OD readings to cell numbers?

Accepted Answer

A standard curve must be generated by correlating OD values with known cell counts.

Question 5

Which of the following statements about autoclave sterilization is correct?

Accepted Answer

Sterilization is achieved by high temperature (121°C) generated by steam under pressure, not by pressure alone.

Question 6

Which equation best represents the decimal reduction time (D-value) for microbial death?

Accepted Answer

D = \frac{t}{\log_{10}($$N_0/N$$)}

Question 7

Which physical method is most effective for sterilizing heat-sensitive (thermolabile) solutions?

Accepted Answer

Filtration using a 0.2 \mu m pore size membrane.

Question 8

A laboratory technician uses the spread plate method to count bacteria. Why is it important to plate serial dilutions?

Accepted Answer

To ensure the number of colonies falls within the ideal range (30–300 CFU) for accurate counting.

Question 9

Which of the following is NOT a mechanism of action for chemical antimicrobial agents?

Accepted Answer

Increasing microbial motility.

Question 10

Which statement best explains why UV radiation is effective for surface sterilization but not for sterilizing thick or bulky materials?

Accepted Answer

UV radiation has low penetration power and cannot reach microbes inside thick materials.

Question 11

A hospital wants to reduce microbial load on surgical instruments to safe public health levels. Which process should be used?

Accepted Answer

Sanitization

Question 12

Which of the following is the most resistant to antimicrobial agents?

Accepted Answer

Prions

Question 13

A microbiologist is culturing an obligate anaerobe. Which of the following would be most effective in creating an anaerobic environment?

Accepted Answer

Using a reducing medium with sodium thioglycolate and an anaerobic jar.

Question 14

Which term describes the minimum concentration of an antimicrobial agent required to inhibit visible growth of a microorganism?

Accepted Answer

Minimum inhibitory concentration (MIC)

Question 15

Which of the following best describes the difference between total cell counting and viable cell counting?

Accepted Answer

Total cell counting includes both living and dead cells, while viable cell counting includes only living cells.

Question 16

Which of the following is a differential medium and what does it indicate?

Accepted Answer

Mannitol Salt Agar; it indicates mannitol fermentation by a color change due to phenol red.

Question 17

A bacterial population is reduced from $$10^8 to$$ $$10^5$$ cells after 15 minutes of exposure to a disinfectant. What is the number of log reductions?

Accepted Answer

3 log reductions

Question 18

Which of the following factors does NOT influence the effectiveness of an antimicrobial agent?

Accepted Answer

The color of the microbial colony.

Question 19

Which method is used to assay antimicrobial activity by measuring the zone of inhibition around a disc?

Accepted Answer

Kirby-Bauer diffusion test

Question 20

Which of the following best describes the effect of refrigeration on microbial growth?

Accepted Answer

It is bacteriostatic, slowing microbial metabolism and growth.

Cultivation of Microbes and Control of Microbial Growth

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