Q: Which enzyme is responsible for breaking down hydrogen peroxide into water and oxygen, and what is the equation for this reaction?

Catalase; $$2\ \mathrm{H}_2\mathrm{O}_2 \xrightarrow{\text{catalase}} 2\ \mathrm{H}_2\mathrm{O} + \mathrm{O}_2$$

Question 1

Which of the following best describes the temperature range and significance of psychrotrophs in food microbiology?

Accepted Answer

Psychrotrophs grow between 0°C and 20–30°C and are significant because they cause food spoilage at refrigeration temperatures.

Question 2

A bacterium is placed in a solution containing 10% NaCl. What is the most likely outcome for a typical non-halophilic bacterial cell?

Accepted Answer

The cell will undergo plasmolysis due to water loss.

Question 3

Which of the following statements about chemically defined media is correct?

Accepted Answer

Chemically defined media have an exact known chemical composition, allowing precise control over nutrient content.

Question 4

Which component in the chemically defined medium for E. coli serves as the main nitrogen source?

Accepted Answer

Ammonium phosphate, monobasic $$\left( \mathrm{NH}_4\mathrm{H}_2\mathrm{PO}_4 \right)$$

Question 5

Which of the following best explains why agar is used as a solidifying agent in microbiological media?

Accepted Answer

Agar remains solid at incubation temperatures and is not generally metabolized by microbes.

Question 6

A facultative anaerobe is inoculated into thioglycollate broth. Where would you expect to see the most growth?

Accepted Answer

Throughout the tube, but more growth at the top where oxygen concentration is highest.

Question 7

Which enzyme is responsible for breaking down hydrogen peroxide into water and oxygen, and what is the equation for this reaction?

Accepted Answer

Catalase; $$2\ \mathrm{H}_2\mathrm{O}_2 \xrightarrow{	ext{catalase}} 2\ \mathrm{H}_2\mathrm{O} + \mathrm{O}_2$$

Question 8

A microbiologist wants to isolate a pure culture from a mixed sample. Which technique should be used and why?

Accepted Answer

Streak plate method, because it separates individual cells on the surface, allowing isolated colonies to form.

Question 9

A sample is diluted in a series of 1:10 dilutions, and 1 mL from the $$10^{-4}$$ dilution is plated. After incubation, 32 colonies are counted. What is the concentration of bacteria in the original sample?

Accepted Answer

$$3.2 	imes 10^6$ CFU/mL

Question 10

Which phase of the bacterial growth curve is characterized by a balance between cell division and cell death, resulting in a stable population size?

Accepted Answer

Stationary phase

Question 11

A water sample is filtered and the filter is placed on a nutrient pad. After incubation, colonies are counted. What is this method best suited for?

Accepted Answer

Counting bacteria in dilute environmental water samples

Question 12

Which of the following best describes the function of selective media?

Accepted Answer

Selective media suppress unwanted microbes and encourage the growth of desired microbes.

Question 13

A bacterial culture is preserved by lyophilization. What does this process involve?

Accepted Answer

Freezing the culture at -54° to -72°C and dehydrating it in a vacuum.

Question 14

Which of the following equations correctly calculates the number of generations in a bacterial culture?

Accepted Answer

$$	ext{Number of generations} = \dfrac{\log(	ext{cells generated}/	ext{initial cells})}{\log 2}$$

Question 15

Which of the following best describes the role of trace elements in microbial growth?

Accepted Answer

Trace elements are inorganic elements required in small amounts, usually as enzyme cofactors.

Question 16

A microbiologist measures the turbidity of a bacterial suspension using a spectrophotometer. What is being measured, and how does it relate to cell density?

Accepted Answer

The amount of light absorbed by the suspension; higher absorbance means higher cell density.

Question 17

Which of the following best explains why obligate anaerobes are unable to grow in the presence of oxygen?

Accepted Answer

They lack enzymes such as superoxide dismutase and catalase to detoxify reactive oxygen species.

Question 18

Which of the following is a correct description of a pure culture?

Accepted Answer

A culture containing only one species or strain of microorganism.

Question 19

Which of the following best describes the function of differential media?

Accepted Answer

Differential media make it easy to distinguish colonies of different microbes based on their biochemical properties.

Microbial Growth: Requirements, Media, and Measurement

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