Q: A sample is diluted serially and plated as shown in the thirteenth image. If 32 colonies are counted on the $$1:100,000$$ dilution plate, what is the number of bacteria per mL in the original sample?

$$32 \times 10^5$ = 3.2 \times 10^6$

Question 1

Which of the following best describes the growth pattern of an obligate anaerobe in a thioglycolate broth tube, as shown in the third image provided?

Accepted Answer

Growth only at the bottom of the tube where oxygen is absent.

Question 2

A microbiologist wants to culture a chemoheterotroph such as Escherichia coli using a chemically defined medium. Which component provides the main source of nitrogen in the medium shown in the sixth image?

Accepted Answer

Ammonium phosphate, monobasic ($$\mathrm{NH_4H_2PO_4})$$

Question 3

Which phase of bacterial growth is characterized by a balance between cell division and cell death, as shown in the twelfth image?

Accepted Answer

Stationary phase

Question 4

A sample is diluted serially and plated as shown in the thirteenth image. If 32 colonies are counted on the $$1:100,000$$ dilution plate, what is the number of bacteria per mL in the original sample?

Accepted Answer

$$32 	imes 10^5$ = 3.2 	imes 10^6$

Question 5

Which of the following best explains why honey and salted fish are preserved from microbial spoilage?

Accepted Answer

High osmotic pressure creates a hypertonic environment, causing plasmolysis in microbes.

Question 6

Which type of medium is both selective and differential, as shown in the ninth image, and is used to distinguish lactose fermenters from non-fermenters?

Accepted Answer

MacConkey agar

Question 7

Which physical method of microbial control is most appropriate for sterilizing heat-sensitive solutions, as depicted in the fifteenth image?

Accepted Answer

Filtration using membrane filters

Question 8

A patient receives antibodies from an immune serum injection after exposure to rabies. Which type of immunity is this?

Accepted Answer

Artificially acquired passive immunity

Question 9

Which immunoglobulin class is most abundant in serum and can cross the placenta to confer passive immunity to the fetus?

Accepted Answer

IgG

Question 10

Which of the following is a function of helper T cells (TH cells) in adaptive immunity?

Accepted Answer

Activating B cells and other immune cells by secreting cytokines

Question 11

Which of the following best describes the action of catalase in aerobic bacteria?

Accepted Answer

Catalase breaks down hydrogen peroxide into water and oxygen: $$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 12

Which of the following is a direct method for measuring microbial growth?

Accepted Answer

Plate count method

Question 13

Which of the following best explains why autoclaving is more effective than boiling for sterilization?

Accepted Answer

Autoclaving uses steam under pressure, allowing temperatures above $$100^\circC to $$kill endospores.

Question 14

Which of the following is a consequence of complement activation in the immune system?

Accepted Answer

Cytolysis of pathogens

Question 15

A bacterial culture is exposed to a chemical disinfectant. After 10 minutes, 90% of the bacteria are killed. If the initial population was $$1 	imes 10^6$, how many bacteria remain?

Accepted Answer

$$1 	imes 10^5$

Question 16

Which of the following best describes the role of lysozyme in innate immunity?

Accepted Answer

It digests peptidoglycan in bacterial cell walls.

Question 17

Which of the following is a characteristic of T-independent antigens?

Accepted Answer

They are typically polysaccharides with repeating subunits.

Question 18

Which of the following best explains why fever can be beneficial during infection?

Accepted Answer

It enhances the effects of interferons and inhibits the growth of some microbes.

Question 19

Which of the following is a function of IgA antibodies?

Accepted Answer

Prevention of microbial attachment to mucosal surfaces

Microbial Growth, Control, and Immunity: Comprehensive Study Notes

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