Q: A bacterial culture is serially diluted and plated. If 12 colonies are counted on a plate from a $$10^{-6}$$ dilution and 0.1 mL was plated, what is the concentration of cells in the original culture?

$$1.2 \times 10$$^{8}$$ cells/mL

Q: Which of the following is a correct formula for calculating the number of bacteria per mL in an original culture after serial dilution and plating?

$$(\text{Number of colonies}) \times (\text{dilution factor}) / (\text{volume plated in mL})$$

Question 1

Which of the following best describes the difference between a prototroph and an auxotroph?

Accepted Answer

A prototroph can grow on minimal medium, while an auxotroph requires a complete medium due to its inability to synthesize one or more essential compounds.

Question 2

A bacterial culture is serially diluted and plated. If 12 colonies are counted on a plate from a $$10^{-6}$$ dilution and 0.1 mL was plated, what is the concentration of cells in the original culture?

Accepted Answer

$$1.2 	imes 10$$^{8}$$ cells/mL

Question 3

Which of the following is NOT a requirement for bacterial conjugation?

Accepted Answer

A filter separating the two strains

Question 4

In an Hfr × F− mating, the order of gene transfer is leu – azi – ton – lac. Which statement is most accurate?

Accepted Answer

leu is adjacent to the point of origin

Question 5

Which of the following best explains why recipient cells in an Hfr × F− cross usually remain F−?

Accepted Answer

Conjugation does not last long enough to transfer the entire F factor.

Question 6

Which process involves the uptake of small pieces of extracellular DNA by a bacterial cell and does NOT require physical contact?

Accepted Answer

Transformation

Question 7

A strain of E. coli O157:H7 acquires the ability to produce a toxin normally found in Shigella. Which mechanism would NOT explain this genetic change?

Accepted Answer

Vertical gene transfer

Question 8

Which of the following statements about R plasmids is correct?

Accepted Answer

They can confer resistance to multiple antibiotics and can be transferred between bacteria.

Question 9

Which of the following is a key feature of the lytic cycle of a bacteriophage?

Accepted Answer

The host cell is lysed, releasing new phage particles.

Question 10

In a plaque assay, what does each plaque on the bacterial lawn represent?

Accepted Answer

A single phage infection event followed by lysis of bacteria.

Question 11

Which of the following is true about generalized transduction?

Accepted Answer

It can transfer any bacterial gene to another bacterium via a phage.

Question 12

A bacterial strain is described as trp−. Which medium would allow it to grow?

Accepted Answer

Minimal medium plus tryptophan

Question 13

Which of the following is a correct formula for calculating the number of bacteria per mL in an original culture after serial dilution and plating?

Accepted Answer

$$(	ext{Number of colonies}) 	imes (	ext{dilution factor}) / (	ext{volume plated in mL})$$

Question 14

Which of the following is NOT a stage of bacterial growth in batch culture?

Accepted Answer

Death phase

Question 15

Which of the following is a consequence of the F factor integrating into the bacterial chromosome?

Accepted Answer

The cell becomes an Hfr strain and can transfer chromosomal genes during conjugation.

Question 16

Which of the following is true about cotransformation?

Accepted Answer

Genes that are close together are more likely to be transferred together during transformation.

Question 17

Which of the following best describes the difference between vertical and horizontal gene transfer?

Accepted Answer

Vertical transfer is from parent to offspring, while horizontal transfer is between unrelated bacteria.

Question 18

Which of the following is a plausible reason why antibiotic resistance can spread rapidly in a hospital setting?

Accepted Answer

Horizontal gene transfer via R plasmids among bacteria.

Question 19

Which of the following is a correct statement about the interrupted mating experiment in E. coli?

Accepted Answer

It allows mapping of gene order based on the time of entry of genes during conjugation.

Question 20

Which of the following is NOT a feature of the lysogenic cycle?

Accepted Answer

Immediate lysis of the host cell

Microbial Genetics: Genetic Analysis and Mapping in Bacteria and Bacteriophages

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