Q: Which equation would you use to calculate the original concentration of bacteria in a sample after performing a 10-fold serial dilution and plating 0.1 mL of the final dilution, if you count 50 colonies?

$$C = \frac{\text{Number of colonies}}{\text{Volume plated (mL)} \times \text{Dilution factor}}$$

Question 1

Which of the following best describes the purpose of aseptic technique during the transfer of bacteria?

Accepted Answer

To ensure that only the desired microorganism is transferred without contamination from the environment or other cultures.

Question 2

Why is it important to use a streak plate technique when isolating bacteria from a mixed sample?

Accepted Answer

To dilute the bacterial sample across the surface of the agar, allowing individual colonies to form from single cells.

Question 3

Which of the following statements about MacConkey agar is correct?

Accepted Answer

It is both selective and differential, inhibiting Gram-positive bacteria and differentiating lactose fermenters from non-fermenters.

Question 4

A microbiologist wants to count the number of viable bacteria in a sample. Which method should they use and why?

Accepted Answer

Spread plate method, because it allows for accurate counting of colonies formed from viable cells.

Question 5

Which equation would you use to calculate the original concentration of bacteria in a sample after performing a 10-fold serial dilution and plating 0.1 mL of the final dilution, if you count 50 colonies?

Accepted Answer

$$C = \frac{	ext{Number of colonies}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}}$$

Question 6

Which of the following best explains why Xylose Lysine Deoxycholate (XLD) agar is considered both selective and differential?

Accepted Answer

It inhibits Gram-positive bacteria and differentiates Salmonella and Shigella based on their ability to ferment xylose and produce hydrogen sulfide.

Question 7

A student is preparing a pour plate to count bacteria. Which step is essential to ensure accurate results?

Accepted Answer

Mixing the diluted sample with molten agar before pouring into the plate.

Question 8

Which of the following best describes the growth pattern of obligate anaerobes in thioglycollate broth?

Accepted Answer

They grow only at the bottom of the tube where oxygen concentration is lowest.

Question 9

Which property of Cysteine Lactose Electrolyte Deficient (CLED) agar makes it useful for isolating urinary pathogens?

Accepted Answer

It prevents the swarming of Proteus species, allowing for easier isolation of other bacteria.

Question 10

Which of the following is a critical step in the aseptic transfer of bacteria to prevent contamination?

Accepted Answer

Flaming the inoculating loop before and after each transfer.

Question 11

A microbiologist observes red colonies with black centers on XLD agar. What does this indicate about the bacteria?

Accepted Answer

The bacteria are likely Salmonella species, which ferment xylose and produce hydrogen sulfide.

Question 12

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

Accepted Answer

It allows for the differentiation of microbial species based on their metabolic or biochemical properties.

Question 13

Which group of bacteria would grow only at the top of a thioglycollate broth tube?

Accepted Answer

Obligate aerobes

Question 14

Why is it important to check for turbidity in broth cultures before using them for further experiments?

Accepted Answer

Turbidity indicates bacterial growth, which confirms the viability of the culture.

Question 15

Which of the following best explains why serial dilutions are performed before plating bacteria for colony counts?

Accepted Answer

To reduce the concentration of bacteria so that individual colonies can be counted accurately.

Question 16

A student streaks a plate and observes isolated colonies in the final quadrant. What does this indicate about their technique?

Accepted Answer

They successfully diluted the sample across the plate, allowing single cells to form colonies.

Question 17

Which of the following is a key feature of selective media?

Accepted Answer

It inhibits the growth of unwanted microorganisms while allowing the growth of the desired ones.

Question 18

Which of the following best describes the role of sodium deoxycholate in XLD agar?

Accepted Answer

It acts as a selective agent, inhibiting the growth of Gram-positive bacteria.

Question 19

A culture medium is designed to allow the growth of Gram-negative bacteria while differentiating those that ferment lactose. Which type of medium is this?

Accepted Answer

Selective and differential medium

Question 20

Which of the following best explains why Robert Koch's development of solid media was significant for microbiology?

Accepted Answer

It allowed for the isolation of pure cultures by separating bacteria into individual colonies.

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