Q: Which of the following equations correctly represents the calculation of the Most Probable Number (MPN) per 100 mL, given the MPN index from the reference table and the sample volumes used?

$$\text{MPN per 100 mL} = \text{MPN index} \times \frac{100}{\text{volume of sample in the middle set of tubes (mL)}}$$

Question 1

Which of the following components in Salmonella-Shigella (S-S) agar is primarily responsible for inhibiting the growth of Gram-positive bacteria?

Accepted Answer

Bile salts

Question 2

A microbiologist observes colorless colonies with black centers on S-S agar after incubating a stool sample. Which organism is most likely present?

Accepted Answer

Salmonella spp.

Question 3

What is the primary function of phenol red in the phenol red-lactose broth used in the MPN method?

Accepted Answer

To act as a pH indicator, changing color in response to acid production from lactose fermentation

Question 4

Which of the following best describes a Durham tube?

Accepted Answer

A small inverted tube placed inside a culture tube to capture gas produced by microorganisms

Question 5

Why are different volumes of a water sample (e.g., 10 mL, 1 mL, 0.1 mL) used to inoculate the MPN tubes?

Accepted Answer

To increase the likelihood of detecting bacteria present at low concentrations by providing a range of dilutions

Question 6

A water sample is tested using the MPN method. After incubation, the following results are observed: 3 positive tubes at 10 mL, 2 positive tubes at 1 mL, and 0 positive tubes at 0.1 mL. Using a standard MPN reference table, what is the next step to determine the MPN value?

Accepted Answer

Locate the combination 3-2-0 in the MPN table to find the corresponding MPN value per 100 mL

Question 7

On S-S agar, which of the following colony appearances is most characteristic of Shigella spp.?

Accepted Answer

Colorless colonies without black centers

Question 8

Which of the following best explains why fecal coliforms produce pink colonies on S-S agar?

Accepted Answer

They ferment lactose, producing acid that reacts with neutral red to form pink colonies

Question 9

A student is performing the MPN test and observes that the phenol red-lactose broth in several tubes has turned yellow and contains a gas bubble in the Durham tube. What does this indicate?

Accepted Answer

Lactose fermentation with gas production, suggesting the presence of coliform bacteria

Question 10

Which of the following equations correctly represents the calculation of the Most Probable Number (MPN) per 100 mL, given the MPN index from the reference table and the sample volumes used?

Accepted Answer

$$	ext{MPN per 100 mL} = 	ext{MPN index} 	imes \frac{100}{	ext{volume of sample in the middle set of tubes (mL)}}$$

Question 11

A public health laboratory is testing a water sample for contamination. The MPN test yields positive results in all 10 mL tubes, half of the 1 mL tubes, and none of the 0.1 mL tubes. What does this pattern most likely indicate about the water sample?

Accepted Answer

The water sample contains a moderate concentration of coliform bacteria

Question 12

Which of the following best describes the role of sodium thiosulfate in S-S agar?

Accepted Answer

It serves as a source of sulfur for detecting hydrogen sulfide production

Question 13

A water sample is tested using the MPN method. The phenol red-lactose broth remains red after incubation. What is the most likely explanation?

Accepted Answer

No lactose fermentation has occurred, indicating the absence of coliform bacteria

Question 14

Which of the following is a critical reason for using a reference table in the MPN method?

Accepted Answer

To statistically estimate the most probable number of bacteria based on the pattern of positive and negative tubes

Question 15

In a scenario where a water sample is highly contaminated, which of the following would you expect to observe in the MPN test tubes?

Accepted Answer

All tubes, regardless of dilution, show positive results for lactose fermentation and gas production

Question 16

Which of the following best explains why S-S agar contains both bile salts and brilliant green dye?

Accepted Answer

To selectively inhibit the growth of Gram-positive and some Gram-negative bacteria, allowing for the isolation of Salmonella and Shigella

Question 17

A laboratory technician is interpreting results from S-S agar. Which of the following observations would most likely indicate the presence of fecal coliforms?

Accepted Answer

Pink colonies due to lactose fermentation

Question 18

Which of the following is the correct sequence of steps for determining the MPN of a water sample?

Accepted Answer

Inoculate tubes with different sample volumes, incubate, record positive tubes, use reference table to estimate MPN

Question 19

A student mistakenly uses only 1 mL of water sample in all MPN tubes instead of using 10 mL, 1 mL, and 0.1 mL. What is the most likely consequence of this error?

Accepted Answer

The test will be less sensitive to detecting low concentrations of bacteria

Microbiological Analysis of Food and Water: Salmonella-Shigella Agar and MPN Method

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