Question 1

Which component in Mannitol Salt Agar (MSA) is responsible for selecting Staphylococcus species by inhibiting the growth of most other bacteria?

Accepted Answer

7.5\% sodium chloride

Question 2

A microbiologist plates a sample on MSA and observes yellow colonies surrounded by yellow medium. What does this result indicate about the organism?

Accepted Answer

It is Staphylococcus aureus, which ferments mannitol and produces acid

Question 3

Why does SM110 medium not change color when Staphylococcus aureus ferments mannitol?

Accepted Answer

It lacks phenol red indicator

Question 4

On a blood agar plate, a student observes greenish colonies. What type of hemolysis is this and what does it indicate?

Accepted Answer

Alpha hemolysis; partial breakdown of red blood cells

Question 5

Which medium would you use to differentiate between lactose fermenters and non-lactose fermenters among Gram-negative bacteria?

Accepted Answer

MacConkey agar

Question 6

A sample is streaked on Hektoen Enteric agar and produces greenish/blue colonies. What is the most likely identity of the organism?

Accepted Answer

Shigella or Salmonella

Question 7

Which of the following media is specifically designed to inhibit the growth of Gram-positive bacteria while allowing Gram-negative pathogens to grow?

Accepted Answer

MacConkey and Hektoen Enteric agar

Question 8

What is the expected appearance of Salmonella colonies on XLD agar?

Accepted Answer

Red with black centers

Question 9

A student streaks a sample on EMB agar and observes colorless colonies. What does this result suggest?

Accepted Answer

The organism is a non-lactose fermenter, possibly Shigella or Salmonella

Question 10

Which equation best represents the acid production by S. aureus on MSA when mannitol is fermented?

Accepted Answer

$$\mathrm{C_6H_{12}O_6$} \rightarrow 2\ \mathrm{C_3H_6O_3}$$

Question 11

Why is trypticase soy broth used for subculturing colonies after hemolysis identification on blood agar?

Accepted Answer

It is a general-purpose medium that supports the growth of many bacteria

Question 12

Which of the following best describes the function of phenol red in MSA?

Accepted Answer

It is a pH indicator that turns yellow in acidic conditions

Question 13

A hospital lab receives a wound swab and plates it on MSA. After incubation, the medium remains red. What is the most likely conclusion?

Accepted Answer

The sample contains a non-mannitol fermenting Staphylococcus species

Question 14

Which medium would you use to observe colony color differences among Staphylococcus strains without a color change in the medium?

Accepted Answer

SM110

Question 15

A student streaks a mixture on MacConkey agar and observes colorless colonies. What does this indicate about the bacteria?

Accepted Answer

They are non-lactose fermenting Gram-negative bacteria, such as Salmonella or Shigella

Question 16

Which of the following is a correct sequence for isolating and identifying beta-hemolytic Streptococcus from a throat swab?

Accepted Answer

Streak on blood agar, observe clear colonies, subculture in trypticase soy broth

Question 17

Which of the following media would be most useful for isolating Salmonella from a stool sample?

Accepted Answer

XLD agar

Question 18

A student is asked to identify the type of hemolysis produced by Streptococcus pneumoniae. Which observation would confirm alpha hemolysis?

Accepted Answer

Greenish discoloration around colonies on blood agar

Question 19

Which of the following best explains why MacConkey and Hektoen Enteric agars are used in clinical labs for stool cultures?

Accepted Answer

They inhibit Gram-positive bacteria and differentiate Gram-negative pathogens based on lactose fermentation

Question 20

Which of the following is a plausible reason for using both MSA and SM110 in the isolation of Staphylococcus species?

Accepted Answer

MSA differentiates mannitol fermenters by color change, while SM110 allows observation of colony color differences without color change in the medium

Isolation and Identification of Staphylococcus, Streptococcus, and Gram-Negative Pathogens

Study Guide - Practice Questions