Question 1

Which of the following best explains why the number of bacterial colonies increased after handwashing in the laboratory experiment, even though the types of colonies decreased?

Accepted Answer

Handwashing removes only the transient bacteria, leaving behind the tightly adhered resident microbiota that can grow more easily on the agar.

Question 2

A patient presents with a sore throat. A throat swab is cultured on blood agar, and colonies surrounded by a clear halo are observed. Which organism is most likely responsible, and what type of hemolysis is this?

Accepted Answer

Streptococcus pyogenes; $$\beta-$$hemolysis

Question 3

Which component of Mannitol Salt Agar (MSA) is responsible for selecting Staphylococcus species?

Accepted Answer

Sodium chloride (NaCl) at 7.5% (w/v)

Question 4

A microbiologist streaks a blood agar plate and observes colonies with a greenish halo. What type of hemolysis is this, and which group of bacteria is most likely responsible?

Accepted Answer

$$\alpha-$$hemolysis; viridans streptococci

Question 5

Which test would you use to differentiate Staphylococcus aureus from other Staphylococcus species, and what is the expected result for S. aureus?

Accepted Answer

Coagulase test; positive for S. aureus

Question 6

A latex agglutination test for Staphylococcus aureus detects which two cell wall components?

Accepted Answer

Bound coagulase and protein A

Question 7

Why is the catalase test not performed directly on blood agar?

Accepted Answer

Blood cells contain catalase, which can cause a false positive result.

Question 8

Which of the following statements about the skin microbiota is correct?

Accepted Answer

Resident microbes prevent colonization by pathogens by occupying host cell receptors.

Question 9

A blood agar plate is streaked and stabbed with a sample. After incubation, a sharp clear zone is seen along the stab, but only a greenish halo on the surface. What does this indicate?

Accepted Answer

The organism produces streptolysin O, which is oxygen-labile and only active in the agar.

Question 10

Which of the following best describes the function of free coagulase produced by Staphylococcus aureus?

Accepted Answer

It activates prothrombin to thrombin, which then converts fibrinogen to fibrin, causing clot formation.

Question 11

A rapid antigen test for Group A Streptococcus is performed. The result shows two reddish-purple lines, one at the control and one at the test line. What does this indicate?

Accepted Answer

Group A streptococcal antigen is present in the specimen.

Question 12

Which of the following is a key difference between resident and transient skin microbiota?

Accepted Answer

Resident microbiota are tightly adhered to skin cells, while transient microbiota are more easily removed by washing.

Question 13

Which of the following best explains why Staphylococcus aureus can be distinguished from other staphylococci on Mannitol Salt Agar?

Accepted Answer

S. aureus ferments mannitol, producing acid and turning the phenol red indicator yellow.

Question 14

A patient with a sore throat has a throat swab cultured on blood agar. The colonies are surrounded by a green halo. Which test would help differentiate between S. pneumoniae and other alpha-hemolytic streptococci?

Accepted Answer

Optochin sensitivity test

Question 15

Which of the following is true regarding the transmission of skin and throat microbiota?

Accepted Answer

Skin microbes are often transmitted by direct contact, while throat microbes are commonly spread by coughing and sneezing.

Question 16

Which of the following best describes the role of the catalase test in differentiating between staphylococci and streptococci?

Accepted Answer

Staphylococci are catalase positive, while streptococci are catalase negative.

Question 17

A blood agar plate shows no change in the medium around the colonies. What type of hemolysis is this, and what does it indicate?

Accepted Answer

$$\gamma-$$hemolysis; no hemolysis

Question 18

Which of the following is a correct statement about opportunistic pathogens among the skin and throat microbiota?

Accepted Answer

They can cause disease when there is a break in the skin or mucous membranes.

Question 19

Which of the following best describes the mechanism by which bound coagulase (clumping factor) acts?

Accepted Answer

It directly activates fibrinogen to form fibrin, causing clumping of cells.

Skin and Throat Microbiota: Laboratory Study Guide

Study Guide - Practice Questions

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