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 transient bacteria, but the resident microbiota are tightly adhered and remain, resulting in more colonies of fewer types.

Question 2

A microbiologist streaks a throat swab on blood agar and observes colonies surrounded by a clear halo. Which type of hemolysis is this, and which organism is most likely responsible?

Accepted Answer

Beta hemolysis; Streptococcus pyogenes

Question 3

Which selective and differential medium is used to distinguish Staphylococcus aureus from other staphylococci, and what is the basis for differentiation?

Accepted Answer

Mannitol Salt Agar; based on mannitol fermentation causing a pH shift detected by phenol red

Question 4

A patient’s throat swab is tested with a rapid Group A Streptococcus antigen test. 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 Streptococcus antigen is present in the specimen.

Question 5

Which enzyme is produced by Staphylococcus aureus but not by most other staphylococci, and what is its clinical significance?

Accepted Answer

Coagulase; it enables S. aureus to clot plasma, aiding in immune evasion.

Question 6

Which of the following best describes the role of resident skin microbiota in preventing infection?

Accepted Answer

They occupy host cell receptors, preventing colonization by pathogenic microbes.

Question 7

A blood agar plate shows colonies with a greenish halo. What type of hemolysis is this, and what does it indicate about the organism?

Accepted Answer

Alpha hemolysis; partial lysis of red blood cells

Question 8

Which test would you use to differentiate between Staphylococcus and Streptococcus species, and what result would you expect for each?

Accepted Answer

Catalase test; Staphylococcus (+), Streptococcus (-)

Question 9

Which of the following statements about Streptolysin O is correct?

Accepted Answer

It is oxygen-labile and detected by stabbing the blood agar.

Question 10

A laboratory technician performs a slide coagulase test and observes agglutination within 2 minutes. What does this indicate?

Accepted Answer

The organism produces bound coagulase and is likely Staphylococcus aureus.

Question 11

Which of the following best describes the function of the selective agent in Mannitol Salt Agar?

Accepted Answer

It inhibits the growth of most bacteria except Staphylococcus spp. due to high NaCl concentration.

Question 12

A patient presents with a sore throat. A throat swab is cultured and shows β-hemolytic colonies that are bacitracin sensitive. Which organism is most likely present?

Accepted Answer

Streptococcus pyogenes (Group A)

Question 13

Which of the following is a plausible explanation for why handwashing is especially important in healthcare and food service industries?

Accepted Answer

It reduces the transmission of potentially harmful microbes by removing transient bacteria.

Question 14

Which of the following equations correctly represents the conversion of fibrinogen to fibrin during the coagulase test?

Accepted Answer

$$	ext{fibrinogen} \xrightarrow{	ext{thrombin}} 	ext{fibrin (clot)}$$

Question 15

Which of the following best describes the difference between free and bound coagulase?

Accepted Answer

Free coagulase activates prothrombin to thrombin; bound coagulase activates fibrinogen directly.

Question 16

Which of the following is a common method of microbe transmission for both skin and throat flora?

Accepted Answer

Direct contact with animate or inanimate surfaces (fomites)

Question 17

A blood agar plate is streaked and stabbed with a throat swab. After incubation, a sharp zone of hemolysis is observed along the stab line but not on the surface. What does this indicate?

Accepted Answer

The organism produces Streptolysin O, which is oxygen labile and detected in low oxygen conditions.

Question 18

Which of the following best describes the role of the phenol red indicator in Mannitol Salt Agar?

Accepted Answer

It changes color from red to yellow if mannitol is fermented, indicating acid production.

Question 19

Which of the following is a correct interpretation of the catalase test?

Accepted Answer

Bubbling upon addition of hydrogen peroxide indicates a positive result, typical of Staphylococcus spp.

Skin and Throat Flora: Microbiota, Pathogenicity, and Laboratory Identification

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