Q: Which of the following equations correctly represents the calculation for the percentage of hemolysis on a blood agar plate if the total area of the plate is $$A_{total}$$ and the area of clear hemolysis is $$A_{clear}$$?

$$\text{Percent Hemolysis} = \frac{A_{clear}}{A_{total}} \times 100$$

Question 1

Which of the following best explains why Mannitol Salt Agar (MSA) is both a selective and differential medium for Staphylococcus species?

Accepted Answer

MSA contains 7.5% sodium chloride, which inhibits most bacteria except Staphylococci, and mannitol, which differentiates species based on fermentation and pH change.

Question 2

A clinical sample is streaked on MSA. After incubation, the medium turns yellow around the colonies. Which organism is most likely present, and what is the biochemical basis for this result?

Accepted Answer

Staphylococcus aureus; it ferments mannitol, producing acid that lowers the pH and turns phenol red yellow.

Question 3

Which test is definitive for confirming the pathogenicity of Staphylococcus aureus, and what is the expected result?

Accepted Answer

Coagulase test; a positive result is indicated by clot formation in plasma.

Question 4

A patient’s blood culture grows Gram-positive cocci in clusters that are catalase positive and coagulase negative. Which organism is most likely present?

Accepted Answer

Staphylococcus epidermidis

Question 5

Which of the following best distinguishes Staphylococcus epidermidis from Staphylococcus saprophyticus in the laboratory?

Accepted Answer

Novobiocin sensitivity; S. epidermidis is sensitive, S. saprophyticus is resistant.

Question 6

A blood agar plate (BAP) shows colonies surrounded by a clear, colorless zone. Which type of hemolysis is this, and which organism is most likely responsible?

Accepted Answer

Beta hemolysis; Streptococcus pyogenes

Question 7

Which of the following statements about Streptococcus pneumoniae is correct?

Accepted Answer

It is alpha-hemolytic and inhibited by optochin (P disk).

Question 8

A throat swab is streaked on blood agar and incubated in a capnophilic environment. Colonies show complete hemolysis and are sensitive to bacitracin. What is the most likely identity of the organism?

Accepted Answer

Streptococcus pyogenes

Question 9

Which of the following is a major virulence factor for Staphylococcus epidermidis, especially in catheter-associated infections?

Accepted Answer

Biofilm formation

Question 10

A rapid strep test is positive for Group A antigen. What is the next recommended step to confirm the diagnosis?

Accepted Answer

Culture the sample on a blood agar plate to observe hemolysis and bacitracin sensitivity.

Question 11

Which of the following equations correctly represents the calculation for the percentage of hemolysis on a blood agar plate if the total area of the plate is $$A_{total}$$ and the area of clear hemolysis is $$A_{clear}$$?

Accepted Answer

$$	ext{Percent Hemolysis} = \frac{A_{clear}}{A_{total}} 	imes 100$$

Question 12

Which of the following best describes the difference between alpha, beta, and gamma hemolysis on blood agar?

Accepted Answer

Alpha: partial lysis (greenish), Beta: complete lysis (clear), Gamma: no lysis (intact red)

Question 13

A patient with a urinary tract infection is found to have Gram-positive cocci that are catalase positive, coagulase negative, and resistant to novobiocin. Which organism is most likely responsible?

Accepted Answer

Staphylococcus saprophyticus

Question 14

Which of the following is the correct sequence of steps for performing a coagulase test?

Accepted Answer

Inoculate plasma with the test organism, cover, incubate, observe for clot formation.

Question 15

Which of the following best explains why Streptococci are catalase negative while Staphylococci are catalase positive?

Accepted Answer

Streptococci lack the enzyme catalase, so they cannot break down hydrogen peroxide into water and oxygen.

Question 16

A blood agar plate is inoculated with a throat swab and incubated in a Brewer’s jar. Why is this environment used, and what type of bacteria does it favor?

Accepted Answer

It creates a capnophilic environment, favoring the growth of Streptococci.

Question 17

Which of the following is true regarding MRSA (Methicillin-resistant Staphylococcus aureus)?

Accepted Answer

MRSA is resistant to most $$\beta-$$lactam antibiotics due to altered penicillin-binding proteins.

Question 18

Which of the following best describes the Lancefield classification system for Streptococci?

Accepted Answer

It is based on antigenic differences in the cell wall carbohydrates.

Question 19

A laboratory technician observes greenish-brown discoloration around colonies on a blood agar plate. Which type of hemolysis is this, and what is the likely organism?

Accepted Answer

Alpha hemolysis; Streptococcus pneumoniae

Isolation and Identification of Staphylococcus and Streptococcus

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