Question 1

Which component of Mannitol Salt Agar (MSA) makes it selective for Staphylococci, and why?

Accepted Answer

7.5% sodium chloride, because Staphylococci are halophiles and can tolerate high salt concentrations.

Question 2

A patient’s wound culture on MSA turns the medium yellow. Which organism is most likely present, and what does this indicate?

Accepted Answer

Staphylococcus aureus; it ferments mannitol producing acid.

Question 3

Which test is definitive for confirming the pathogenicity of Staphylococcus aureus?

Accepted Answer

Coagulase test

Question 4

A blood agar plate (BAP) shows a clear zone around colonies. What type of hemolysis is this, and which Streptococcus species is most likely responsible?

Accepted Answer

Beta hemolysis; Streptococcus pyogenes

Question 5

Which of the following best describes the result and interpretation of a positive coagulase test?

Accepted Answer

Clot forms; the organism is Staphylococcus aureus.

Question 6

A throat swab is streaked on a BAP and incubated in a capnophilic environment. Why is this incubation condition used?

Accepted Answer

Streptococci are facultative anaerobes and grow better with increased CO$$_2.$$

Question 7

Which test would you use to distinguish Streptococcus pneumoniae from other alpha-hemolytic streptococci?

Accepted Answer

Optochin sensitivity test

Question 8

A Gram-positive coccus is catalase negative and produces complete hemolysis on BAP. Which organism is most likely?

Accepted Answer

Streptococcus pyogenes

Question 9

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

Accepted Answer

It is resistant to most β-lactam antibiotics.

Question 10

A urine sample from a young woman grows Gram-positive cocci that are catalase positive, coagulase negative, and resistant to novobiocin. Which organism is most likely?

Accepted Answer

Staphylococcus saprophyticus

Question 11

Which Lancefield group is associated with Streptococcus pyogenes, and what is the significance of this grouping?

Accepted Answer

Group A; it is the most virulent group.

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

Question 13

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

Accepted Answer

Culture the sample on a blood agar plate.

Question 14

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

Accepted Answer

Biofilm formation

Question 15

A catalase test is performed on a Gram-positive coccus. Bubbling is observed. What does this indicate?

Accepted Answer

The organism is likely a Staphylococcus species.

Question 16

Which of the following best explains why S. aureus can be easily identified on MSA?

Accepted Answer

It ferments mannitol, turning the medium yellow due to acid production.

Question 17

A blood agar plate is streaked with a throat swab and shows greenish-brown discoloration around colonies. Which organism and hemolysis type is most likely?

Accepted Answer

Streptococcus pneumoniae; alpha hemolysis

Question 18

Which test would you use to distinguish Staphylococcus epidermidis from Staphylococcus saprophyticus?

Accepted Answer

Novobiocin sensitivity test

Question 19

A patient with a prosthetic heart valve develops endocarditis. Which organism is most likely, and what is its key virulence factor?

Accepted Answer

Staphylococcus epidermidis; biofilm formation

Isolation and Identification of Staphylococcus and Streptococcus

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