AP-Micro Microbiology: Bacteriology: Gram-Positive Identification 2

Question 1

Which enzyme produced by Staphylococcus aureus converts fibrinogen to fibrin, forming a clot around the bacteria?

Accepted Answer

Coagulase

Answer

Coagulase is a key virulence factor of S. aureus that clots plasma, distinguishing it from coagulase-negative staphylococci.

Question 2

A gram-positive rod that forms a central spore and appears 'drumstick'-shaped on smear is most consistent with:

Accepted Answer

Clostridium tetani

Answer

C. tetani produces a terminal (subterminal) spore that swells the cell end, creating the classic drumstick morphology.

Question 3

Which characteristic distinguishes Enterococcus faecalis from Streptococcus pyogenes on laboratory media?

Accepted Answer

Growth in 6.5% NaCl broth

Answer

Enterococci are salt-tolerant and grow in 6.5% NaCl, while Streptococcus pyogenes does not.

Question 4

The CAMP test is used to presumptively identify which gram-positive organism?

Accepted Answer

Streptococcus agalactiae

Answer

S. agalactiae (Group B Strep) produces CAMP factor that enhances the hemolysis of S. aureus, creating an arrowhead pattern.

Question 5

A gram-positive coccus that is alpha-hemolytic, optochin-resistant, and bile-insoluble is most likely:

Accepted Answer

Viridans streptococci

Answer

Viridans streptococci are alpha-hemolytic like S. pneumoniae but are resistant to optochin and not lysed by bile salts.

AP Practice Test