Microbiology: Bacteriology: Gram-Positive Identification Flashcards

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

Answer: Coagulase

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

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

Answer: Clostridium tetani

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

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

Answer: Growth in 6.5% NaCl broth

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

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

Answer: Streptococcus agalactiae

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

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

Answer: Viridans streptococci

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

Which gram-positive bacillus causes disease by producing a toxin that ADP-ribosylates EF-2, halting protein synthesis?

Answer: Corynebacterium diphtheriae

C. diphtheriae exotoxin ADP-ribosylates elongation factor 2 (EF-2), blocking protein synthesis in host cells.

On Mannitol Salt Agar (MSA), a yellow halo around colonies indicates:

Answer: Mannitol fermentation by S. aureus

S. aureus ferments mannitol, producing acid that turns the phenol red indicator yellow on MSA.