AP-Micro Microbiology: Bacteriology: Gram-Positive Identification 3

Question 1

Which virulence factor of Streptococcus pyogenes mimics host proteins to evade immune recognition?

Accepted Answer

M protein

Answer

M protein on the surface of S. pyogenes has regions that resemble human cardiac proteins, contributing to molecular mimicry and rheumatic fever.

Question 2

Bacillus anthracis is differentiated from other Bacillus species by its:

Accepted Answer

Non-hemolytic, non-motile, capsule-forming colonies

Answer

B. anthracis is uniquely non-hemolytic, non-motile, and produces a poly-D-glutamate capsule that inhibits phagocytosis.

Question 3

A newborn presents with meningitis; CSF Gram stain shows gram-positive rods with tumbling motility. The most likely pathogen is:

Accepted Answer

Listeria monocytogenes

Answer

L. monocytogenes is a key cause of neonatal meningitis and is characterized by end-over-end tumbling motility at room temperature.

Question 4

The PYR test (pyrrolidonyl arylamidase) is positive for which two organisms?

Accepted Answer

Streptococcus pyogenes and Enterococcus spp.

Answer

Both S. pyogenes (Group A Strep) and Enterococcus spp. are PYR-positive, helping distinguish them from other streptococci.

Question 5

Clostridium difficile produces two main toxins that cause pseudomembranous colitis. What do Toxin A and Toxin B primarily do?

Accepted Answer

Glucosylate Rho GTPases, disrupting the cytoskeleton

Answer

Both C. difficile toxins A and B glucosylate Rho family GTPases, destroying the actin cytoskeleton and causing cell death.

AP Practice Test