AP-Micro Microbiology: Microbial Ecology and Pathogenesis 3 — Questions and Answers
Question 1: Which virulence factor allows Streptococcus pyogenes to resist phagocytosis by white blood cells?
- Endotoxin (LPS)
- M protein (Correct answer)
- Flagella
- Pili
Correct answer: M protein
The M protein on the surface of S. pyogenes inhibits complement activation and opsonization, thereby resisting phagocytosis.
Question 2: In the sulfur cycle, which process converts sulfate (SO₄²⁻) back to hydrogen sulfide (H₂S) under anaerobic conditions?
- Sulfur oxidation
- Sulfate reduction (Correct answer)
- Nitrification
- Denitrification
Correct answer: Sulfate reduction
Sulfate-reducing bacteria such as Desulfovibrio use sulfate as a terminal electron acceptor under anaerobic conditions, producing H₂S.
Question 3: Quorum sensing in bacteria refers to:
- The random mutation rate in a population
- Cell-density-dependent regulation of gene expression via chemical signals (Correct answer)
- Phage-mediated gene transfer between bacteria
- The process of forming endospores under stress
Correct answer: Cell-density-dependent regulation of gene expression via chemical signals
Quorum sensing allows bacteria to coordinate behavior — such as biofilm formation or toxin production — by detecting population density via autoinducer molecules.
Question 4: Which of the following best defines 'pathogenicity'?
- The dose of a pathogen required to kill 50% of hosts
- The ability of a microorganism to cause disease in a host (Correct answer)
- The range of hosts a pathogen can infect
- The rate at which a pathogen spreads through a population
Correct answer: The ability of a microorganism to cause disease in a host
Pathogenicity is the inherent ability of a microorganism to cause disease, distinguishing pathogens from non-pathogens.
Question 5: Horizontal gene transfer via direct cell-to-cell contact through a sex pilus is called:
- Transformation
- Transduction
- Conjugation (Correct answer)
- Transposition
Correct answer: Conjugation
Conjugation requires physical contact between donor and recipient cells via a pilus, through which plasmid DNA is transferred.
Question 6: A patient has a wound infected with a bacterium that secretes a toxin destroying red blood cells. This toxin is best described as a:
- Neurotoxin
- Enterotoxin
- Hemolysin (Correct answer)
- Cytotoxin
Correct answer: Hemolysin
Hemolysins are exotoxins that lyse red blood cells, and many are detectable by hemolysis zones on blood agar plates.
Question 7: Extreme halophiles, thermophiles, and methanogens are all examples of:
- Bacteria adapted to mild environments
- Archaea found in extreme environments (Correct answer)
- Eukaryotic microorganisms
- Obligate intracellular parasites
Correct answer: Archaea found in extreme environments
These are all archaea — a domain of prokaryotes distinct from bacteria — that thrive in extreme environmental conditions.
AP-Micro Microbiology: Microbial Ecology and Pathogenesis 3
Which virulence factor allows Streptococcus pyogenes to resist phagocytosis by white blood cells?