COT Microbiology 3 — Questions and Answers
Question 1: A 25-year-old woman presents with a vaginal discharge and microscopy reveals motile, pear-shaped organisms with multiple flagella and an undulating membrane. Which of the following best describes the replication strategy of this pathogen?
- Binary fission only — no sexual stage exists
- Schizogony producing merozoites within host red blood cells
- Binary fission; cyst form used for transmission
- Binary fission; no cyst form — trophozoite is the infective stage (Correct answer)
Correct answer: Binary fission; no cyst form — trophozoite is the infective stage
The description — motile, pear-shaped, flagellated with an undulating membrane — is classic for Trichomonas vaginalis. It reproduces by binary fission and, unlike Giardia or Entamoeba, does NOT form cysts. The trophozoite itself is the infective and transmissible form, passed through sexual contact. Schizogony is used by Plasmodium, not Trichomonas.
Question 2: A clinical laboratory receives a sputum sample from a patient with suspected tuberculosis. The acid-fast stain shows red, beaded rods against a blue background. Which characteristic of Mycobacterium tuberculosis accounts for its acid-fast staining property?
- High lipid content of mycolic acids in the cell wall (Correct answer)
- Presence of a polysaccharide capsule surrounding the bacterium
- Production of endospores resistant to decolorization
- Gram-positive peptidoglycan layer retaining carbol fuchsin
Correct answer: High lipid content of mycolic acids in the cell wall
Mycobacterium tuberculosis is acid-fast because its cell wall contains large amounts of mycolic acids — long-chain fatty acids that bind carbol fuchsin dye so strongly that even acid-alcohol decolorization cannot remove it. This waxy, lipid-rich wall is unique to mycobacteria and related genera (Nocardia, Corynebacterium). Capsules, endospores, and Gram-positive walls do not explain acid-fastness.
Question 3: A microbiologist is studying a newly isolated bacterium and finds it grows only in the presence of both pyruvate and low concentrations of oxygen. Under which atmospheric category is this organism best classified?
- Obligate aerobe
- Obligate anaerobe
- Microaerophile (Correct answer)
- Facultative anaerobe
Correct answer: Microaerophile
Microaerophiles require oxygen for growth but are damaged by normal atmospheric O₂ levels (21%). They thrive at reduced oxygen concentrations (2–10%). Campylobacter jejuni is a classic example. Obligate aerobes need full oxygen; obligate anaerobes cannot tolerate oxygen; facultative anaerobes grow with or without oxygen.
Question 4: During a foodborne illness investigation, Clostridium perfringens is identified as the causative agent. Which virulence mechanism is primarily responsible for the watery diarrhea produced by this organism?
- Production of a cytotoxic exotoxin that destroys enterocytes
- Enterotoxin released during sporulation that alters ion transport (Correct answer)
- Invasion of intestinal epithelium triggering inflammatory diarrhea
- Lipopolysaccharide endotoxin stimulating fluid secretion
Correct answer: Enterotoxin released during sporulation that alters ion transport
C. perfringens type A food poisoning is caused by an enterotoxin produced when vegetative cells sporulate in the intestine after ingestion. This enterotoxin acts as a pore-forming protein that disrupts tight junctions and alters ion/fluid transport, causing watery diarrhea without significant inflammation. It is not an endotoxin (C. perfringens is Gram-positive) and invasion is not the primary mechanism for this food-poisoning form.
Question 5: A batch of canned vegetables is implicated in an outbreak of botulism. Testing confirms Clostridium botulinum toxin type A. At which step of neuromuscular junction physiology does botulinum toxin exert its effect?
- Blocking nicotinic acetylcholine receptors on the motor end plate
- Cleaving SNARE proteins required for acetylcholine vesicle fusion (Correct answer)
- Inhibiting acetylcholinesterase, causing prolonged muscle contraction
- Depolarizing the motor neuron membrane, preventing action potential propagation
Correct answer: Cleaving SNARE proteins required for acetylcholine vesicle fusion
Botulinum toxin is a zinc-dependent metalloprotease that cleaves specific SNARE proteins (SNAP-25, synaptobrevin, or syntaxin depending on toxin serotype). SNARE proteins are essential for docking and fusing acetylcholine-containing vesicles with the presynaptic membrane. Cleavage prevents ACh release, causing flaccid paralysis. It does not block receptors (that's curare), inhibit acetylcholinesterase (that's nerve agents/organophosphates), or depolarize the membrane.
Question 6: A researcher is performing a transformation experiment using heat-killed smooth (S) Streptococcus pneumoniae and live rough (R) cells. After injection into mice, some mice die and smooth colonies are recovered. Which molecular component is transferred from S to R cells to restore virulence?
- RNA carrying instructions for capsule synthesis
- Proteins from the S-strain outer membrane
- DNA encoding capsule biosynthesis genes (Correct answer)
- Lipopolysaccharide from the S-strain cell wall
Correct answer: DNA encoding capsule biosynthesis genes
This describes Griffith's classic 1928 transformation experiment. The 'transforming principle' — later confirmed by Avery, MacLeod, and McCarty in 1944 to be DNA — consists of genes encoding polysaccharide capsule synthesis. When R cells take up this exogenous DNA and incorporate it, they express the capsule and regain virulence. RNA, outer-membrane proteins, and LPS were ruled out as the transforming principle in the Avery experiments.
COT Microbiology 3
A 25-year-old woman presents with a vaginal discharge and microscopy reveals motile, pear-shaped organisms with multiple flagella and an undulating membrane.
Which of the following best describes the replication strategy of this pathogen?