COT Microbiology 3

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?

Accepted Answer

Binary fission; no cyst form — trophozoite is the infective stage

Answer

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?

Accepted Answer

High lipid content of mycolic acids in the cell wall

Answer

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?

Accepted Answer

Microaerophile

Answer

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?

Accepted Answer

Enterotoxin released during sporulation that alters ion transport

Answer

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?

Accepted Answer

Cleaving SNARE proteins required for acetylcholine vesicle fusion

Answer

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.

COT - Certified Ophthalmic Technician Practice Test

COT - Certified Ophthalmic Technician Practice Test

COT Microbiology 3