Microbiology 3 Flashcards

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?

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.

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?

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.

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?

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.

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?

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.

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?

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.

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?

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.