CMLA Microbiology Basics 4 — Questions and Answers
Question 1: The coagulase test is used primarily to identify which pathogen?
- Staphylococcus epidermidis
- Staphylococcus aureus (Correct answer)
- Streptococcus pyogenes
- Enterococcus faecalis
Correct answer: Staphylococcus aureus
Staphylococcus aureus is coagulase-positive, clotting plasma by converting fibrinogen to fibrin, which distinguishes it from coagulase-negative staphylococci like S. epidermidis.
Question 2: Which culture medium is used for the primary isolation of Neisseria gonorrhoeae?
- MacConkey agar
- Thayer-Martin agar (Correct answer)
- Sabouraud dextrose agar
- Löwenstein-Jensen agar
Correct answer: Thayer-Martin agar
Thayer-Martin agar is a modified chocolate agar with antibiotics (VCN: vancomycin, colistin, nystatin) that selectively isolates Neisseria gonorrhoeae from genital specimens.
Question 3: A spore-forming, Gram-positive rod that causes food poisoning via a heat-stable enterotoxin is:
- Clostridium perfringens
- Bacillus cereus (Correct answer)
- Listeria monocytogenes
- Staphylococcus aureus
Correct answer: Bacillus cereus
Bacillus cereus produces a heat-stable emetic toxin associated with reheated rice and a heat-labile diarrheal toxin; it forms spores allowing survival of cooking.
Question 4: The KOH (potassium hydroxide) preparation is used in microbiology to:
- Visualize acid-fast organisms
- Clear debris and visualize fungal elements (Correct answer)
- Detect viral inclusion bodies
- Stain bacterial flagella
Correct answer: Clear debris and visualize fungal elements
KOH dissolves keratin and cellular debris in skin, nail, or hair samples, allowing fungal hyphae and spores to be seen clearly under microscopy.
Question 5: Which test confirms that an isolate ferments lactose on MacConkey agar by producing a pink/red colony?
- The organism is oxidase-positive
- The organism is a lactose fermenter producing acid (Correct answer)
- The organism has reduced nitrates
- The organism is urease-positive
Correct answer: The organism is a lactose fermenter producing acid
MacConkey agar contains lactose and neutral red dye; lactose-fermenting organisms produce acid that turns the pH indicator, making colonies appear pink to red.
Question 6: Which parasite is transmitted via the fecal-oral route and infects the small intestine, causing malabsorption and greasy stools?
- Plasmodium falciparum
- Giardia lamblia (Correct answer)
- Wuchereria bancrofti
- Trypanosoma cruzi
Correct answer: Giardia lamblia
Giardia lamblia cysts are ingested via contaminated water or food, excyst in the small intestine, and cause malabsorption with characteristic greasy, foul-smelling stools.
Question 7: A disk diffusion test shows no zone of inhibition around an antibiotic disk. This indicates the organism is:
- Susceptible
- Intermediate
- Resistant (Correct answer)
- Indeterminate
Correct answer: Resistant
No zone of inhibition means the antibiotic did not inhibit bacterial growth around the disk, indicating the organism is resistant to that antibiotic.
CMLA Microbiology Basics 4
The coagulase test is used primarily to identify which pathogen?