Free Medical Laboratory Technician Clinical Microbiology Questions and Answers — Questions and Answers
Question 1: A medical laboratory technician performs a Gram stain on a positive blood culture bottle. Microscopic examination reveals spherical bacteria arranged in grape-like clusters that stain purple. Which of the following organisms is most suggested by this morphology?
- Streptococcus pyogenes
- Neisseria meningitidis
- Staphylococcus aureus (Correct answer)
- Escherichia coli
Correct answer: Staphylococcus aureus
The key descriptors are "Gram-positive (purple) cocci (spherical) in grape-like clusters." This morphology is characteristic of Staphylococcus species. Streptococcus species are also Gram-positive cocci but typically form chains. Neisseria are Gram-negative diplococci, and E. coli is a Gram-negative rod.
Question 2: An MLT inoculates a urine specimen onto a culture medium that is designed to inhibit the growth of Gram-positive organisms while differentiating between bacteria that can and cannot ferment lactose. Lactose fermenters produce pink-to-red colonies on this medium. What is this type of agar called?
- MacConkey agar (Correct answer)
- Blood agar
- Chocolate agar
- Mannitol salt agar
Correct answer: MacConkey agar
MacConkey agar is both selective and differential. It contains bile salts and crystal violet to inhibit most Gram-positive bacteria, making it selective. It also contains lactose and the pH indicator neutral red, which differentiates lactose-fermenting bacteria (which turn pink/red) from non-lactose fermenters.
Question 3: A Gram-positive coccus isolated from a skin abscess is tested in the lab. The organism produces bubbles when a drop of hydrogen peroxide is added (catalase-positive) and causes rabbit plasma to clot (coagulase-positive). What is the most likely identification of this organism?
- Streptococcus pyogenes
- Staphylococcus epidermidis
- Enterococcus faecalis
- Staphylococcus aureus (Correct answer)
Correct answer: Staphylococcus aureus
The catalase test is used to differentiate Staphylococcus spp. (positive) from Streptococcus and Enterococcus spp. (negative). The coagulase test is then used to differentiate Staphylococcus aureus (positive) from other coagulase-negative staphylococci, like Staphylococcus epidermidis. Therefore, a catalase-positive, coagulase-positive coccus is presumptively identified as S. aureus.
Question 4: Which of the following best describes the principle of the Kirby-Bauer disk diffusion susceptibility test?
- The minimum concentration of an antibiotic that kills a bacterium is determined by serial dilutions in broth.
- Antibiotic-impregnated disks are placed on an inoculated agar plate, and the diameter of the zone of growth inhibition is measured after incubation. (Correct answer)
- The presence of specific resistance genes in the bacterial DNA is detected using molecular amplification techniques.
- A colorimetric change indicates the metabolic activity of bacteria in the presence of an antibiotic.
Correct answer: Antibiotic-impregnated disks are placed on an inoculated agar plate, and the diameter of the zone of growth inhibition is measured after incubation.
The Kirby-Bauer method involves inoculating a standardized suspension of bacteria onto a Mueller-Hinton agar plate. Paper disks containing known concentrations of antibiotics are applied, and the plate is incubated. The antibiotic diffuses into the agar, and if the organism is susceptible, a zone of inhibition (no growth) forms around the disk. The diameter of this zone is measured to determine if the organism is susceptible, intermediate, or resistant.
Question 5: While examining a trichrome-stained stool smear for ova and parasites, a technologist identifies a cyst that is approximately 14 µm in diameter. It is spherical and contains three nuclei, each with a small, central karyosome. A chromatoidal bar with bluntly rounded ends is also observed. This morphology is most characteristic of which organism?
- Giardia lamblia
- Entamoeba coli
- Entamoeba histolytica/dispar (Correct answer)
- Cryptosporidium parvum
Correct answer: Entamoeba histolytica/dispar
The key features for identifying Entamoeba histolytica/dispar cysts are their size (typically 10-20 µm), the presence of up to four nuclei with small, central karyosomes, and chromatoidal bars with blunt, rounded ends. Entamoeba coli cysts are typically larger, have up to eight nuclei with eccentric karyosomes, and chromatoidal bars with splintered ends.
Question 6: A patient presents with vulvovaginitis, and a culture of the vaginal discharge grows creamy, white colonies on Sabouraud dextrose agar. A wet mount prepared from a colony reveals oval-shaped budding cells and elongated structures that look like links of sausage, referred to as pseudohyphae. What is the most likely pathogen?
- Candida albicans (Correct answer)
- Aspergillus fumigatus
- Cryptococcus neoformans
- Trichomonas vaginalis
Correct answer: Candida albicans
Candida albicans is a polymorphic fungus that is a common cause of vaginitis. It is characterized by its ability to grow as budding yeast cells and to form pseudohyphae and true hyphae. The observation of both budding yeast and pseudohyphae is highly suggestive of Candida albicans. Aspergillus is a mold, Cryptococcus is an encapsulated yeast that does not typically form pseudohyphae, and Trichomonas is a protozoan parasite.
