COT Microbiology 6 ā Questions and Answers
Question 1: Trachoma, the leading infectious cause of preventable blindness worldwide, is caused by Chlamydia trachomatis. Which serotypes (serovars) are responsible for ocular trachoma?
- Serotypes A, B, and C (Correct answer)
- Serotypes D through K
- Serotypes L1, L2, and L3
- Serotype B only
Correct answer: Serotypes A, B, and C
Chlamydia trachomatis serotypes A, B, and C are responsible for endemic trachoma, causing repeated conjunctival inflammation that leads to scarring, trichiasis, and corneal opacification. Serotypes DāK cause inclusion conjunctivitis and genital infections, while serotypes L1āL3 cause lymphogranuloma venereum (LGV), a systemic sexually transmitted infection.
Question 2: A contact lens wearer presents with a rapidly progressive corneal ulcer and severe pain. Corneal scrapings reveal large, double-walled cysts on Giemsa stain. Which organism is the most likely causative agent?
- Pseudomonas aeruginosa
- Acanthamoeba species (Correct answer)
- Fusarium solani
- Herpes simplex virus
Correct answer: Acanthamoeba species
Acanthamoeba species are free-living protozoa that form characteristic double-walled cysts visible on Giemsa or calcofluor white staining. They cause a devastating keratitis almost exclusively in contact lens wearers, often associated with tap water or homemade saline exposure. The cyst wall distinguishes them from bacterial or fungal etiologies.
Question 3: Which of the following best describes the mechanism by which fluoroquinolone antibiotics (e.g., ciprofloxacin) kill bacteria?
- Inhibiting cell wall synthesis by binding penicillin-binding proteins
- Inhibiting DNA gyrase and topoisomerase IV, preventing DNA replication (Correct answer)
- Disrupting the bacterial cell membrane, causing lysis
- Binding the 30S ribosomal subunit and blocking protein synthesis
Correct answer: Inhibiting DNA gyrase and topoisomerase IV, preventing DNA replication
Fluoroquinolones inhibit bacterial DNA gyrase (topoisomerase II) and topoisomerase IV, enzymes essential for DNA supercoiling and replication. This halts DNA replication and transcription, resulting in bacterial cell death. This mechanism is distinct from beta-lactams (cell wall), aminoglycosides (30S ribosome), or polymyxins (membrane disruption).
Question 4: A patient develops acute follicular conjunctivitis with preauricular lymphadenopathy one week after swimming in a public pool. Which pathogen is the most likely cause?
- Neisseria gonorrhoeae
- Chlamydia trachomatis serotypes A-C
- Adenovirus (Correct answer)
- Staphylococcus aureus
Correct answer: Adenovirus
Adenovirus is the classic cause of epidemic keratoconjunctivitis (EKC) and pharyngoconjunctival fever, frequently transmitted in swimming pools ('swimming pool conjunctivitis'). It produces follicular conjunctivitis with a prominent preauricular lymph node ā a key clinical sign. Chlamydia causes chronic follicular conjunctivitis, while gonococcal disease causes hyperacute purulent discharge.
Question 5: In the context of ophthalmic instrument disinfection, which method achieves the highest level of microbial kill, including bacterial endospores?
- High-level disinfection with glutaraldehyde for 20 minutes
- Sterilization via autoclave (steam under pressure) (Correct answer)
- Intermediate-level disinfection with 70% isopropyl alcohol
- Low-level disinfection with quaternary ammonium compounds
Correct answer: Sterilization via autoclave (steam under pressure)
Autoclaving (steam sterilization at 121Ā°C, 15 psi for 15ā30 minutes) is the gold standard for sterilization ā it destroys all microbial life including highly resistant bacterial endospores (e.g., Bacillus and Clostridium). Glutaraldehyde achieves sterilization only after extended contact (10 hours), not 20 minutes. Alcohols and quaternary ammonium compounds do not reliably kill spores.
Question 6: A corneal scraping from a patient with suspected fungal keratitis is plated on Sabouraud dextrose agar. After 5 days, fluffy white colonies with a powdery surface appear. Microscopy shows septate hyphae with flask-shaped phialides. Which organism is most consistent with these findings?
- Candida albicans
- Aspergillus fumigatus (Correct answer)
- Mucor species
- Cryptococcus neoformans
Correct answer: Aspergillus fumigatus
Aspergillus fumigatus produces septate hyphae and characteristic flask-shaped conidiophores (phialides) bearing chains of conidia. It grows rapidly on Sabouraud agar with a powdery, initially white then gray-green surface. Candida produces yeast forms with pseudohyphae, Mucor has non-septate (aseptate/pauci-septate) hyphae, and Cryptococcus is an encapsulated yeast.
COT Microbiology 6
Trachoma, the leading infectious cause of preventable blindness worldwide, is caused by Chlamydia trachomatis.
Which serotypes (serovars) are responsible for ocular trachoma?