Microbiology 9 Flashcards

A contact lens wearer presents with severe pain, ring-shaped corneal infiltrate, and a history of swimming in a lake. Corneal scrapings are obtained. Which staining technique provides the HIGHEST sensitivity for detecting the causative organism's cysts?

Answer: Calcofluor white fluorescent stain

Calcofluor white binds to cellulose and chitin in the cell wall of Acanthamoeba cysts, producing bright fluorescence under UV microscopy. This makes it the most sensitive rapid stain for detecting Acanthamoeba — the organism responsible for this clinical picture. Gram and Ziehl-Neelsen stains are used for bacteria and mycobacteria respectively, while PAS is less sensitive for Acanthamoeba than calcofluor white.

A patient develops a white, hypopyon, and decreased vision 8 weeks after uncomplicated cataract surgery. There was no early postoperative inflammation. Vitreous tap grows a slow, non-spore-forming, anaerobic gram-positive rod in 10 days. Which organism BEST fits this presentation?

Answer: Cutibacterium acnes (formerly Propionibacterium acnes)

Cutibacterium acnes (formerly P. acnes) is the classic cause of delayed-onset (weeks to months) post-cataract endophthalmitis. It is a slow-growing, anaerobic, gram-positive rod that can be sequestered within residual lens cortex inside the capsular bag, producing a smoldering, low-grade infection. Its slow growth explains the delayed presentation and the 10-day culture time. Clostridium is spore-forming; S. epidermidis and Streptococcus are not anaerobes of this profile.

Which statement CORRECTLY distinguishes the antifungal mechanism of natamycin from that of voriconazole in the treatment of fungal keratitis?

Answer: Natamycin binds directly to ergosterol, disrupting membrane integrity; voriconazole inhibits lanosterol 14α-demethylase, blocking ergosterol synthesis

Natamycin is a polyene antifungal that binds directly to ergosterol already present in the fungal cell membrane, forming pores that disrupt osmotic stability. Voriconazole is a triazole that inhibits the cytochrome P450 enzyme lanosterol 14α-demethylase (CYP51), blocking ergosterol biosynthesis. These are distinct mechanisms — polyenes target the end product; azoles block its production. β-glucan synthesis inhibition is the mechanism of echinocandins, not natamycin.

A biologic indicator using Geobacillus stearothermophilus spores shows GROWTH after a completed autoclave sterilization cycle. The run log shows the chamber reached 121°C. Which explanation is MOST likely responsible for the sterilization failure?

Answer: Residual air in the chamber prevented steam penetration, creating a cool pocket that never reached lethal temperature

G. stearothermophilus is the standard biologic indicator for gravity-displacement and pre-vacuum autoclaves because its spores are highly heat-resistant but reliably destroyed at 121°C with adequate steam contact. If growth occurs despite a logged 121°C reading, the most common cause is incomplete air evacuation — trapped air creates insulating pockets that prevent steam from contacting the load at the lethal combination of temperature, moisture, and time. The thermocouple measures chamber air temperature, not load temperature inside an air pocket.

Fluoroquinolone resistance has been increasing in ocular Staphylococcus aureus isolates. Which molecular mechanism is the PRIMARY driver of clinically significant resistance to fourth-generation fluoroquinolones (e.g., moxifloxacin) in this organism?

Answer: Sequential mutations in gyrA (DNA gyrase subunit A) and parC (topoisomerase IV subunit C)

Fluoroquinolones target two bacterial type II topoisomerases: DNA gyrase (GyrA/GyrB) and topoisomerase IV (ParC/ParE). In S. aureus, topoisomerase IV is the primary target. Clinically significant high-level resistance requires sequential chromosomal mutations in both parC and gyrA, creating a double mutant with markedly reduced fluoroquinolone binding. A single mutation typically confers only low-level resistance. MexAB-OprM is specific to Pseudomonas aeruginosa; PBP alterations are relevant to β-lactam resistance; aminoglycoside-modifying enzymes do not act on fluoroquinolones.

Corneal scrapings from a patient with a chronic, non-healing ulcer are plated and grow filamentous colonies with chalky-white aerial hyphae that partially dip into the agar after 5–7 days at 35°C. Modified acid-fast staining is weakly positive. Which organism and its MOST appropriate primary treatment are correctly paired?

Answer: Nocardia asteroides — treated with topical trimethoprim-sulfamethoxazole or amikacin

Nocardia asteroides is an aerobic, weakly gram-positive, partially acid-fast actinomycete. Key morphology: thin, branching filaments with aerial hyphae that embed into agar (unlike true fungi). Weak acid-fast positivity (modified Kinyoun stain) is a hallmark. It causes chronic, indolent keratitis, particularly after trauma. Treatment is topical trimethoprim-sulfamethoxazole or amikacin (sometimes combined). Fusarium keratitis is acid-fast negative and not filamentous in the same manner; Actinomyces is anaerobic and not acid-fast; Streptomyces would not be weakly acid-fast positive and is rarely a clinical corneal pathogen.