Microbiology 7 Flashcards

A contact lens wearer presents with severe pain disproportionate to clinical findings, a ring-shaped stromal infiltrate, and a history of swimming in a lake while wearing soft lenses. PCR confirms Acanthamoeba keratitis. Which property of Acanthamoeba cysts makes this infection particularly resistant to standard disinfection protocols?

Answer: The cyst wall is composed of cellulose and protein, rendering it impermeable to most quaternary ammonium compounds and hydrogen peroxide at standard concentrations

Acanthamoeba cysts have a double-layered wall composed of cellulose (outer ectocyst) and protein (inner endocyst), which makes them highly resistant to quaternary ammonium compounds (found in older multipurpose solutions) and even to hydrogen peroxide systems at standard 3% concentration with short contact times. This structural impermeability — not efflux pumps or metabolic dormancy — is the key resistance mechanism, which is why prolonged biguanide (PHMB) or polyhexamethylene biguanide therapy is required for treatment.

During a post-cataract surgery endophthalmitis workup, vitreous tap cultures grow a gram-positive, catalase-positive, coagulase-negative organism. The isolate shows resistance to oxacillin but susceptibility to vancomycin. Which organism and resistance mechanism best describes this finding?

Answer: Staphylococcus epidermidis with mecA-mediated resistance causing altered penicillin-binding protein 2a (PBP2a)

The profile — gram-positive, catalase-positive, coagulase-negative, oxacillin-resistant, vancomycin-susceptible — is classic for methicillin-resistant Staphylococcus epidermidis (MRSE), the most common cause of delayed-onset post-cataract endophthalmitis. The mecA gene encodes PBP2a, a modified penicillin-binding protein with low affinity for beta-lactam antibiotics, conferring resistance to all penicillins and cephalosporins. Streptococcus is catalase-negative; Enterococcus with vanA would be vancomycin-resistant; Propionibacterium acnes is gram-positive but anaerobic and non-catalase-positive.

A patient presents with hyperacute purulent conjunctivitis within 12 hours of sexual exposure. Gram stain of conjunctival scrapings shows intracellular gram-negative diplococci within polymorphonuclear cells. Which characteristic most distinguishes Neisseria gonorrhoeae from other bacterial conjunctival pathogens and drives the urgency of treatment?

Answer: Its ability to penetrate an intact, non-ulcerated corneal epithelium via IgA1 protease secretion and opacity protein-mediated invasion

Neisseria gonorrhoeae is one of the very few bacterial pathogens capable of penetrating an intact corneal epithelium — no prior abrasion or surface defect is needed. This invasion is mediated in part by IgA1 protease (which cleaves protective secretory IgA), outer membrane opacity (Opa) proteins that bind host cell receptors, and pili enabling adherence. This property makes hyperacute gonococcal conjunctivitis a true ocular emergency: corneal perforation can occur within 24–48 hours without systemic antibiotics (typically ceftriaxone IM plus copious saline irrigation).

An ophthalmic technician is processing a flexible fiber-optic indirect ophthalmoscope light pipe that cannot tolerate heat sterilization. The manufacturer specifies high-level disinfection. Which agent achieves high-level disinfection appropriate for this semi-critical instrument, and what is its primary mechanism of action?

Answer: 2% glutaraldehyde (Cidex) for a minimum 20-minute contact time; it alkylates nucleic acids and proteins by crosslinking free amino and hydroxyl groups

Glutaraldehyde 2% is an established high-level disinfectant for heat-sensitive semi-critical instruments. Its mechanism is alkylation — it crosslinks free amino (-NH2) and hydroxyl (-OH) groups in microbial proteins and nucleic acids, disrupting structural and enzymatic function. At 20 minutes it achieves high-level disinfection (kills vegetative bacteria, mycobacteria, most viruses, fungi); at 10 hours it achieves sterilization. Isopropyl alcohol is only an intermediate-level disinfectant and does not reliably kill non-enveloped viruses or mycobacteria. Sodium hypochlorite is corrosive to delicate instruments. Quaternary ammonium compounds are low-level disinfectants only.

Corneal scrapings from a patient with post-traumatic keratitis (soil contamination) grow a septate mold with dichotomous 45-degree angle branching on lactophenol cotton blue stain. The organism produces conidial heads described as 'aspergillus heads.' Sensitivity testing shows resistance to fluconazole. Which agent is MOST appropriate for this infection, and why does fluconazole fail?

Answer: Natamycin 5% topical; fluconazole fails because Aspergillus lacks the ERG11-encoded lanosterol 14-alpha-demethylase target that azoles require, making fluconazole intrinsically ineffective against most filamentous fungi

Aspergillus species have intrinsic resistance to fluconazole. Fluconazole inhibits lanosterol 14-alpha-demethylase (encoded by ERG11 in Candida), the enzyme responsible for ergosterol synthesis. However, Aspergillus has a different cytochrome P450 enzyme (CYP51A) with structural differences that result in very poor fluconazole binding affinity — this is intrinsic, not acquired resistance. Natamycin 5% (pimaricin) is the gold-standard topical antifungal for filamentous fungal keratitis because it binds ergosterol directly (bypassing the enzymatic target fluconazole blocks). FKS1 mutations affect echinocandins, not azoles; melanin-sequestration is not the primary fluconazole resistance mechanism.

A neonate presents on day 3 of life with bilateral chemosis, copious mucopurulent discharge, and marked eyelid edema. Gram stain shows gram-negative intracellular diplococci. The mother received standard prenatal care but reported a penicillin allergy. The neonatologist notes the infant did not receive standard ocular prophylaxis at birth due to a protocol error. Which statement about the prophylaxis that was omitted is most accurate?

Answer: Erythromycin 0.5% ophthalmic ointment is the current CDC-recommended prophylaxis and reduces gonococcal ophthalmia neonatorum risk, though it does not reliably prevent chlamydial conjunctivitis

Erythromycin 0.5% ophthalmic ointment is the only neonatal ocular prophylaxis currently approved by the FDA and recommended in the US (CDC/AAP guidelines). It is effective against Neisseria gonorrhoeae but has limited efficacy against Chlamydia trachomatis — neonates born to mothers with chlamydial infection still require systemic oral erythromycin treatment even after prophylaxis. Silver nitrate 1% was historically used but was discontinued in the US due to chemical conjunctivitis and lack of chlamydial coverage. Povidone-iodine 2.5% is used in some countries but is not FDA-approved for this indication in the US. Tetracycline ointment is a WHO alternative but is not considered interchangeable in current US guidelines for chlamydial coverage.