Microbiology 10 Flashcards

A clinical microbiology lab receives a CSF sample from a patient with meningitis. Gram stain reveals gram-negative diplococci. The organism oxidase-positive, grows on chocolate agar but NOT on blood agar, and ferments both glucose and maltose but NOT lactose or sucrose. Which of the following virulence factors is MOST responsible for this organism's ability to evade complement-mediated killing in non-immune individuals?

Answer: Outer membrane protein factor H binding protein that recruits host complement regulator

Neisseria meningitidis evades complement-mediated serum killing primarily through factor H binding proteins (fHbp), which recruit the host complement regulatory protein factor H to the bacterial surface. Factor H accelerates decay of the alternative pathway C3 convertase, preventing complement deposition. While the capsule inhibits phagocytosis, it is less directly responsible for complement evasion in serum. LOS sialylation also contributes but is less potent than fHbp-mediated factor H recruitment. IgA1 protease operates at mucosal surfaces, not in the bloodstream.

A COT student is reviewing a culture of a strict anaerobe isolated from a deep wound infection. The organism produces black colonies on Bacteroides bile esculin (BBE) agar, is resistant to kanamycin and vancomycin, but sensitive to colistin. It also produces a unique enzyme that degrades heparin. Which species BEST fits this profile?

Answer: Porphyromonas gingivalis

Porphyromonas gingivalis produces black-pigmented colonies on BBE agar due to heme accumulation, is resistant to kanamycin and vancomycin (key Bacteroidetes traits) but sensitive to colistin, distinguishing it from Bacteroides fragilis (colistin resistant). Crucially, P. gingivalis produces a heparanase that degrades heparin — a virulence factor linked to periodontal tissue invasion. Prevotella melaninogenica also produces pigment but is colistin sensitive AND kanamycin sensitive. Fusobacterium does not produce black pigment on BBE.

During sterilization validation of an autoclave, a Biological Indicator (BI) containing Geobacillus stearothermophilus spores shows growth (positive) after a full 134°C/18-minute cycle. The chemical indicator strip inside the same pack turned the appropriate color. Which is the MOST likely explanation?

Answer: The steam did not reach adequate temperature or contact time at the BI location, indicating a sterilization failure despite the chemical indicator color change

Chemical (CI) and biological indicators (BI) measure different parameters: CIs detect physical/chemical conditions (temperature, steam presence) while BIs measure actual microbicidal efficacy on a standardized spore population. A positive BI with a passed CI indicates a sterilization failure — steam likely did not adequately penetrate to the BI location (e.g., due to air entrapment, overloading, or improper pack placement), even though surface conditions appeared adequate to the CI. This is a critical distinction in sterilization monitoring. Spores do not spontaneously revert after sterilization; the BI standard is exactly 10⁶ spores; and CIs do NOT outperform BIs in sensitivity for actual sterilization efficacy.

A researcher studying prion diseases notes that PrPSc (scrapie form) is highly resistant to standard decontamination. Which of the following mechanisms BEST explains why prions resist destruction by conventional autoclaving at 121°C/15 minutes?

Answer: The beta-sheet rich tertiary structure of PrPSc aggregates into amyloid fibrils with extraordinary thermodynamic stability, allowing partial renaturation of infectious conformation

PrPSc consists entirely of misfolded protein with an abnormally high beta-sheet content that forms amyloid aggregates. These aggregates are thermodynamically highly stable — conventional autoclaving denatures but does not fully destroy the infectious conformation, and some molecules can renature into infectious form during cool-down. Prions contain NO nucleic acid (ruling out option C) and do NOT encode enzymes (ruling out option A). While chaperones exist, they play no role in prion renaturation post-autoclaving (option D). The recommended decontamination is 134°C for ≥18 minutes or 1N NaOH treatment.

A urine culture from a catheterized patient grows >10⁵ CFU/mL of an organism that is urease-positive, produces a characteristic 'swarming' motility on non-inhibitory agar, and forms struvite crystals in alkaline urine. Antibiotic susceptibility testing shows resistance to trimethoprim-sulfamethoxazole and ampicillin but sensitivity to nitrofurantoin. Which of the following is the MOST accurate statement about this organism's pathogenicity mechanism?

Answer: Urease converts urea to ammonia, raising urine pH above 7.2, precipitating magnesium ammonium phosphate (struvite) and forming biofilm-embedded stones that shield the organism from antibiotics

This organism is Proteus mirabilis. Its urease hydrolyzes urea → NH₃ + CO₂, alkalinizing urine above pH 7.0–7.2. This precipitates magnesium ammonium phosphate (struvite = 'triple phosphate') crystals, which embed bacteria in a mineral biofilm matrix that physically shields them from antibiotics and immune cells — a major reason catheter-associated UTIs from P. mirabilis are so refractory. Swarming motility in P. mirabilis is mediated by flagella, not type IV pili (option B is wrong). Nitrofurantoin sensitivity does not exclude biofilm formation (option C is incorrect). Struvite is a precipitation phenomenon, not protein-mediated chelation (option D is wrong).

A COT is reviewing quality control records for an ultrasonic cleaner used in instrument reprocessing. The daily aluminum foil erosion test shows minimal pitting after a 5-minute run. Which is the MOST appropriate action?

Answer: Investigate the unit immediately — insufficient foil erosion/pitting indicates cavitation failure, meaning the ultrasonic energy is inadequate to remove bioburden from instrument lumens

The aluminum foil erosion test assesses cavitation — the rapid formation and collapse of microscopic vapor bubbles that provide the mechanical cleaning action in ultrasonic cleaners. Adequate cavitation produces uniform, fine pitting/erosion across the foil surface. Minimal or absent pitting indicates cavitation failure, meaning the unit is NOT generating sufficient mechanical energy to dislodge bioburden from instrument crevices and lumens. This is a significant reprocessing failure requiring immediate investigation (e.g., transducer malfunction, improper solution level, temperature, or degassing issues). More pitting = better function; less pitting = unit malfunction. Increasing solution concentration does not compensate for absent mechanical energy.