Pseudomonas

Pseudomonas is a genus of gram-negative rod-shaped bacteria found widely in soil, water, and moist environments. The clinically dominant species is Pseudomonas aeruginosa, an aerobe — it can respire anaerobically using nitrate, but it does not ferment — that is oxidase-positive, non-lactose fermenting, and motile. It produces distinctive pigments — pyocyanin (blue-green) and pyoverdine — and cultures often give off a grape-like odor.

P. aeruginosa is a classic opportunist: it rarely infects healthy people but exploits compromised defenses and hospital environments. High-yield associations include pneumonia in patients with cystic fibrosis (where biofilm-forming mucoid strains chronically colonize the airways), burn wound infections, otitis externa (including malignant otitis externa in patients with diabetes), hot tub folliculitis, contact lens-associated keratitis, catheter-related urinary tract infections, ventilator-associated pneumonia, and ecthyma gangrenosum in neutropenic patients.

Its virulence rests on factors like exotoxin A (which inhibits protein synthesis via elongation factor EF-2, similar to diphtheria toxin), elastases, phospholipase C, and an endotoxin-bearing outer membrane. Just as important is its antibiotic resistance: efflux pumps, low outer membrane permeability, and biofilm formation make Pseudomonas infections difficult to treat, requiring specific antipseudomonal agents such as piperacillin-tazobactam, cefepime, ceftazidime, carbapenems, or aminoglycosides.

The USMLE Step 1 tests Pseudomonas among the gram-negative bacilli — its lab identification, patient associations, toxins, and the antipseudomonal drug list. The CCMA exam touches Pseudomonas in urinalysis, where it is one of the organisms behind urinary tract infections and abnormal urine findings.