Clinical Pathology: Microbiology

• Burn patients are at risk for severe disease because their protective skin barrier has been compromised and their circulatory system is disrupted. In addition to Pseudomonas aeruginosa, other common pathogens recovered from burn patients include Staphylococcus aureus, Enterococcus species, Acinetobacter, and other members of the Enterobacteriaceae.

• P. aeruginosa are aerobic, motile, long, slender, gram-negative bacilli. Macroscopic features include flat colonies with a metallic sheen, β-hemolysis on sheep blood agar, and a characteristic sweet grape-like odor. P. aeruginosa can produce diffusible pigments resulting in colonies that appear green, red, yellow, or brown. Most Pseudomonas species are oxidase positive, with the exception of P. luteola and P. oryzihabitans. P. aeruginosa is unique among the more common species because of the ability to grow at 42^oC.

• P. aeruginosa is an opportunistic pathogen that causes various infections, including ventilation-associated pneumonia, meningitis, malignant otitis externa, sepsis, endocarditis, osteomyelitis, osteochondritis, and folliculitis. Virulence factors produced by P. aeruginosa include exotoxin A (like diphtheria toxin) and cytotoxins.

• A mucoid phenotype of P. aeruginosa is prevalent in cystic fibrosis patients. This is due to over-production of alginate. Other characteristics of mucoid P. aeruginosa include slow growth, loss of motility, and loss of pigment production.

• Treatment of burn wound infections includes debridement of necrotic tissue, early grafting, and pathogen-specific antimicrobial therapy. Effective antibiotics for P. aeruginosa include piperacillin/tazobactam, ceftazidime, and a combination of an aminoglycoside (e.g., tobramycin) with an anti-pseudomonal β-lactam, such as ticarcillin.