biofilm

Biofilms form when planktonic (free-floating) bacteria adhere to a surface and secrete an extracellular polymeric substance (EPS) matrix composed of polysaccharides, proteins, and extracellular DNA. This matrix protects bacteria from antibiotics, desiccation, and host immune cells (phagocytes and antibodies have difficulty penetrating the matrix). Bacteria within biofilms exhibit altered gene expression, reduced metabolic rate, and can develop persister cells that are phenotypically tolerant to antibiotics without genetic resistance mutations. This significantly increases minimum inhibitory concentrations (MIC) required for treatment, often 10-1000x higher than planktonic bacteria.

Clinically, biofilms are critical in: (1) Prosthetic device infections - central venous catheters, urinary catheters, prosthetic joints, and prosthetic heart valves, commonly caused by Staphylococcus epidermidis (coagulase-negative staph) which produces slime layer, and Staphylococcus aureus; (2) Native valve endocarditis - viridans streptococci adhere to damaged valves; (3) Cystic fibrosis - Pseudomonas aeruginosa forms biofilms in the lungs, contributing to chronic, difficult-to-treat pulmonary infections; (4) Dental plaque - Streptococcus mutans forms biofilms on tooth surfaces leading to caries; (5) Chronic wound infections.

Management typically requires removal of the infected foreign material (e.g., catheter, prosthetic device) since antibiotics alone often cannot eradicate biofilm-embedded organisms. Combination antibiotic therapy and prolonged treatment courses are often necessary. Key USMLE associations: S. epidermidis + IV catheters/prosthetic devices, S. mutans + dental caries, Pseudomonas + CF lungs and contact lens keratitis.