Biofilm

A biofilm is a structured community of microorganisms attached to a surface and held together in a slimy matrix that the cells themselves secrete. Biofilms grow on stones in streams, inside water pipes, on teeth as dental plaque, on living tissue and on medical devices. In nature, most bacteria are thought to live in biofilms rather than as free-floating, or planktonic, cells.

A biofilm develops in stages. Free-floating cells first attach loosely to a surface, then attach firmly, divide and start producing extracellular polymeric substances (EPS), a water-rich gel made mostly of polysaccharides. The matrix keeps the cells from drying out, shields them from predators and contains channels that carry water and nutrients. The cells coordinate their behavior by quorum sensing, releasing signal molecules whose concentration tells them how crowded they are. Eventually some cells break away and colonize new surfaces.

Biofilms matter in medicine because bacteria inside them can tolerate antibiotics and disinfectants far better than the same cells living freely. The matrix slows the entry of drugs, cells deep inside grow slowly, and resistance genes pass easily between neighbors. Biofilms on catheters, artificial joints and the airways of people with cystic fibrosis cause stubborn, long-lasting infections.