Biofilms are surface-attached communities of microorganisms encased in a self-produced extracellular polymeric substance (EPS). The EPS matrix, composed of polysaccharides, proteins, nucleic acids, and lipids, provides structural integrity and protects resident cells from environmental stresses, antibiotics, and host immune defenses.
Biofilm formation proceeds through five stages. Initial reversible attachment of planktonic cells to a surface is mediated by electrostatic interactions, van der Waals forces, and hydrophobic interactions. Irreversible attachment involves adhesins and production of EPS. During maturation stage I, microcolonies form and the biofilm architecture develops under the control of quorum sensing. Maturation stage II produces a three-dimensional mushroom-shaped structure with water channels for nutrient delivery and waste removal. In the dispersal stage, cells detach from the biofilm and colonize new surfaces.
The biofilm phenotype is fundamentally different from planktonic growth. Cells within biofilms exhibit altered gene expression, reduced metabolic activity, and increased antibiotic tolerance. Nutrient and oxygen gradients create heterogeneous microenvironments, with cells in the biofilm interior entering a stationary-phase-like state.
Antibiotic tolerance in biofilms arises from multiple mechanisms. Reduced penetration through the EPS matrix limits drug access. Slower growth rates make cells less susceptible to antibiotics that target actively dividing cells. Persister cells are a subpopulation that enter a dormant, antibiotic-tolerant state. The biofilm environment induces stress responses that increase tolerance.
Clinically important biofilm-forming organisms include Pseudomonas aeruginosa in cystic fibrosis lung infections and ventilator-associated pneumonia. Staphylococcus aureus and S. epidermidis form biofilms on indwelling medical devices including catheters, prosthetic joints, and heart valves. Candida species biofilm on central venous catheters cause candidemia. Dental plaque is a polymicrobial biofilm causing caries and periodontitis.
Laboratory study of biofilms uses the Calgary biofilm device for high-throughput susceptibility testing, flow cell systems for real-time confocal microscopy, and 96-well plate crystal violet staining for biomass quantification. Minimum biofilm eradication concentration (MBEC) assays determine the antibiotic concentration required to kill biofilm cells.
