Practice question

Biofilms represent structured multicellular communities where bacteria irreversibly attach to biotic or abiotic surfaces and embed themselves in self-produced extracellular polymeric substances composed of exopolysaccharides such as alginate and Psl, extracellular DNA released via lysis, proteins, amyloids and lipids anchored via adhesins like CsgA curlin and Bap. Within biofilm microenvironments, steep gradients of oxygen, nutrients and pH create heterogeneous populations including dormant persister cells with low metabolic activity and downregulated macromolecular synthesis that are phenotypically tolerant to antibiotics targeting active growth such as beta-lactams. The anionic matrix restricts diffusion of cationic antibiotics like aminoglycosides and vancomycin by electrostatic adsorption, chelates cations required for drug activity and contains enzymes like beta-lactamases that locally degrade antibiotics. Cell-to-cell proximity on the order of microns enhances horizontal gene transfer of resistance plasmids via conjugation, induces expression of efflux pumps and stress sigma factors RpoS and RpoE, and quorum sensing via acyl-homoserine lactones regulates biofilm maturation and dispersal. Consequently minimum inhibitory concentration for biofilm bacteria increases 10- to 1000-fold compared to isogenic planktonic cells, leading to chronic persistent infections of central venous catheters, cystic fibrosis lungs colonized by Pseudomonas and infective endocarditis, where eradication typically requires combined mechanical removal and prolonged high-dose combination therapy.