Biofilms are microbial cities that can resist antibiotics thousands of times better
Dental plaque is a biofilm: a crowd of microbes glued together in a slime they manufacture themselves. Inside that matrix, bacteria share food, trade genes and shrug off threats, in some cases becoming up to 5,000 times more resistant to antibiotics. The fossil record suggests they have been living this way for about 3.25 billion years.
The slime is a matrix of extracellular polymeric substances, typically a blend of polysaccharides, proteins, lipids and DNA, which holds many times its own dry weight in water. Because the structure is three-dimensional and communal, researchers have likened biofilms to cities for microbes. Cells living in one behave differently from free-swimming, or planktonic, cells of the same species, switching large sets of genes on or off when they settle.
Formation follows a sequence. Colonists first cling loosely through weak van der Waals forces and hydrophobic effects, then anchor firmly with hair-like pili. Some archaea in oxygen-free groundwater use hami, tubes tipped with three hooks. As the colony grows through division and recruitment, bacteria signal each other with quorum-sensing molecules. Five stages are commonly described, running from reversible attachment through maturation into clustered micro-colonies and ending in dispersal.
Dispersal is more than simple escape. Pseudomonas aeruginosa cells leaving a biofilm are physiologically unlike both settled and planktonic cells: highly virulent towards macrophages, yet vulnerable to iron stress. A fatty acid messenger it secretes, cis-2-decenoic acid, can trigger dispersal in several bacteria and in the yeast Candida albicans. Low, non-toxic levels of nitric oxide can also break biofilms apart, which makes it a candidate treatment for chronic infections.
Biofilms coat rocks, leaves in humid climates, industrial pipes and hospital equipment, and can mix bacteria, archaea, fungi, algae and protozoa. Their matrix acts as an external digestive system, trapping enzymes near the cells, and some contain water channels that carry nutrients. Under the right conditions the matrix is tough enough to fossilise, producing the layered rocks called stromatolites.
Source: Biofilm
