| Biofilms |
We have formed many of our ideas about bacteria by studying pure cultures - homogenous populations growing in broths. In the wild, however, microorganisms live alongside, in, or on other organisms and often produce proteins not apparent in the laboratory. Bacteria communicate chemically with their neighbors and respond to signals they receive. An understanding of communication among bacteria - including those within bacterial communities - is shaping medical treatments, strategies for bacterial control, and providing a new perspective of the interrelationships between species.
One form of bacterial community is the biofilm. An example is the coating of bacteria on your teeth. Biofilms are "living veneers" composed of microcolonies of bacteria, surrounded by a gooey extracellular matrix that the bacteria secrete. A network of water channels provides nutrients and efficiently removes waste products for the bacteria on the surface. Deeper down, cells rely on diffusion for nutrient delivery and waste removal. Oxygen concentrations vary within a biofilm; cells buried deeper can be oxygen deprived. This variation in environment means that members of a biofilm community, even genetically identical individuals, differ in their metabolic states. In fact, those buried deep within the film are effectively dormant.
"There's a real transformation that takes place and the bacteria start acting like a community... a whole different organism. And there are significant differences in the level of expression of genes within the biofilm because of the different environments within the microcolonies."
(Anne Camper, Center for Biofilm Engineering)
Biofilms of Pseudomonas aeruginosa in the lungs of cystic fibrosis patients can be life-threatening. The thick mucus that this inherited disorder produces provides a suitable environment for an infection to become established. However, this is not a simple infection. The bacteria organize themselves into a biofilm and, as they do, some become less susceptible to antibiotics. For patients, the result is a prolonged infection that is very difficult to treat.
Why do bacteria in biofilms survive much higher concentrations of antibiotics and disinfectants than free-living organisms? One reason involves the dormant bacteria in the biofilm. Many antibiotics - penicillin, for example - act only on actively growing cells. Cells that were dormant can serve to reestablish a biofilm once the antimicrobial is no longer present.
Another mechanism for survival is the layered nature of a biofilm. The effectiveness of a disinfectant, such as bleach, is depleted as it acts on outer layers of the film; bacteria located in inner layers may survive. A third mechanism for survival involves the generation of proteins that provide antimicrobial resistance, such as enzymes that inactivate hydrogen peroxide. Some biofilms are able to manufacture larger quantities of such enzymes so they become more resistant than planktonic (free-floating) bacteria.