Biofilms: The Social Life of Microorganisms
Microorganisms typically do not live as single cells in pure cultures. The vast majority lives in mixed populations, organized in aggregates which are termed “biofilms”. This includes films at interfaces, flocks (floating films), sludge’s and microbial mats. They all have emergent properties in common that only can be developed in the aggregated form: they generate a matrix of extracellular polymeric substances which keeps them together, allows for the development of stable, synergistic micro consortia and for intense cell-cell communication.
The matrix is activated by extracellular enzymes which are retained in it, representing an external digestion system. This system also sequesters nutrients from the environment and serves as ultimate recycling yard and nutrient source. Horizontal gene exchange is facilitated with a vast gene pool present. In biofilms, organisms differentiate rapidly, forming phenotypically different subpopulations, a mechanism which contributes to ecological fitness.
Resistance to biocides is enhanced by a range of mechanisms. On the other hand, strong competition prevails in which attacking and defence strategies evolved, including the formation of antibiotics and bacteriocins. Grazing organisms can limit biofilm growth but also stimulate ecological fitness. Even “programmed cell death” is observed, leading to a more porous matrix which allows for better access of nutrients for organisms in the depth of the matrix.
Under stress conditions, cells can transform into a viable but not cultivable (VBNC) state which is of relevance for public hygiene because they cannot be detected with the methods designed for their determination but can resuscitate. Biofilms represent the oldest, most abundant and successful form of life on Earth, displaying aspects of multicellularity. Life evolved from biofilms and they are involved in the biogeochemical cycles of all major elements.
In biofilms, photosynthesis was developed biofilms they are responsible for the self-purification mechanisms of soils, sediments and water. However, biofilms can occur in the wrong place and time, causing bio fouling, bio corrosion and bio deterioration, leading to substantial economic loss and supporting an entire industry dedicated to cleaning and disinfection. In medicine, they are cause of persistent infections and are related to many diseases. In general, biofilms are of fundamental but mostly unaccounted relevance for our life.
