The figure shows a schematic picture of how a biofilm is formed. Two different bacterial species (A and B) are first planktonic. One of the bacteria (1), which has adhesion ability, adheres to the surface and begins to multiply there (2). After a while, a monolayer has been formed and polysaccharides (glycocalyx) are secreted (3). Then the monolayer begins to grow in height and a colony, which is encapsulated in glycocalix, is formed (4). This is the beginning of a biofilm, which may contain several different bacterial species and also fungi (yeast or mould). When the biofilm has grown to a certain size, bacteria (6) or clumps of bacteria are released, which can attach to new places and form more biofilm.
Image: Karl-Erik Johansson (BVF, SLU) - Click on the image to enlarge it.
Bacteria that grow freely in liquid media are said to be planktonic, but some bacteria can also grow on solid substrates, forming a so-called biofilm. Biofilms can exist on live or dead surfaces and occur in nature, on materials in industrial processes and in different health care situations. Biofilms consist of population(s) of bacteria, which adhere to a surface and to each other and are enclosed in a network (matrix) of biopolymers. The formation of a biofilm starts with bacteria that adhere to a surface by means of e.g. fimbriae (pili) where they bind irreversibly and initially grow as a monolayer. Then they form several layers and start to produce some kind of a biopolymer (extracellular matrix) that often consists of the same material as the capsule, but in a looser structure. The biopolymer is made up of polysaccharides and is called glycocalyx (capsule). Dextran is one example of such a polysaccharide. Biofilms consist of one or more bacterial populations (species), glycocalyx, DNA and proteins.
A bacterial species that can not itself adhere to a surface, can often become attached to pre-existing bacterial glycocalyx and grow as a biofilm. Bacteria in biofilms are more resistant to antibiotics, detergents and phagocytosis than planktonic bacteria. Bacteria (Streptococcus spp.) can grow as biofilms on teeth (= plaques), on implants (e.g. heart valves), in plastic tubes, which transport nutrients etc. In patients with cystic fibrosis Pseudomonas aeruginosa may grow as a biofilm in the lungs. In biofilms, bacteria can communicate with each other by means of chemical signal substances (quorum sensing) to control gene expression in the whole population.
Link to a video montage on YouTube which is recommended:
Biofilm: "What Are Bacterial Biofilms?"
Quorum sensing: "Quorum sensing, bacteria talks"