<P> Biofilms are usually found on solid substrates submerged in or exposed to an aqueous solution, although they can form as floating mats on liquid surfaces and also on the surface of leaves, particularly in high humidity climates . Given sufficient resources for growth, a biofilm will quickly grow to be macroscopic (visible to the naked eye). Biofilms can contain many different types of microorganism, e.g. bacteria, archaea, protozoa, fungi and algae; each group performs specialized metabolic functions . However, some organisms will form single - species films under certain conditions . The social structure (cooperation / competition) within a biofilm depends highly on the different species present . </P> <P> The EPS matrix consists of exopolysaccharides, proteins and nucleic acids . A large proportion of the EPS is more or less strongly hydrated, however, hydrophobic EPS also occur; one example is cellulose which is produced by a range of microorganisms . This matrix encases the cells within it and facilitates communication among them through biochemical signals as well as gene exchange . The EPS matrix also traps extracellular enzymes and keeps them in close proximity to the cells . Thus, the matrix represents an external digestion system and allows for stable synergistic microconsortia of different species (Wingender and Flemming, Nat. Rev. Microbiol. 8, 623 - 633). Some biofilms have been found to contain water channels that help distribute nutrients and signalling molecules . This matrix is strong enough that under certain conditions, biofilms can become fossilized (Stromatolites). </P> <P> Bacteria living in a biofilm usually have significantly different properties from free - floating bacteria of the same species, as the dense and protected environment of the film allows them to cooperate and interact in various ways . One benefit of this environment is increased resistance to detergents and antibiotics, as the dense extracellular matrix and the outer layer of cells protect the interior of the community . In some cases antibiotic resistance can be increased a thousandfold . Lateral gene transfer is often facilitated within bacterial and archaeal biofilms and leads to a more stable biofilm structure . Extracellular DNA is a major structural component of many different microbial biofilms . Enzymatic degradation of extracellular DNA can weaken the biofilm structure and release microbial cells from the surface . </P> <P> However, biofilms are not always less susceptible to antibiotics . For instance, the biofilm form of Pseudomonas aeruginosa has no greater resistance to antimicrobials than do stationary - phase planktonic cells, although when the biofilm is compared to logarithmic - phase planktonic cells, the biofilm does have greater resistance to antimicrobials . This resistance to antibiotics in both stationary - phase cells and biofilms may be due to the presence of persister cells . </P>

Why do bacteria in a biofilm have different characteristics than free-floating bacteria