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A biofilm is any group of microorganisms in which cells stick to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS). Biofilm extracellular polymeric substance, which is also referred to as slime (although not everything described as slime is a biofilm), is a polymeric conglomeration generally composed of extracellular DNA, proteins, and polysaccharides. Biofilms may form on living or non-living surfaces and can be prevalent in natural, industrial and hospital settings. The microbial cells growing in a biofilm are physiologically distinct from planktonic cells of the same organism, which, by contrast, are single-cells that may float or swim in a liquid medium. Microbes form a biofilm in response to many factors, which may include cellular recognition of specific or non-specific attachment sites on a surface, nutritional cues, or in some cases, by exposure of planktonic cells to sub-inhibitory concentrations of antibiotics.〔 (primary source)〕 When a cell switches to the biofilm mode of growth, it undergoes a phenotypic shift in behavior in which large suites of genes are differentially regulated. ==Formation== Formation of a biofilm begins with the attachment of free-floating microorganisms to a surface. These first colonists adhere to the surface initially through weak, reversible adhesion via van der Waals forces. If the colonists are not immediately separated from the surface, they can anchor themselves more permanently using cell adhesion structures such as pili. Hydrophobicity also plays an important role in determining the ability of bacteria to form biofilms, as those with increased hydrophobicity have reduced repulsion between the extracellular matrix and the bacterium.〔Donlan, Rodney M. 2002. Biofilms: Microbial Life on Surfaces. Emerging Infectious Diseases. Vol. 8, No. 9: pg. 881-890.〕 Some species are not able to attach to a surface on their own but are instead able to anchor themselves to the matrix or directly to earlier colonists. It is during this colonization that the cells are able to communicate via quorum sensing (QS) using products such as AHL. Some bacteria are unable to form biofilms as successfully due to their limited motility. Non-motile bacteria cannot recognize the surface or aggregate together as easily as motile bacteria.〔 Once colonization has begun, the biofilm grows through a combination of cell division and recruitment. Polysaccharide matrices typically enclose bacterial biofilms. In addition to the polysaccharides, these matrices may also contain material from the surrounding environment, including but not limited to minerals, soil particles, and blood components, such as erythrocytes and fibrin.〔 The final stage of biofilm formation is known as dispersion, and is the stage in which the biofilm is established and may only change in shape and size. The development of a biofilm may allow for an aggregate cell colony (or colonies) to be increasingly resistant to antibiotics. Cell-cell communication or quorum sensing has been shown to be involved in the formation of biofilm in several bacterial species.〔Quorum-Sensing Regulation of the Biofilm Matrix Genes (pel) of Pseudomonas aeruginosa〕 ''A. baumannii'' is infamous for its ability to form biofilms both on inanimate objects as well as biotic surfaces. ''A. baumannii'' has been reported to commence secretion of exopolysaccharides once it has successfully adhered to a surface, be it hydrophilic or hydrophobic like glass or plastic, respectively, or surfaces of living cells. Previous findings indicate that within the protective environment of the biofilm, the pathogen remains protected from starvation, desiccation and the action of antibiotics. As such, the ability to form biofilms alone may be linked to the increased virulence in some of the strains. Direct evidence to support this hypothesis is still lacking. However, reports have shown that multi-drug resistant strains are efficient biofilm producers, indicating a direct relationship between biofilm formation and antibiotic resistance. Other reports have shown that the biofilm-associated protein (BAP) in ''A. baumannii'', involved in biofilm formation, is capable of stimulating humoral response in mice, which suggests that it may have a role in virulence. The ability of ''A. baumannii'' to form biofilms has been shown to be related to certain outer membrane surface-associated proteins like OmpA and BAP as well as certain pili-associated adhesins. The presence of metal cations has also been reported to be required for biofilm formation as indicated by the reduced ability of ''A. baumannii'' to produce biofilms in presence of chelators like ethylenediaminetetraacetic acid (EDTA). The formation of CsuA/BABCDE-dependent pili appears to be essential for biofilm adherence to and formation on abiotic surfaces and the assembly of these pili seems to involve chaperone and usher-like proteins. However, this system does not seem to be involved in the adherence of ''A. baumannii'' to living cells, and the underlying mechanisms governing attachment to biotic surfaces still remain to be elucidated. Formation of biofilm by ''A. baumannii'' is under tight regulation both at the transcriptional and translational levels involving highly efficient and cross-linked two-component regulatory systems. However, the identification of these regulatory systems has not yet been achieved. Quorum sensing has also been implicated in the regulation of biofilm formation. ''A. baumannii'' has been shown to be capable of producing quorum-sensing molecules, namely N-acylhomoserine lactones of various chain length with -(3-hydroxydodecanoyl)-L-HSL reported as the primary signal molecule. However, only a single autoinducer synthase gene named abaI has been identified to date. Quorum sensing is the main method of communication between the bacterial cells within the biofilm and may also serve as a mechanism to coordinate and regulate the multiple virulence factors in ''A. baumannii''. There are reports indicating that quorum sensing might possibly be involved in host–pathogen interactions as well. Thus, biofilm formation and quorum sensing are important components in the wide arsenal of virulence determinants produced by ''A. baumannii''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「biofilm」の詳細全文を読む スポンサード リンク
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