10. Biofilms – strategy of a survival of bacteria in environment. Characterize structure of biofilms. Explain the increased resistance of bacteria in biofilms.

The view of bacteria as unicellular organisms has strong roots in the tradition of culturing bacteria in liquid media. However, in nature microbial activity is mainly associated with surfaces where bacteria form highly structured and cooperative consortia which are commonly referred to as biofilms. The ability of bacteria to organize structurally and to distribute metabolic activities between the different members of the consortium demands a high degree of coordinated cell-cell interaction. Recent work has established that many bacteria employ sophisticated intercellular communication systems that rely on small signal molecules to control the expression of multiple target genes. In Gram-negative bacteria, the most intensively investigated signal molecules are N-acyl-L-homoserine lactones (AHLs), which are utilized by the bacteria to monitor their own population densities in a process known as 'quorum sensing'. These density-dependent regulatory systems rely on two proteins, an AHL synthase, usually a member of the LuxI family of proteins, and an AHL receptor protein belonging to the LuxR family of transcriptional regulators. At low population densities cells produce a basal level of AHL via the activity of an AHL synthase. As the cell density increases, AHL accumulates in the growth medium. On reaching a critical threshold concentration, the AHL molecule binds to its cognate receptor which in turn leads to the induction/repression of AHL-regulated phenotypes (e.g. bioluminescence in Vibrio fischeri).

Quorum sensing is a method of bacterial communication and allows the co-ordination of gene expression in response to fluctuations in cell density. This enables populations of cells to control a diverse array of biological processes in synchrony, from biofilm formation to bioluminescence. In order to quorum sense individual cells produce and respond to small signalling molecules, termed autoinducers, that accumulate in the cell’s external environment (Surette et al, 1999). The concentration of autoinducer increases, both intra and extra-cellularly, with increasing cell density and once a minimal threshold concentration is reached, gene expression is altered.

Many bacteria monitor population density in this way, including the marine bacterium vibrio fischeri. This particular bacterium possesses two quorum sensing systems, ain and lux; both systems use acyl homoserine lactones (AHLs) as signalling molecules. In the lux system, synthesis of AHL is directed by the product of the LuxI gene, using the substrates S-adenosylmethionine and acylated acyl protein (SAM and ACP). Once a critical concentration of AHL is reached, it associates with the transcriptional activator, LuxR, to regulate transcription of the Luminescence (lux) operon.