Listening in on bacteria: acyl-homoserine lactone signalling

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Abstract
Bacteria can communicate with each other using diffusible chemical signals and can co-ordinate their behaviour to function as a group. Interbacterial signalling that functions to reflect population density is described as quorum sensing. Among Gram-negative bacteria, acylated-homoserine lactones (acyl-HSLs) are common quorum-sensing signals that regulate a diverse range of target functions, which are often involved in host interactions. Under certain circumstances, at least, quorum sensing has a role in the development of sessile microbial populations — called microbial biofilms — for several different bacteria, including the opportunistic human pathogens Pseudomonas aeruginosa and Burkholderia cepacia. Acyl-HSLs are synthesized from common fatty-acid and amino-acid precursors, most commonly through proteins of the LuxI family of acyl-HSL synthases, although alternate biosynthetic routes have been identified in a few cases. Increases in acyl-HSL concentration are generally perceived through the activity of LuxR-type transcription factors that bind the signals and are subsequently modified in their DNA-binding activity. Most LuxR proteins are transcriptional activators, although a few acyl-HSL-responsive repressors have also been identified. The recently obtained three-dimensional structure of a LuxR-type protein reveals potential mechanisms for its interactions with both the relevant acyl-HSL and the DNA sequences upstream of regulated target genes. Acyl-HSL quorum sensing might provide new targets for drug therapies. Other well-studied mechanisms of bacterial quorum-sensing include oligopeptide signal molecules in Gram-positive bacteria and the potential interspecies signal, formerly known as autoinducer-2 (furanosyl boron diester), which is produced by a diverse range of microorganisms.