THE BACTERIAL ENIGMA - CRACKING THE CODE OF CELL-CELL COMMUNICATION

In recent years it has become clear that the production of N-acyl homo serine lactones (N-AHLs) is widespread in Gram-negative bacteria. Thes e molecules act as diffusible chemical communication signals (bacteria l pheromones) which regulate diverse physiological processes including bioluminescence, antibiotic production, plasmid conjugal transfer and synthesis of exoenzyme virulence factors in plant and animal pathogen s. The paradigm for N-AHL production is in the bioluminescence (lux) p henotype of Photobacterium fischeri (formerly classified as Vibrio fis cheri) where the signalling molecule N-(3-oxohexanoyl)-L-homoserine la ctone (OHHL) is synthesized by the action of the Luxl protein. OHHL is thought to bind to the LuxR protein, allowing it to act as a positive transcriptional activator in an autoinduction process that physiologi cally couples cell density (and growth phase) to the expression of the bioluminescence genes. Based on the growing information on Luxl and L uxR homologues in other N-AHL-producing bacterial species such as Erwi nia carotovora, Pseudomonas aeruginosa, Yersinia enterocolitica, Agrob acterium tumefaciens and Rhizobium leguminoserum, it seems that analog ues of the P. fischeri lux autoinducer sensing system are widely distr ibuted in bacteria. The general physiological function of these simple chemical signalling systems appears to be the modulation of discrete and diverse metabolic processes in concert with cell density. In an ev olutionary sense, the elaboration and action of these bacterial pherom ones can be viewed as an example of multicellularity in prokaryotic po pulations.