Cell-to-cell communication in Escherichia coli and Salmonella typhimurium: they may be talking, but who's listening?

The closely related enteric bacteria Escherichia coli and Salmonella typhimurium are among the best understood microorganisms, and much of our knowledge of prokaryotic physiology and genetics has derived from their study. Despite their utility as model prokaryotes, however, it is clear that E. coli and S. typhimurium do not represent all aspects of microbial physiology and behavior. One such underrepresented area is that of the microbial cell-cell interaction. There are a variety of microorganisms that have evolved elaborate means by which individual cells communicate and coordinate their actions. Often, these bacteria produce and release extracellular signal molecules that allow them to gauge their own population density and respond by altering expression of specific genes, a process generally described as quorum sensing. Examples of intercellular communication systems include oligopeptide-based signaling used by a variety of Gram-positive bacteria, A factor production during fruiting body development in Myxocccus xanthus, butanolide control of antibiotic biosynthesis in Streptomyces spp., and a volatile fatty acid methyl ester signal that regulates virulence in the plant pathogen Ralstonia solanacearum (see refs. 1–4). In Gram-negative bacteria, by far the most common form of quorum sensing is mediated by production and subsequent perception of acylated homoserine lactones (acyl HSLs) (5, 6).