Cell–cell signalling in microbial communities

Cambridge University Press, 1999. £65.00/$115.00 hbk (xiv + 365 pages)ISBN 0 521 65261 8It has been argued that the tide of World War II turned upon the decision to equip Russian tanks with radios. Soviet armies could subsequently achieve an almost organismal degree of coordinated behaviour in their advances across Eastern Europe. Microorganisms have also learned the importance of accurate communication, such that a population of single cells can sometimes coordinate its behaviour to the point of resembling a multicellular organism. Although just ten years ago microbial signalling was thought to be the exception rather than the rule, it is becoming increasingly clear that many, or perhaps most, microorganisms have this capability. Thus, organisms that were once thought to live solitary lives are now known to create and inhabit rich social environments. This theme is the topic of a recent book, Microbial Signalling and Communication, which commemorates the 57th Symposium of the Society for General Microbiology, held in Edinburgh in April 1999.This valuable collection of papers includes several chapters describing signalling via acyl-homoserine lactones, including the archetypal example, the LuxR/LuxI regulators of Vibrio fischeri. These proteins activate transcription of the bioluminescence operon in V. fischeri, and do so only at the high population densities found in the light organs of particular host fish or squid. This phenomenon of switching on certain genes at high population densities is a recurring theme sometimes referred to as quorum sensing. The chapter by Greenberg describes the synthesis of the V. fischeri pheromone, called an autoinducer, as well as autoinducer binding to LuxR, the transcriptional activator. Several other chapters focus on similar regulatory systems, each composed of a LuxR homologue and a LuxI homologue, found in a variety of human and animal pathogens including Pseudomonas, Aeromonas and Yersinia, in plant-associated bacteria such as Erwinia, Agrobacterium and Rhizobium, and in more than a dozen other genera of microorganisms.Although LuxR/LuxI-type regulatory systems are widespread among proteobacteria, they have not yet been reported in other groups of organisms. However, a different class of pheromone-dependent regulatory system is widespread among low G+C Gram-positive microorganisms, each using a specific oligopeptide as a signal molecule. Peptide pheromones are used to regulate sporulation by Bacillus subtilis, genetic competence by B. subtilis and Streptococcus pneumoniae, the production of bacteriocins by Lactococcus, Lactobacillus and Carnobacterium and to regulate expression of pathogenesis genes in Staphyloccus aureus. Curiously, peptide-dependent signalling regulatory systems seem under-represented in this volume, although one chapter by Dunny and colleagues describes the regulation of conjugation genes by Enterococcus faecalis. Another chapter reviews the interesting evidence that a protein released by Micrococcus luteus stimulates cell division.The several chapters devoted to signalling in eukaryotic microorganisms are especially welcome. Mating pheromones in the fission yeast Schizosaccharomyces pombe are nicely reviewed, although little mention is made of the corresponding system in budding yeast. A wider discussion is included on signalling in filamentous fungi during vegetative growth and the sexual cycle, and another chapter describes the pheromones of Dictyostellium that mediate fruiting-body formation. This is a good review, although the mathematical modelling of this phenomenon might be beyond the reach of the general reader.In some parts, the focus of this book might be too wide. The chapter on signalling in dinoflagellates says nothing about intraspecific signal exchange; rather, it concentrates on the detection of signals from inanimate sources (e.g. light detection), from prey organisms, from endosymbiotic prokaryotes or from host animals. Similarly, the chapters on enteropathogenic Escherichia coli and on Yersinia, although both excellent reviews, have no information at all on intraspecific signalling, despite that fact that the existence of this phenomenon is suspected in the former organism and well established in the latter.As a greater understanding of examples of microbial signalling is achieved, one of the challenges will be the practical application of this information. For example, one goal will be to block signalling systems in bacterial pathogens, and the search for pheromone antagonists is already under way. Conversely, attempts have been made to stimulate those systems that are found in beneficial microorganisms and, to this end, transgenic plants have been developed that secrete autoinducer pheromones to stimulate gene expression in symbiotic bacteria. This book will do a good job of bringing the uninitiated up to speed and also describes important goals for the new century. It will be an important addition to research libraries and extremely useful for students of microbial signalling.