Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, UK

Until relatively recently, analyses of bacterial pathogens of plants and animals were conducted largely in two cultures of intellectual isolation. There was comparatively little benefit derived from communication between phytobacteriologists and medical/veterinary bacteriologists. This was not surprising as the microbes being investigated attack hosts - plants or animals - which are undeniably different, and so it was not at all obvious that there might be some common principles in bacterial pathogenesis of the respective hosts. However, in the past few years there has been an explosion in the widespread application of recombinant DNA methods and the adaptation of a spectrum of bacterial genetic tools to diverse bacterial pathogens. The outcomes of such applications of molecular genetic analysis to investigation of bacterial pathogenesis have revealed several common themes in bacterial attack of plants and animals. In no area is this convergence of concepts more transparent then in the study of gene regulation systems in bacterial pathogens. Indeed, advances in the molecular biological analysis of prokaryotic pathogens generally has shown that bacterial pathogens do very similar things, physiologically, to many other bacteria and display very few truly 'unique' characteristics. As with saprophytic bacteria, the physiological processes in bacterial pathogens are driven by nutritional need - the only real difference being that, for at least some of their life cycle, pathogens have to derive some of their nutrition from a particular eukaryotic host(s). Fundamentally, therefore, the phenomenon called 'pathogenesis' can be reduced, in some senses, to an aspect of microbial ecology in which the disease process is ultimately a consequence of the bacterium's ability/need to sense localized environmental and nutritional signals, to adapt physiologically then multiply in/on specific hosts.

In an attempt to set the scene for the various aspects of gene regulation processes in phytopathogenic bacteria discussed by the other speakers, I will review multiple examples from prokaryotic phytopathology and human/animal pathogenesis which highlight the diversity of gene regulation systems in bacteria. Wherever appropriate, the similarities between systems used by bacterial pathogens will be demonstrated. Examples to be discussed will include the sensing of, and response to, (i) external signals from host cells; (ii) signals from the bacterial community; and (iii) environmental and nutritional cues. In addition, I will review some of the common features of global gene regulation mechanisms and virulence factor targeting systems in bacterial pathogens, where molecular studies of plant pathogens and human/animal pathogens have been mutually enriching and have produced rapid advances in the past few years.