INTRODUCTION

In nature, colonization of habitats by mixtures of bacterial populations is the rule rather than the exception. Diverse groups of micro-organisms are invariably isolated from samples from environmentally exposed habitats. Evidence is accumulating that such mixtures of organisms are not merely passive neighbours but that they are involved in a wide range of dynamic physical and metabolic interactions. Indeed, these interactions appear to be essential for the attachment, growth and survival of species at a site, and also enable organisms to persist in what often appear to be overtly hostile environments.

Such interacting mixtures of micro-organisms are termed microbial communities, and are generally found on a surface, spatially organized as a biofilm (see other chapters in this volume). Microbial communities have been described from habitats ranging from aquatic environments, anaerobic digesters, plant surfaces and soil particles, to the digestive tract of humans and animals. Although the nomenclature of the species from these diverse habitats is different, this chapter will demonstrate that the function (or niche; Alexander, 1971) of the community members is often similar.

Existence within a microbial community can have profound consequences for the component populations. These include (a) a broader habitat range for colonization, (b) an increased metabolic diversity and efficiency, so that substrates normally recalcitrant to catabolism by individual organisms can often be broken down by consortia to simpler products, (c) increased resistance to environmental stress and to host defence factors (Caldwell et al., 1997a, b; Shapiro, 1998) and, in some cases, (d) an increased ability to cause disease (pathogenic synergism; van Steenbergen et al., 1984; Brook, 1987a).