Abstract A genuinely new dimension to microbiology concerns the fate of synthetic compounds dispersed throughout the biosphere, with xenobiotic compounds representing a major challenge to the metabolic versatility of microorganisms. While microbial bio-degradation of many of these compounds proceeds by the activity of single species of microorganisms, this paper develops the argument that microbial communities function by synergistic mechanisms to effect the breakdown of many of these compounds. The degradation of some compounds, especially xenobiotic compounds, depends on the use of gratuitous metabolic pathways evolved for natural compounds, which in many instances may be evolved through the combined metabolic activity of two or more microorganisms. The role of plasmid coded pathways is also considered in the context of the evolution of novel pathways.