Basic knowledge and perspectives of bioelimination of xenobiotic compounds

Almost every natural product, irrespective of its molecular weight or structural complexity, is degraded by one or another microbial species in some particular environment. The omnipotence of microorganisms also extends to the majority of synthetic compounds, which are funneled into the natural metabolic cycles. Certain substituents such as halogen, sulfo-, azo- or nitro-groups, particularly the accumulation of such groups and specific substitution patterns confer xenobiotic character to a synthetic compound. Moreover, the electron-withdrawing character of these substituents generates electron deficiency and thus makes the compounds less susceptible to oxidative catabolism. As a consequence, many of these chemicals tend to persist under aerobic environmental conditions. When enzymes with low substrate specificity encounter foreign compounds, highly reactive species may be generated. On the one hand, these can eliminate some of the xenophors spontaneously generating less persistent metabolites. On the other band, gratuitous metabolism of complex structures may give rise to extensive chemical misrouting, generating dead products of high molecular weight. Biodegradation of a xenobiotic substance can be accomplished when the catabolic activities, present in mixed microbial communities, complement each other. Thus, syntrophic interactions can lead to complete mineralization of even complex xenobiotic compounds. Mineralization of xenobiotics by a single organism can be achieved by taking advantage of natural or induced gene transfer to construct hybrid degradative pathways. Mobilization of blocks of genes, encoding catabolic bottleneck reactions, may speed up the evolutionary potential of natural communities so that under appropriate selective conditions new multifunctional pathways are generated. As a consequence of the electron deficiency of xenobiotic compounds such as polychlorinated arenes or ethenes, azo dyes or polynitroaromatics, the combination of the reductive potential of anaerobic microbes with subsequent oxidative processes opens a hitherto largely unexploited technology for biological treatment of waste water and for soil bioremediation. Copyright (C) 1996 Elsevier Science B.V.