Catabolic mobile genetic elements and their potential use in bioaugmentation of polluted soils and waters

Genes that encode the degradation of both naturally occurring and xenobiotic organic compounds are often located on plasmids, transposons or other mobile and/or integrative elements. The list of published reports of such mobile genetic elements (MGEs) keeps growing as researchers continue to isolate and characterize new degrading bacteria and their corresponding degradative genes. There is also growing evidence that horizontal exchange of catabolic (degradative) genes among bacteria in microbial communities plays an important role in the evolution of catabolic pathways. Around 10 years ago the hypothesis was raised that we might be able to accelerate this natural gene exchange and pathway construction by introducing and subsequently spreading degradative genes, located on MGEs, into well established, competitive indigenous microbial populations as a means of bioaugmentation of polluted soils and waters. During the last decade, only a few reports on successful MGE- mediated bioaugmentation have been published. After summarizing the diversity of degradative MGEs, this review presents an overview of studies that have monitored the transfer of degradative genes in soil microcosms and in activated sludge and other wastewater treatment reactors, with emphasis on those that have clearly shown a direct effect of gene transfer on accelerated biodegradation. A few successful cases suggest that the strategy could indeed work under specific conditions, such as when the in situ degradation potential is absent and the pollutant degrading transconjugants can grow and become numerically dominant populations in the bacterial community. Further studies in this area are obviously needed to improve our current knowledge on the efficiency of gene dissemination as a tool in bioremediation.