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

Abstract

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.

Genetic bioaugmentation as an effective method for in situ bioremediation

“…[1][2][3] As previously suggested by Top et al (2002), 4 HGT could be useful for bioremediation to shift microbial communities in favor of degrading contaminants of concern. In particular, plasmid conjugal transfer is one of the dominant and well-studied mechanisms of HGT in bacteria, by which resistance to toxicants such as metals and antibiotics as well as the ability to degrade complex organic compounds may be spread.…”

Horizontal gene transfer between bacteria

“…Key abiotic and biotic factors that affect the extent of HGT in hot spots in the phytosphere or in soil have been reviewed recently (Van Elsas et al, 2003;. The importance of selection on the impact of gene transfer processes in soils was discussed in a review by Top et al (2002). Recently, Heuer and Smalla (2007) showed that treating soils with manure or with manure supplemented with sulfadiazine resulted in significantly increased abundance of genes conferring resistance to sulfadiazine, of MGEs (integron gene cassettes or plasmids), and increased transfer frequencies of exogenously isolated MGEs conferring antibiotic resistances.…”

Transfer of Plasmid pJP4 from <i>Escherichia coli</i> to Activated Sludge Bacteria by Filter Mating

“…However, the introduced population is often unable to adapt and rapidly declines in microbial ecosystems such as activated sludge by biotic and abiotic factors 1,3) . Those experiences suggest that plasmid transfer from introduced bacteria to indigenous predominant bacteria in activated sludge will enhance the bioaugmentation possibility by providing ecologically stable hosts for the plasmid: socalled plasmid-mediated augmentation 2,4) . Furthermore, indigenous transconjugants might highly express metal resistance or contaminant degradation abilities than the introduced donor strain by rearrangement or combination of the plasmid genes with preexisting genes 2) .…”

“…Those experiences suggest that plasmid transfer from introduced bacteria to indigenous predominant bacteria in activated sludge will enhance the bioaugmentation possibility by providing ecologically stable hosts for the plasmid: socalled plasmid-mediated augmentation 2,4) . Furthermore, indigenous transconjugants might highly express metal resistance or contaminant degradation abilities than the introduced donor strain by rearrangement or combination of the plasmid genes with preexisting genes 2) . However, a few studies evaluated the potential for plasmid transfer from introduced donors to indigenous recipients in activated sludge.…”

“…Bacterial inoculants used for bioaugmentation in wastewater treatment often contain such plasmids 1,2) . However, the introduced population is often unable to adapt and rapidly declines in microbial ecosystems such as activated sludge by biotic and abiotic factors 1,3) .…”