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.
In the past decades, large amounts of non-insecticidal hexachlorocyclohexane (HCH) isomers (alpha-, beta-, delta- and epsilon-HCH) have been dumped as side-products of the insecticide gamma-HCH (lindane). This study investigates the effect of HCH isomers on methane oxidation, an important soil function performed by methanotrophic bacteria. Both activity and structure of the methanotrophic community were assessed, using methane oxidation assays and PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) respectively. Methane oxidation assays with historically polluted soils revealed that on the long-term methane oxidation was inhibited by HCH pollution. PCR-DGGE and diversity analysis based on Lorenz curves showed that the type I methanotrophic community was less evenly distributed in historically HCH-polluted soils compared with less polluted reference soils. Short-term experiments with methane-enriched consortia further demonstrated that only gamma- and delta-isomers inhibited methane oxidation. Type I methanotrophs of methane-enriched microbial consortia that received gamma- or delta-HCH evolved towards higher species richness. Apparently, for historically HCH-polluted soils, a narrow community remained after long-term exposure while in case of short-term exposures, methane-enriched consortia were converted into less active, but richer communities when they were stressed by the presence of gamma- or delta-HCH. This work demonstrates the importance of incorporating all isomers and possible other side-products in risk assessment studies of persistent organic pollutants and the use of structural analysis of type I methanotrophic communities as evaluating tool.
The diversity of bacterial groups of activated sludge samples that received wastewater from four different types of industry was investigated by a nested PCR-DGGE (denaturing gradient gel electrophoresis) approach. Specific 16S rRNA primers were chosen for large bacterial groups (Bacteria and alpha-Proteobacteria in particular), which dominate activated sludge communities, as well as for actinomycetes, ammonium oxidisers and methanotrophs (types I and II). In addition primers for the new Acidobacterium kingdom were used to observe their community structure in activated sludge. After this first PCR amplification, a second PCR with bacterial primers yielded 16S rRNA gene fragments that were subsequently separated by DGGE, thus generating 'group-specific DGGE patterns'. The community structure and diversity of the bacterial groups from the different samples was further analysed using different techniques, such as statistical analysis and Shannon diversity index evaluation of the band patterns. By combining the seven DGGE gels, cluster analysis, multidimensional scaling and principal component analysis clearly clustered two of the four activated sludge types separately. It was shown that the combination of molecular and statistical methods can be very useful to differentiate microbial communities.
Self-healing concrete has been scrutinized by several researchers and some industrial concrete producers in relation to the remediation of the occurrence of micro-cracks. Such cracks are a quite well known problem that can lead to corrosion of the steel reinforcement and thus to the possible failure of the entire concrete structure. The need to repair these cracks as soon as possible leads to maintenance costs which can be of the order of €130 (direct costs) per m3 of concrete. Recent scientific studies indicate that a Microbial Induced Carbonate Precipitation (MICP), using microbial spores as active agent, can be an alternative for the actual repair methods. However, the production of bacterial spores is yet imposing considerable costs. According to some concrete producers they would be willing to pay about €15 to €20 per m3 of concrete for a bio-based self-healing product. However, the actual cost of spores production and encapsulation represent a total cost which is orders of magnitude higher. This article analyzes the costs for the biological self-healing in concrete and evaluates the industrial challenges it faces. There is an urgent need to develop the production of a bio-additive at much lower costs to make the biological self-healing industrial applicable. Axenic production and a possible non-axenic process to obtain ureolytic spores were analyzed and the costs calculations are presented in this paper.
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