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Bioremediation of contaminated sites has been accepted as an efficient and cheaper alternative to physicochemical means of remediation in several cases. Although chemotactic behaviour of many bacteria has been studied earlier and assays have been developed to study bacterial chemotaxis in semi-solid media, this phenomenon has never been demonstrated in soil. For bioremediation application it is important to know whether bacteria actually migrate through the heterogenous soil medium towards a gradient of a particular chemoattractant. In the present study we have successfully demonstrated bacterial chemotaxis of a Ralstonia sp. SJ98 in soil microcosm using qualitative and quantitative plate and tray assays. The migration of bacteria has been established using several methods such as plate counting, vital staining and flow cytometry and slot blot hybridization. A non-chemotactic p-nitrophenol utilizing strain Burkholderia cepacia RKJ200 has been used as negative control. Our work clearly substantiates the hypothesis that chemotactic bacteria may enhance in situ bioremediation of toxic pollutants from soils and sediments. [ABSTRACT FROM AUTHOR]

Biofilms are assemblages of single or multiple populations that are attached to abiotic or biotic surfaces through extracellular polymeric substances. Gene expression in biofilm cells differs from planktonic stage expression and these differentially expressed genes regulate biofilm formation and development. Biofilm systems are especially suitable for the treatment of recalcitrant compounds because of their high microbial biomass and ability to immobilize compounds. Bioremediation is also facilitated by enhanced gene transfer among biofilm organisms and by the increased bioavailability of pollutants for degradation as a result of bacterial chemotaxis. Strategies for improving bioremediation efficiency include genetic engineering to improve strains and chemotactic ability, the use of mixed population biofilms and optimization of physico–chemical conditions. Here, we review the formation and regulation of biofilms, the importance of gene transfer and discuss applications of biofilm-mediated bioremediation processes. [Copyright &y& Elsevier]

The introduction of culture-independent molecular screening techniques, especially based on 16S rRNA gene sequences, has allowed microbiologists to examine a facet of microbial diversity not necessarily reflected by the results of culturing studies. The bacterial community structure was studied for a pesticide-contaminated site that was subsequently remediated using an efficient degradative strain Arthrobacter protophormiae RKJ100. The efficiency of the bioremediation process was assessed by monitoring the depletion of the pollutant, and the effect of addition of an exogenous strain on the existing soil community structure was determined using molecular techniques. The 16S rRNA gene pool amplified from the soil metagenome was cloned and restriction fragment length polymorphism studies revealed 46 different phylotypes on the basis of similar banding patterns. Sequencing of representative clones of each phylotype showed that the community structure of the pesticide-contaminated soil was mainly constituted by Proteobacteria and Actinomycetes. Terminal restriction fragment length polymorphism analysis showed only nonsignificant changes in community structure during the process of bioremediation. Immobilized cells of strain RKJ100 enhanced pollutant degradation but seemed to have no detectable effects on the existing bacterial community structure. [ABSTRACT FROM AUTHOR]

Discusses the designing of safer genetically engineered organisms for environmental release with specific emphasis on the use of bacterial plasmid addiction systems to limit their survival thus minimizing the anticipated risk. Conceptualization of a novel strategy to construct suicidal genetically engineered microorganisms by exploring/combining the knowledge of different plasmid addiction systems.

Biological methods for decontamination promise an improved substitute for ineffective and costly physico-chemical remediation methods, although so far only a fraction of the total microbial diversity (i.e. the culturable fraction with metabolic potential) has been harnessed for this purpose. Exploring and exploiting the ‘overlooked’ genetic resource might ameliorate concerns associated with the degradation of recalcitrant and xenobiotic pollutants that are not degraded or only poorly degraded by known culturable bacteria. Recent advances in the molecular genetics of biodegradation and in knowledge-based methods of rational protein modification provide insight into the development of ‘designer biocatalysts’ for environmental restoration. The application of such genetically engineered microorganisms (GEMs) in the environment has been limited, however, owing to the risks associated with uncontrolled growth and proliferation of the introduced biocatalyst and horizontal gene transfer. Programming rapid death of the biocatalyst soon after the depletion of the pollutant could minimize the risks in developing these technologies for successful bioremediation. [Copyright &y& Elsevier]

Abstract: Use of genetically modified microorganisms (GEMs) for pollution abatement has been limited because of risks associated with their release in the environment. Recent developments in the area of recombinant DNA technologies have paved the way for conceptualizing “suicidal genetically engineered microorganisms” (S-GEMS) to minimize such anticipated hazards and to achieve efficient and safer bioremediation of contaminated sites. Our strategy of designing a novel S-GEM is based on the knowledge of killer–anti-killer gene(s) that would be susceptible to programmed cell death after detoxification of any given contaminated site(s). [Copyright &y& Elsevier]

Biodegradation of p-nitrophenol (PNP), a priority pollutant, was studied as a model system for bioremediation of sites contaminated with nitroaromatic/organic compounds. Bioremediation of PNP-containing soil was first carried out in pots using immobilized and free cells of Arthrnbacter protophormiae RKJ100 in order to ascertain the role of a suitable carrier material. Results showed that stability of the introduced strain was enhanced upon immobilization and that the rate of PNP depletion decreased with increasing depth of soil. Small-scale field studies (in one square meter plots) were then conducted in which PNP-contaminated soil from an agricultural field was bioaugmented with strain RKJ100 under natural environmental conditions. PNP was totally depleted in 5 days by immobilized cells, whereas free cells were able to deplete 75% of PNP in the same time period. The fate of the released strain as monitored by plate counts, hybridization studies, and real- time polymerase chain reaction revealed fairly stable population of the cells upon immobilization on corncob powder throughout the period of study. [ABSTRACT FROM AUTHOR]