There is major international concern over the wide-scale contamination of soil and associated ground water by persistent explosives residues. 2,4,6-Trinitrotoluene (TNT) is one of the most recalcitrant and toxic of all the military explosives. The lack of affordable and effective cleanup technologies for explosives contamination requires the development of better processes. Significant effort has recently been directed toward the use of plants to extract and detoxify TNT. To explore the possibility of overcoming the high phytotoxic effects of TNT, we expressed bacterial nitroreductase in tobacco plants. Nitroreductase catalyzes the reduction of TNT to hydroxyaminodinitrotoluene (HADNT), which is subsequently reduced to aminodinitrotoluene derivatives (ADNTs). Transgenic plants expressing nitroreductase show a striking increase in ability to tolerate, take up, and detoxify TNT. Our work suggests that expression of nitroreductase (NR) in plants suitable for phytoremediation could facilitate the effective cleanup of sites contaminated with high levels of explosives.

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An unstable chromosomal element encoding multiple antibiotic resistance in Shigella flexneri serotype 2a was found to include sequences homologous to the csg genes encoding curli in Escherichia coli and Salmonella enterica serovar Typhimurium. As curli have been implicated in the virulence of serovar Typhimurium, we investigated the csg loci in all four species of Shigella. DNA sequencing and PCR analysis showed that the csg loci of a wide range of Shigella strains, of diverse serotypes and different geographical distributions, were almost universally disrupted by deletions or insertions, indicating the existence of a strong selective pressure against the expression of curli. Strains of enteroinvasive E. coli (EIEC), which share virulence traits with Shigella spp. and cause similar diseases in humans, also possessed insertions or deletions in the csg locus or were otherwise unable to produce curli. Since the production of curli is a widespread trait in environmental isolates of E. coli, our results suggest that genetic lesions that abolish curli production in the closely related genus Shigella and in EIEC are pathoadaptive mutations.

Environmental contamination with recalcitrant toxic chemicals presents a serious and widespread problem to the functional capacity of soil. Soil bacteria play an essential role in ecosystem processes, such as nutrient cycling and decomposition; thus a decrease in their biomass and community diversity, resulting from exposure to toxic chemicals, negatively affects the functioning of soil. Plants provide the primary energy source to soil microorganisms and affect the size and composition of microbial communities, which in turn have an effect on vegetation dynamics. We have found that transgenic tobacco plants overexpressing a bacterial nitroreductase gene detoxify soil contaminated with the high explosive 2,4,6-trinitrotoluene (TNT), with a significantly increased microbial community biomass and metabolic activity in the rhizosphere of transgenic plants compared with wild type plants. This is the first report to demonstrate that transgenic plants engineered for the phytoremediation of organic pollutants can increase the functional and genetic diversity of the rhizosphere bacterial community in acutely polluted soil compared to wild type plants.