Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a cyclic nitroamine explosive that is a major component in many military high-explosive formulations. In this study, two aerobic bacteria that are capable of using RDX as the sole source of carbon and nitrogen to support their growth were isolated from surface soil. These bacterial strains were identified by their fatty acid profiles and 16S ribosomal gene sequences as Williamsia sp. KTR4 and Gordonia sp. KTR9. The physiology of each strain was characterized with respect to the rates of RDX degradation and [U- Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a cyclic nitroamine that is one of the most powerful and commonly used military explosives. RDX has contaminated soils and groundwater at army ammunition plants and other military sites through its manufacture, use on testing and firing ranges, and disposal (22). RDX is a toxic compound that affects the central nervous system of laboratory animals, causing convulsions (10, 37), and it has been proposed as a possible human carcinogen (38). Information on the environmental factors that determine the transport and fate of RDX in soils is needed to accurately assess the risk posed by RDX and to ensure the sustainability of live-fire training exercises.Anaerobic biodegradation of RDX has been extensively studied (1,6,21,23,25,26,29,(42)(43)(44)(45). McCormick et al. (29) proposed a pathway where RDX undergoes sequential reduction of the nitro groups to form hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine; hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine; and hexahydro-1,3,5-trinitroso-1,3,5-triazine. According to the proposed pathway, this transformation then produces formaldehyde, methanol, hydrazine, and dimethyl hydrazine. Hawari et al. (23) proposed another pathway where ring cleavage occurs before sequential reduction, producing the metabolites methylenedinitramine and bis(hydroxymethyl)nitramine.Although most of the initial strains isolated could only degrade RDX anaerobically (25,32,41,42), several pure strains have subsequently been isolated that aerobically degrade RDX as a nitrogen source. Binks et al. (8) isolated Stenotrophomonas maltophilia PB1 and tentatively identified an RDX metabolite as methylene-N-(hydroxymethyl)-hydroxylamine-NЈ-(hydroxymethyl)nitroamine. Rhodococcus sp. strain DN22 was isolated by Coleman et al. (12) and was found to produce nitrite as a metabolite. Fournier et al. (19) found that DN22 also produces the metabolites ammonia, nitrous oxide, formaldehyde, carbon dioxide, and a dead-end product with a molecular weight of 119 that was later identified as 4-nitro-2,4-diazabutanal by Bhushan et al. (7). Fournier et al. (19) also proposed a pathway for aerobic RDX degradation in which denitration occurs, followed by spontaneous ring cleavage. Further experiments determined that the enzyme responsible for the degradation of RDX by DN22 is a cytochrome P450 enzyme (7, 13). Rhodococcus rhodochrous strain 11Y, which was isolated by SethSmith et al. (33), was found to produce the metabol...

Cyclic nitramine explosives are synthesized globally mainly as military munitions, and their use has resulted in environmental contamination. Several biodegradation pathways have been proposed, and these are based mainly on end-product characterization because many of the metabolic intermediates are hypothetical and unstable in water. Biodegradation mechanisms for cyclic nitramines include (a) formation of a nitramine free radical and loss of nitro functional groups, (b) reduction of nitro functional groups, (c) direct enzymatic cleavage, (d) alpha-hydroxylation, or (e) hydride ion transfer. Pathway intermediates spontaneously decompose in water producing nitrite, nitrous oxide, formaldehyde, or formic acid as common end-products. In vitro enzyme and functional gene expression studies have implicated a limited number of enzymes/genes involved in cyclic nitramine catabolism. Advances in molecular biology methods such as high-throughput DNA sequencing, microarray analysis, and nucleic acid sample preparation are providing access to biochemical and genetic information on cultivable and uncultivable microorganisms. This information can provide the knowledge base for rational engineering of bioremediation strategies, biosensor development, environmental monitoring, and green biosynthesis of explosives. This paper reviews recent developments on the biodegradation of cyclic nitramines and the potential of genomics to identify novel functional genes of explosive metabolism.

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