Aerobic biodegradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) as a nitrogen source by a Rhodococcus sp., strain DN22

Abstract

An aerobic, Gram-positive bacterium was isolated from explosives-contaminated soil by enrichment culture, using the nitramine explosive, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), as the sole added N source. The organism, identified by 16S rDNA analysis as a Rhodococcus sp., strain DN22, grew exponentially with a mean generation time of 6.5 h at 25°C in minimal medium containing RDX as the sole N source. The growth medium was depleted of RDX 24 h after inoculation but growth did not cease until 20 h later. It was concluded that strain DN22 was using NO − 2, released from RDX, as a growth substrate because, following inoculation of strain DN22 into RDX-containing minimal medium, the concentration of NO − 2 increased during the first 10 h of incubation and declined to undetectable amounts 20 h later. The ratio of the growth yields of strain DN22 grown on RDX or NO − 3 as N sources indicated that three of the six N atoms of RDX were being incorporated into biomass. Increased concentrations of NH + 4 in the growth medium reduced the extent of RDX degradation. Nitrite was produced from RDX by resting cell suspensions grown on RDX, NO − 3, NO − 2 or glutamine as N sources, but not by cells grown on NH + 4–N or in peptone-yeast extract medium. Resting cells grown on RDX showed the highest degradative activity, compared to cells grown on alternative N sources, indicating that the RDX degradation system is inducible. When soil was inoculated with RDX-grown cells of strain DN22rr, a rifampicin-resistant derivative of strain DN22, there was significant biodegradation of RDX. The addition of oat chaff to soil stimulated the growth of strain DN22rr and enhanced biodegradation of RDX, resulting in 90% degradation of the explosive in 21 d.