Novel Denitrifying Bacterium Ochrobactrum anthropi YD50.2 Tolerates High Levels of Reactive Nitrogen Oxides

Abstract

ABSTRACT Most studies of bacterial denitrification have used nitrate (NO 3 − ) as the first electron acceptor, whereas relatively less is understood about nitrite (NO 2 − ) denitrification. We isolated novel bacteria that proliferated in the presence of high levels of NO 2 − (72 mM). Strain YD50.2, among several isolates, was taxonomically positioned within the α subclass of Proteobacteria and identified as Ochrobactrum anthropi YD50.2. This strain denitrified NO 2 − , as well as NO 3 − . The gene clusters for denitrification ( nar , nir , nor , and nos ) were cloned from O. anthropi YD50.2, in which the nir and nor operons were linked. We confirmed that nirK in the nir - nor operon produced a functional NO 2 − reductase containing copper that was involved in bacterial NO 2 − reduction. The strain denitrified up to 40 mM NO 2 − to dinitrogen under anaerobic conditions in which other denitrifiers or NO 3 − reducers such as Pseudomonas aeruginosa and Ralstonia eutropha and nitrate-respiring Escherichia coli neither proliferated nor reduced NO 2 − . Under nondenitrifying aerobic conditions, O. anthropi YD50.2 and its type strain ATCC 49188 T proliferated even in the presence of higher levels of NO 2 − (100 mM), and both were considerably more resistant to acidic NO 2 − than were the other strains noted above. These results indicated that O. anthropi YD50.2 is a novel denitrifier that has evolved reactive nitrogen oxide tolerance mechanisms.