Energy content decrease and viable-not-culturable status induced by oxygen limitation coupled to the presence of nitrogen oxides in Rhizobium

Rhizobium "hedysari" HCNT1 and Sinorhizobium meliloti 41 were investigated and compared for their ability to shift from a typical aerobic, growth-supporting metabolism to O2–limiting, low-energy-expending, basal activities. Such metabolic conversion leads bacteria to stop reproduction although allows them to survive. Once anaerobic, both rhizobia started to consume their internal energy budget and most of the cells remained metabolically active for a long time, as revealed by microscope-based analyses. However, although R. "hedysari" HCNT1 also maintained the same number of culturable cells, S. meliloti 41 started to reduce this number almost immediately when anaerobic incubation took place. In the presence of NO2– the ability of R. "hedysari" HCNT1 to restore normal growth reduced drastically. A mutant strain, previously obtained by inactivation of the gene encoding nitrite reductase (nirK), did not burn up internal adenosine triphosphate when exposed to the same O2–limiting conditions in the presence of NO2–. This finding indicated that NO2–-reduction activity in the wild type strain, HCNT1, results in a decrease in cell energy content and culturability. Therefore, R. "hedysari" HCNT1 and S. meliloti 41 follow different paths to reduce the internal energy pool towards the so-called viable-not-culturable state that can be reached within a relatively large interval of internal energy charge, depending upon the bacterial strain.