A unique intracellular compartment formed during the oligotrophic growth of Rhodococcus erythropolis N9T-4

Rhodococcus erythropolis N9T-4, isolated from stored crude oil, shows extremely oligotrophic features and can grow on a basal medium without any additional carbon, nitrogen, sulfur, and energy sources, but requires CO₂ for its oligotrophic growth. Transmission electron microscopic observation showed that a relatively large and spherical compartment was observed in a N9T-4 cell grown under oligotrophic conditions. In most cases, only one compartment was observed per cell, but in some cases, it was localized at each pole of the cell, suggesting that it divides at cell division. We termed this unique bacterial compartment an oligobody. The oligobody was not observed or very rarely observed in small sizes under nutrient rich conditions, whereas additional carbon sources did not affect oligobody formation. Energy dispersive X-ray spectroscopy analysis revealed remarkable peaks corresponding to phosphorus and potassium in the oligobody. The oligobodies in N9T-4 cells could be stained by Toluidine blue, suggesting that the oligobody is composed of inorganic polyphosphate and is a type of acidocalcisome. Two genes-encoding polyphosphate kinases, ppk1 and ppk2, were found in the N9T-4 genome: ppk1 disruption caused a negative effect on the formation of the oligobody. Although it was suggested that the oligobody plays an important role for the oligotrophic growth, both ppk-deleted mutants showed the same level of oligotrophic growth as the wild-type strain.