Growth and pathogenicity characteristics of Ralstonia solanacearum strain RS1100 in long-term stationary phase culture

Nonsporulating gram-negative bacteria remain metabolically active and also develop increased resistance to a variety of environmental stresses after exponential growth has stopped and cells enter stationary phase. Ralstonia solanacearum is a nonsporulating gram-negative soil-borne pathogen that causes lethal wilt diseases of many plants around the world. Its growth and pathogenicity characteristics for entering longterm stationary phase were investigated by a prolonged 20- day laboratory culture. The data obtained from viability trial revealed that ≈99% of R. solanacearum strain RS1100 (race 1) died at day 14 in SPA medium and then survivors maintained at ≈105 cfu ml –1 until the 20th day. Binary fission was observed and two modes of death procedures were assumed for growth of R. solanacearum under transmission electronic microscope, nucleoid condensation or nucleoid fragmentation and cytolysis. The latent period for R. solanacearum causing plant wilt prolonged while the cell density reduced gradually. The pathogen retained its aggressiveness at low density as 1.13 × 10 5 cfu ml –1 at 20 th day. Both mucoid and nonmucoid forms of R. solanacearum were coexistent throughout the experiment with varying ratio. The results here confirm that upon entering long-term stationary phase R. solanacearum can survive the stress conditions of nutrient starvation during prolonged stationary phase.