Survival of Salmonella species in river water

The survival of four Salmonella strains in river water microcosms was monit ored by culturing techniques, direct counts, whale-cell hybridization, scan ning electron microscopy, and resuscitation techniques via the direct viabl e count method and flow cytometry. Plate counts of bacteria resuspended in filtered and untreated river water decreased several orders of magnitude wi thin the first week of incubation, while they did not decrease as rapidly i n autoclaved water. In situ hybridization studies suggested a rapid decreas e in ribosomal content, as determined by the drastic decrease in the number of detectable cells after 72 h. In contrast, direct counts remained relati vely constant during 45 days in all microcosoms. Although the culturable co unts of two bacterial strains in filtered water after 31 days represented a pproximately 0.001% of the total counts, direct viable counts and resuscita tion studies with a dilution series suggested that the number of viable bac teria was at least four orders of magnitude higher. Additionally, notable c hanges in forward scatter and in nucleic acid content were observed only af ter 4 h of nutrient amendments by flow cytometry. However, cells from the r esuscitation experiments did not grow on solid media unless cell-free super natant from viable cultures was added during the resuscitation period. The results in this study suggest the presence of a not immediately culturable status in Salmonella.