AN IMPROVED FILTER METHOD FOR DIRECT VIABLE COUNT OF SALMONELLA IN SEAWATER

The method of Kogure et al. (1984), which employed measurement of enlarged cells in water samples enriched with low concentrations of nutrient, combined with detection by fluorescent antibodies was used to obtain direct viable counts of Salmonella typhimurium. Technical conditions of the method have been optimized for S. typhimurium, i.e., incubation time of 6 h at 37-degrees-C and addition of nalidixic acid at a concentration of 12 mg/l. Natural seawater samples with a salinity of 35 parts per thousand were inoculated with S. typhimurium at concentrations of 10(4)-10(5) cells/ml and kept at 4-degrees-C up to 60 h. When yeast extract and nalidixic acid were added to the seeded samples, elongated cells could not be detected. Minimal inhibitory concentrations of nalidixic acid were higher for S. typhimurium at full strength seawater salinity (250 mg/1) compared with fresh water (15.6 mg/1). This effect on the direct viable count was eliminated by filtering samples, using 0.2-mum pore size Nuclepore black filters and modifying the procedure. The method developed in this study permitted observation of elongated (viable) cells of S. typhimurium in the full strength seawater samples (17.3-28.3% elongated cells). The method is suitable for conditions in which the salinity of the samples poses a problem in obtaining direct viable counts.