Influence of freshwater sediment on the survival of Escherichia coli and Salmonella sp. as measured by three methods of enumeration.

Growth and survival of enteric bacteria in freshwater sediments should be of concern to public health officials because of potential contributions of bacteria to the water column. Bacteria densities were measured in sediment and water using direct counts (DC), direct viable counts (DVC) and standard plate counts (PC). Both Salmonella sp. and Escherichia coli survived in microcosms containing autoclaved water and sediment for at least 28 d, as measured by all three methods of enumeration. Moreover, when bacteria were enumerated from sediment-containing microcosms, the DC, DVC and PC values were equivalent. Plate counts showed that both organisms were present at high densities on day 56. In addition, parallel platings on MacConkey agar of E. coli from microcosms containing sediment gave results identical to PC values, indicating that E. coli was not stressed in these microcosms. In microcosms containing water only, E. coli densities declined gradually over 2 weeks by all three measures of enumeration. By day 14, only 58% of total DC was viable as measured by DVC. Due to the protective nature of sediments, the use of standard media may be adequate to enumerate E. coli from some freshwater sediments.