Use of the Direct Viable Count (D.V.C.) For the Assessment of Survival of E. Coli in Marine Environments

The survival of the enterotoxigenic Escherichia coli H 10407 was observed in different marine seawaters, both in situ and in vitro by use of dialysis chambers and microcosms, exposed and not exposed to light. Survival was monitored during several days by measurement of plate counts (culturable bacteria), direct viable counts with yeast extract and nalidixic acid (DVC : viable bacteria), and acridine orange direct counts (total bacteria). Without light, culturable E. coli counts decreased slowly (1.5 log unit), whereas viable and total bacteria counts remained almost the same through 8 days. When light stress emphasized the sea-water stress, the difference between culturable and viable counts increased from the very first two days (4 log units). In all the experiments, the number of viable bacteria remained very high. On the other hand, one experiment using dialysis chambers held in a turbid and brackish estuarine water showed no marked decrease in the initial counts. This better survival was probably due to the presence of organic matter. So culturability and viability of E. coli can change according to its environment. And because of the consequences for sanitary monitoring of seawaters, it must be kept in mind that viability of E. coli can be preserved within several days, in drastic conditions.