Some factors affecting the survival of faecal bacteria in estuarine water

A seasonal variation in the numbers of faecal indicator bacteria in the Derwent Estuary was not observed, although a decrease in numbers along the Estuary associated with a decrease in human population was noted. There were no consistent significant correlations between the numbers of indicator bacteria and predacious microorganisms, temperature, salinity, solar radiation or rainfall. The survival of E. coli in estuarine water samples, however, exhibited a marked seasonal variation. This variation was not correlated with changes in salinity or microbial predators, but appeared to be associated with changes in water temperature with greater survival during the colder, winter months (April-July) than in the warmer, summer months (December-February). There was no significant variation in E. cal: survival or the growth of predators from sites subject to previous sewage pollution to sites free from previous sewage pollution. The introduction of faecal bacteria into estuarine water samples produced a homeostatic response from a sequence of the indigenous microbial predators. These organisms increased markedly in numbers, bringing about a marked decrease and often complete destruction of the prey bacteria. Following the exhaustion of food supply, the predacious microorganisms gradually returned to their original level. In pure culture studies involving individual predator and prey species, a similar pattern of predator growth and prey destruction also occurred. Once prey numbers had been reduced to a certain level, predator numbers also declined as the food supply declined, until the predatory pressure was removed from the prey population, resulting in the cryptic growth of the prey species. Bacterial decline following the inhibition of protozoan predators indicated that bacterial predators also contributed to prey destruction, but in natural estuarine water samples were maintained at lower levels due to \grazing\" by predacious protozoa. The periodic inhibition of protozoan predators revealed that their major effect on the prey population and on bacterial predators was exerted during the first 2 days of a 10 day decline period. The initial concentration of E. coli prey present influenced the size of the predator population and the sequence of microbial predators which developed. The survival of faecal indicator bacteria in separate estuarine water samples varied from one organism to another as follows: Enterobacter aerogenes Streptococcus faecium > E. coli Salmonella typhimurium > Klebsiella pneumoniae. When incubated together prey resistance and prey selection by microbial predators resulted in different prey survival patterns: S. typhimurium > E. coli E. coli > K. pneumoniae and E. coli > S. faecium. E. coli and S. typhimurium exhibited similar survival curves and their presence resulted in the growth of comparable numbers of predacious microorganisms at a range of incubation temperatures. Bacterial decline was found to be dependent on the presence of both bacterial and protozoan predators the latter having a temperature optimum of 15-20°C and the former becoming more important as the incubation temperature increased. The decline of E. coIi cells in estuarine water samples was found to be significantly greater in the presence of both . naturally occurring microbial predators and solar radiation than when each of these factors was acting independently. The effect of solar radiation on microbial. predators was negligible while the resistance of bacteria to light-induced decay varied from one organism to another as follows: S. typhimurium S. faecium E. aerogenes E. herbicola >E. coli > K. - pneumoniae."