A microcosm study of the survival of Escherichia coli and Salmonella typhimurium in brackish water

The study of the survival of Escherichia coli and Salmonella typhimurium in sewage treated by stabilization ponds, and then mixed more or less rapidly with coastal marine water, has been experimentally tested in microcosms (membrane diffusion chambers). The results obtained show that under the same experiment conditions (slow saline, marine and rapid saline stress) the survival time of the two bacteria varies considerably. In sewage treated by stabilization ponds, Salmonella typhimurium and Escherichia coli exhibit the similar survival rates. T90 evaluated for Escherichia coli and Salmonella typhimurium in the winter period was, respectively, 125 and 132 h. In brackish water the survival rate of Salmonella typhimurium was generally higher than that of Escherichia coli. When these two bacteria were suspended in stabilization ponds effluent and rapidly mixed with brackish water, survival time was particularly short, whereas it was prolonged when the bacteria were submitted to a gradual increase in salinity. Biological factors, and in particular predation, can play an important role in eliminating enteric bacteria during pond system treatment, but their efficiency is supposed to diminish significantly when there is an environmental change such as the discharge of pond system effluent into brackish water. This can explain the increase of enteric bacteria survival.