The introduction of Escherichia coli into natural seawater results in a direct response from the natural microbial parasites and predators, which increase in number and rapidly destroy the alien coliform bacteria. In a predictable successional sequence, the decline in numbers of E. coli was followed by a reduction in the parasite and predator populations resulting from the absence of suitable host organisms and from predation by larger protozoa.

E. coli numbers declined more rapidly in seawater taken from near a sewage outlet, because the parasites and predators were enriched by the constant availability of host organisms. A slower rate of decline of E. coli numbers in seawater samples taken further from the outlet was attributed to lower initial populations of parasites and predators. Two common marine predators of E. coli, a fruiting myxobacter (Polyangium) and a small amoeba (Vexillifera), have been characterized.