In several well-authenticated instances intestinal pathogens, including Salmonella species, have been isolated from water in the absence of bacteria, such as Escherichia coli, commonly used to detect faecal pollution. The present study examines certain anaerobic, non-sporing commensals of the intestinal tract as alternative ‘indicator’ bacteria. The numbers of bifidobacteria and Bacteroides fragilis recovered from faecal specimens were between 109 and 1010 per gram while the Gram-positive, anaerobic cocci numbered only about 106 per gram. In sewage, this numerical difference was eliminated by a rapid loss of viability of bifidobacteria and B. fragilis so that the counts of all three types of bacteria approximated to those of E. coli. Storage tests with aerated and non-aerated sewage established that further loss of viability of bifidobacteria and B. fragilis occurred only fairly slowly. In samples of water from the Dighty Water and River Tay Estuary, shown to be faecally polluted, the three anaerobic types of bacteria were recovered in numbers roughly equal to those of E. coli and their persistence in the surface of the bed of the Dighty Water was also at least equal to that of E. coli.

These results demonstrated that, in spite of their strictly anaerobic growth requirements, Bifidobacteria, B. fragilis and the Gram-positive, anaerobic cocci persist in aerobic, aqueous environments. If their habitat can be shown to be reasonably restricted to the intestinal tract they are likely, under special circumstances, to be useful indicators of faecal pollution.