Pathogenic bacteria excreted in human feces are found in low counts in wastewater and are known to be more sensitive to environmental conditions than Escherichia coli. The latter, excreted both by humans and warm-blooded animals, are used as an indicator of fecal wastes which may contain the pathogen, but the validity of this practice is often questioned. In wells recharged with chlorinated lake water, regrowth of coliform bacteria reached 10⁵/100 ml when the water was re-pumped 7 to 28 days later. Very often, the more sensitive E. coli die away and are replaced by other coliforms, so that counts do not in themselves indicate a change in water quality.

Recently, low numbers of Samonella have been detected with the aid of large sampling procedures. On the other hand, there has been little additional data on the efficiency of wastewater treatment plants. Survival varied from 0.003 up to 29 percent in a recent study. S. typhimurium survived in a sand column up to 44 days, while S. typhi did not last more than seven days. Shigella flexneri, introduced into an experimental oxidation pond, were recovered from the effluent but were killed by application of 8 mg/1 chlorine after a short contact time.

The possibility of using coliphage as a means for identifying E. coli has been considered in an additional investigation. Coliphage are found in wastewater effluents in counts ranging from 10⁴ to 10⁵/100 ml. They are quite durable under adverse conditions and have been found to be highly resistant to chlorination. Enterovirus densities in wastewater vary between 100 and 2000/100 ml. Comparative studies have indicated that coliphage are found whenever enteroviruses are, and their ratio may vary from 1000 to one to as low as ten to one. It is believed that this recent technique employing indicator microorganisms will permit more rapid and precise evaluations of water quality and potential health hazards.