THE QUALITY OF WATER AND CONFIRMATORY TESTS FOR B. COLI.

THE QUALITY OF WATER AND CONFIRMATORY TESTS FOR B. COLI.

The reliability of the presumptive test as an index of the presence of B. coli in water in the light of recent investigation, has been seriously questioned. The United States Hygienic Laboratory in its Potomac River report finds that the presumptive test varies in its importance as a measure of pollution, with the degree of pollution itself. Dr. Frost, in the Ohio River investigation, concludes that the error in the presumptive test is greatest in the examination of treated waters. A. H. Creel and Edward Bartow, working on waters of similar character, that is, on drinking water from railroad trains, obtained results entirely at variance. The former, for instance, confirmed only 21 per cent. of positive tests for gas formation, while the latter isolated B. coli in 83 per cent, of the tubes showing gas formation. Graf and Nolte conclude, on the other hand, that the bile test is a better index of the presence of B. coli the more polluted the water.

The writer, having at hand analyses of various types of waters in the State of Maryland, made during periods in 1915 and 1916, thought it would be of interest to tabulate the results obtained in colon determinations and to attempt to draw possible conclusions regarding the efficacy of presumptive tests as an index of the presence of B. coli. In the table the results for the raw waters and plant effluents of three filtration plants have been tabulated. The raw waters of all three are from surface streams, the pollution of the watersheds of each varying in intensity according to the rating given in the table. This rating has been based upon a knowledge of sanitary conditions resulting from a study of the three watersheds. It is of striking interest to note that the percentage of tubes confirmed arrange themselves in the same grouping,—A, B, C—as the sanitary survey of the streams, in a qualitative manner only, had already indicated. If we were to attempt to rate these raw waters by presumptive tests alone, it is at once obvious that the arrangement would be different and apparently not as accurate, since a study .of the isolation tests indicates that the measure of the pollution of the streams varies in the same way as determined upon by the percentage of tubes confirmed. In the effluent samples from these same plants, the order of decreasing purity, as determined by presumptive tests, is C, A, B, by isolation tests—A, C, B, and by percentage of tubes confirmed—A, B, C. Assuming that isolation tests for B. coli serve as a legitimate quantitative index of pollution, an apparent discrepancy in the scoring of the plant effluents would seem to appear. The explanation for such a discrepancy, however, would seem to the writer to be due to the insufficient number of tubes showing gas in plant B effluent, making its use of little statistical value.