Evaluation of the Reliability of Coliform Density Tests

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Abstract
It is the purpose of this paper to compare colony count procedure and multitube dilution procedure as to their theoretical soundness and practical application, and particularly to supplement the pertinent theoretical discussions in 10th Edition, Standard Methods for the Examination of Water and Sewage, by experimental data. Four major objectives were examined. The magnitude of the bias of the MPN test was shown to be of the same order as that predicted by theory, but its magnitude could be shown to vary with the statistical treatment chosen. The dispersion of MPN''s of replicate tests about a "true" density was also shown to approach the theoretical. Reproducibility of direct plate counts was at least 3 times as great as that of the replicate tube procedure. The reproducibility of pairs of MPN determinations approached very closely the theoretical dispersion. The study indicates that the MPN is still the best means available for assessment of sanitary quality but it also indicates the necessity for a method which will embody greater precision than the MPN, with the membrane filter technique offering the greatest potential.

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