The effect of oxidase positive bacteria on total coliform density estimates

The most probable number method for enumerating coliforms was shown to give a false estimate of the coliform density. The inflated estimates were due to lactose positive, non-coliform bacteria whose presence was detected by an oxidase test. Thirty-six percent of all the samples examined contained oxidase positive bacteria that were able to produce gas from lactose. Their presence resulted in coliform density overestimates which ranged from 2 to 33 times the true value. The frequency of occurrence of inaccurate estimates was shown to be affected by water temperature but unrelated to environmental source.