The Ac/tc Bacterial Ratio: a Tool for Watershed Quality Management

An inexpensive tool for indicating microbial quality in watersheds is presented that utilizes the conventional total coliform test, comparing the relative concentrations of different colonies that form on a membrane filter fed by m-Endo media. These bacterial colonies can be classified into 3 types; typical (TC), atypical (AC), and background. The ratio of the concentrations of AC colonies to those of TC is related to water quality, fecal loadings, and fecal age. The AC/TC ratio relies upon shifts in populations between indigenous and introduced bacteria, with the indigenous bacteria providing a baseline against which the concentrations of the introduced are evaluated. When the AC/TC ratio is low (<5), fresh fecal material is in the water and pathogen risk can be expected to be higher. As time passes, the AC/TC ratio increases (>20) and can be related to healthier water quality conditions. Different types of runoff have different AC/TC values with human sewage at the lowest end of the spectrum with a value of 1.5 under normal conditions.