Comparison of two direct-count methods for determining metabolizing bacteria in freshwater.

Planktonic bacteria collected from several freshwater environments and cultured bacteria were used to compare two methods for determining the numbers of metabolizing bacteria. The methods used were (i) reduction of 2-(rho-iodophenyl)-3-(rho-nitrophenyl)-5-phenyl tetrazolium chloride 2-(rho-iodophenyl)-3-(rho-nitrophenyl)-5-phenyl to tetrazolium chloride-formazan and (ii) elongation of cells by using yeast extract and nalidixic acid. No statistically significant difference was found between methods in determining metabolizing bacteria, although significant differences (P < 0.05) were found when comparing numbers of total bacteria. A combination of the two methods yielded significant changes, both positive and negative, in the numbers of metabolizing bacteria.