The modification and evaluation of some cytochemical techniques for the enumeration of metabolically active heterotrophic bacteria in the aquatic environment

Variants of the tetrazolium-reduction, nalidixic acid-inhibition and fluorescein diacetate-hydrolysis techniques for enumeration of metabolically active bacteria were compared, using samples of planktonic, benthic, and epiphytic freshwater bacteria. Results obtained by these methods generally showed statistically significant differences. However, an INT reduction technique, without added substrate, and a slightly modified nalidixic acid procedure gave values which did not differ at the 5% level. The results indicated that from 10 to 40% of total bacteria in the samples examined were metabolically active. These values were up to two orders of magnitude higher than those obtained by conventional plate count techniques.