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

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Abstract
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. A 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride reduction technique, without added substrate, and a slightly modified nalidixic acid procedure gave values which did not differ at the 5% level. From 10-40% of total bacteria in the samples examined were metabolically active. These values were up to 2 orders of magnitude higher than those obtained by conventional plate count techniques.