RELEVANCE OF THE INT TEST RESPONSE AS AN INDICATOR OF ETS ACTIVITY INMONITORING HETEROTROPHIC AEROBIC BACTERIAL-POPULATIONS IN ACTIVATED SLUDGES

The activity of electron transport systems (ETS) assayed with 2-para ( iodo-phenyl)-3(nitrophenyl)-5(phenyl) tetrazolium chloride (INT) has b een considered to be a specific indicator of the respiration of aerobi c microorganisms and an appropriate way to monitor such populations in activated sludges. We have attempted to define the conditions for mea suring ETS activity, as well as its relevance in monitoring these syst ems. We measured INT response and quantified microorganism populations in activated sludges under extreme conditions, such as oxygen starvat ion and deflocculation. We also measured the INT response in each of t he following microbial populations: aerobic bacteria, anaerobic hetero trophic bacteria, protozoans, and metazoans, as well as in fragments o f disrupted bacteria. The results reveal that strictly anaerobic bacte ria in activated sludge also have a positive response to the INT rest, which increases when flocs are dissociated or bacteria are disrupted. The INT reagent seems to have limited access to electron transport ch ains. The INT test in activated sludges thus does not appear to be sui table for monitoring the metabolism of aerobic heterotrophic bacteria, their principal population. The INT test therefore seems to be inappr opriate for detecting a shift towards anaerobiosis. (C) 1998 Elsevier Science Ltd. All rights reserved.