Relevance of the INT test response as an indicator of ETS activity in monitoring 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 been considered to be a specific indicator of the respiration of aerobic microorganisms and an appropriate way to monitor such populations in activated sludges. We have attempted to define the conditions for measuring ETS activity, as well as its relevance in monitoring these systems. We measured INT response and quantified microorganism populations in activated sludges under extreme conditions, such as oxygen starvation and deflocculation. We also measured the INT response in each of the following microbial populations: aerobic bacteria, anaerobic heterotrophic bacteria, protozoans, and metazoans, as well as in fragments of disrupted bacteria. The results reveal that strictly anaerobic bacteria in activated sludge also have a positive response to the INT test, which increases when flocs are dissociated or bacteria are disrupted. The INT reagent seems to have limited access to electron transport chains. The INT test in activated sludges thus does not appear to be suitable for monitoring the metabolism of aerobic heterotrophic bacteria, their principal population. The INT test therefore seems to be inappropriate for detecting a shift towards anaerobiosis.