Organic matter quality and survival of coliforms in Low‐Nutritive Waters

Abstract

The objective of this work was to underscore the importance of the quality of dissolved organic matter (DOM) present in specific, low‐nutritive waters conveying organic fractions—of different origins from which an unknown part is likely to pass through the treatment barrier—on coliform bacteria behavior. Ten coliform bacteria species coming from drinking and river waters were inoculated into three water samples from different origins (drinking water, diluted river water after a heavy rainfall, and diluted algal culture supernatant called bloom water) conveying specific organic fractions from which an undefined part was likely to pass through the drinking water treatment process. Inoculation was done in the presence of autochthonous bacteria from the Nancy, France, drinking water supply. Assays were carried out at identical dissolved organic carbon (DOC) concentrations to those of drinking water, i.e., 1.6–1.8 mg/L. Bacterial growth in the three water samples, measured by microscopic counting, was observed. Inversely, bacteria (coliforms and autochthonous heterotrophic bacteria) lost their culturability on nutritive media in less than two days in drinking water, whereas the culturability of these same bacteria was maintained in bloom and river waters for the complete duration of the test (28 days). The proportion of DOM consumed by the bacterial inoculum in the different water samples may explain this difference: 32% of DOC in bloom water, 20% in river water, and only 6% in drinking water. Identification of coliform bacteria species that survived in bloom and river waters emphasized that only four species were culturable: Citrobacter freundii, Citrobacter youngae, Klebsiella oxytoca, and Enterobacter cloacae (dominant species). These results confirm the important effect of the DOM origin on the microbiological stability in low‐nutritive waters such as drinking waters.