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Abundance of culturable versus viable Escherichia coli in freshwater

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Abstract:

Approved methods traditionally used for Escherichia coli enumeration in waters are culture-based. However, these methods can underestimate the E. coli abundance in aquatic systems because they do not take into account cells that remain viable but have lost the ability to grow in or on culture media. We investigated, in freshwater samples, the abundance of (i) culturable E. coli, enumerated by the most probable number microplate method and (ii) viable E. coli, estimated using a procedure called DVC-FISH, which couples fluorescent in situ hybridization (FISH) and a viability testing technique (direct viable count (DVC)). The ratio of culturable to viable E. coli was close to 1 in highly contaminated waters (samples with a high concentration of culturable E. coli), but decreased drastically for weakly contaminated samples. This indicates a large fraction of viable but nonculturable (VBNC) E. coli in the latter samples. Microcosm experiments showed that some environmental factors, such as nutrient scarcity and solar irradiation, could lead to the presence of a high proportion of VBNC E. coli.

Les méthodes approuvées traditionnellement utilisées pour l’énumération des Escherichia coli dans l’eau sont basées sur la mise en culture. Cependant, ces méthodes peuvent sous-estimer l’abondance de E. coli dans les systèmes aquatiques car elles ne prennent pas en compte les cellules qui demeurent viables mais qui ont perdu leur capacité de croître dans ou sur les milieux de culture. Nous avons examiné, dans des échantillons d’eau douce, l’abondance des (i) E. coli cultivables, comptées par la méthode MPN en microplaques, et (ii) E. coli viables, estimées par une méthode appelée DVC-FISH, qui couple l’hybridation in situ en fluorescence et un test de viabilité. Le ratio des cellules E. coli cultivables sur les cellules viables était proche de 1 dans les eaux fortement contaminées (échantillons comportant de hautes concentrations de E. coli cultivables), mais diminuait radicalement dans les échantillons faiblement contaminés. Ceci indique la présence d’une haute proportion de E. coli viables mais non cultivables dans ces échantillons. Des expériences en microcosme ont montré que certains facteurs environnementaux tels que la faible teneur en nutriments et l’irradiation solaire pouvaient mener à la présence d’une haute proportion de E. coli viables mais non cultivables.

Document Type: Research Article

Publication date: July 1, 2009

More about this publication?
  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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