During an outbreak of gastroenteritis in 28 children living in a small neighbourhood of Cuernavaca city (Mexico), a survey was performed to evaluate the confidence in coliform bacteria as sole indicators of potability of drinking waters. A primary infection by E. coli and a secondary by Pseudomonas aeruginosa was diagnosed in five of the children and the drinking water provided by a well was suspected as a transmission source. General and household distribution systems, household filters and bottled waters were evaluated for total and faecal coliforms, family Enterobacteriaceae, Paeruginosa and residual chlorine. In every sample, pathogenic/opportunistic bacteria were isolated even in the absence of coliforms and in the presence of residual chlorine. Arbitrarily assigned “pollution/risk levels” indicated that the most elevated risk was most frequently associated with storage in tanks and with bacterial colonisation in the pipeline system and commercial household filters where high levels of Paeruginosa were determined. A probability of correspondence between the presence of this bacterium and the secondary gastrointestinal infection diagnosed was found pointing towards a need for the inclusion of other microorganisms, one of which may be Paeruginosa, as indicators of health risk associated with drinking waters in Mexico.

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