[Bacterial contamination and presence of Sinorhizobium meliloti in irrigation canal water from the Neuquén River].

A survey of the changes in populations of heterotrophic bacteria, coliform microorganisms and S. meliloti was conducted in samples taken from the water irrigation channels of the Neuquén River (Argentina). Fifty-six water samples were collected during the spring-summer seasons of 1997-1999 years. Both the heterotrophic plate count bacterial and the number of coliforms oscillated between 110-5050 CFU/ml and 8-1400 CFU/100 ml, respectively, during the period this study was carried out. Fecal coliforms were detected in 91.1% of the water samples investigated. Moreover, the results showed that S. meliloti capable of nodulating alfalfa (Medicago sativa L.) Cuf 101 were present in 68% of the water samples and in effectiveness studies, no isolate out of 25 evaluated could be classified as superior N fixers. That is, they did not produce plants equal in weight to nitrate-grown plants (KNO3 0.05%). All the S. meliloti strains were resistant to novobiocin and bacitracin, while 72% of the microsymbionts demonstrated resistance to between seven and ten antibiotics. Results presented in this study showed that irrigation waters of the Neuquén river could act as dispersal agents of both ineffective S. meliloti strains and thermotolerant coliform bacteria.