High prevalence of Salmonella spp. in wastewater reused for irrigation assessed by molecular methods.

Salmonella spp. is one of the most important causal agents of food-borne illness in developed countries and its presence in irrigation water poses a risk to public health. Its detection in environmental samples is not easy when culture methods are used, and molecular techniques such as PCR or ribosomal rRNA probe hybridization (Fluorescent in situ Hybridization, FISH) are outstanding alternatives. The aim of this work was to determine the environmental risk due to the presence of Salmonella spp. in wastewater by culture, PCR and FISH. A new specific rDNA probe for Salmonella was designed and its efficiency was compared with the rest of methods Serotype and antibiotic resistance of isolated strains were determined. Forty-five wastewater samples (collected from two secondary wastewater treatment plants) were analysed. Salmonella strains were isolated in 24 wastewater samples (53%), two of them after disinfection treatment. Twenty-three Salmonella strains exhibited resistance to one or more antimicrobial agent. Analysis of wastewater samples yielded PCR positive results for Salmonella in 28 out of the 45 wastewater samples (62%). FISH analysis allowed for the detection of Salmonella in 27 (60%) samples. By using molecular methods, Salmonella was detected in four samples after disinfection treatment. These results show the prevalence of Salmonella in reclaimed wastewater even after U.V. disinfection, what is a matter of public health concern, the high rates of resistance to antibiotics and the adequacy of molecular methods for its rapid detection. FISH method, with SA23 probe developed and assayed in this work provides a tool for detecting Salmonella in water within few hours, with a high rate of effectiveness.