Phenotypic Characterization of Salmonella and Escherichia coli Strains Isolated from Wastewater and Market Gardening Crops Along the River Chari in N’Djamena, Chad

Abstract

Water and food-borne diseases caused by Salmonella and Escherichia coli are real public health problems. In N'Djamena; Chad, and many similar cities in Africa, urban agriculture routinely is based polluted wastewater to irrigate market garden crops. This untreated wastewater is discharged directly into the river Chari, thus constituting a major factor in the recrudescence of several diseases. The aim of this study was to isolate the Salmonella and Escherichia coli strains in the wastewater discharged into the river Chari in relation to the risks associated with the re-use of such water in market gardening. The wastewater and market garden products were sampled at the following study sites: Gassi slaughterhouse area, Channel of Ardepdjoumal, Channel of roundabout works  and Channel of Military Base Hospital. Salmonella and Escherichia coli were tested using standardized methods NF ISO 6579 and V08-053 respectively. E. coli strains were isolated from wastewater in 29.66% of samples, with loads ranging from 32 to 7.103 CFU/mL, and Salmonella sp. strains in 19.49% of cases. These results showed that the Canal of Base Hospital was the site most contaminated with Salmonella sp. strains (28.57%). Analysis of the raw vegetables showed very high levels of E. coli contamination, with loads ranging from 2.102 to 16.102 CFU/mL. The highest Salmonella contamination rate was observed in samples of rocket (61.53%) and lettuce (50%), mainly from the site near by the Channel of Military Base Hospital. The results from the wastewater were higher than the WHO standards values for market gardening, which are 103 CFU/100 mL of E. coli and no Salmonella. As a result, the re-use of this wastewater in market gardening is not recommended, and its discharge into the river Chari is prohibited by Chad's public health code. Consumption of these market garden products therefore increases the risk of transmitting water-borne and food-borne diseases if consumers do not comply with basic hygiene rules.