Detection of salmonella in shellfish grown in polluted seawater

Abstract

Three bays along the South African coast were studied for the presence of Salmonella spp in seawater, effluent and storm water discharges into the bays and in shellfish harvested at the same sites. The microbial quality of water and shellfish samples was studied using faecal coliforms and enterococci as indicators of pollution. A combination of Rappaport-Vassiliadis enrichment broth and bismuth sulphite agar was found to provide the best enumeration method for Salmonella spp in samples from a wide range of origins. A poor relationship between levels of indicator organisms and the presence of Salmonella spp was observed at both low and high densities of indicators. A correlation between the presence of Salmonella spp in shellfish and surrounding seawater was frequently found and in most instances Salmonella contamination could be traced to discharges in the vicinity. The findings of this study question the suitability of faecal coliform limits as guidelines for microbial quality of shellfish and shellfish harvesting grounds. Most seawater sites did not comply with the recommended indicator limits. Salmonella spp were detected in seawater and shellfish samples which were free of faecal coliforms.