Enhanced Survival of Escherichia coli in Subtropical Beach Sand and Implications for Water Quality Managers

Abstract

Several health problems such as gastroenteritis, upper respiratory infections and skin, eye, and ear infections can be associated with swimming in polluted waters. In order to assess potential health risks, authorities regularly monitor the quality of recreational waters by analyzing for indicator organisms. Total coliforms, fecal coliforms, enterococci and E. coli, naturally found in the intestinal flora of warmblooded animals, are amongst the most frequently used bacterial indicators for sewage pollution. These organisms are thought to be representative of more virulent pathogens and their densities in the water are indicative of the amount of sewage contamination present. Current guidelines for beaches in the United States recommend routine testing of beach water samples, but risks associated with bacterial numbers in the sand have never been considered. This is surprising because many beachgoers spend most of their time around the sand. As part of an EPA-funded study to investigate microorganisms on the beach, levels of E. coli in the sand and the water were recorded over a lO-month period at two different beaches in South Florida. Densities were consistently higher in the dry sand relative to the wet sand and sand had more bacteria than water. Mesocosm experiments conducted in the laboratory showed that, regardless of conditions, E. coli levels significantly increased within 24 h to 48 h in sterile sand but died when added to sterile seawater. Because of the high sand counts, the potential for bacteria to wash out of sand was studied at Hollywood Beach. Results showed that beach bacteria that grow in situ and wash out into the water influence the densities of E. coli in the water. This has consequences for water managers assessing beach quality. Future protocols for sampling might include water samples collected well away from the swash zone. Attempts were made to isolate a eukaryotic pathogen, Entamoeba histolytica, from wet sand because previous work had shown that sand samples were occasionally positive for E. histolytica using a commercially available test kit. Even although Entamoeba-like organisms were present, ELISA test revealed that these were not E. histolytica. Preliminary experiments were conducted using alternative methods for assessing fecal pollution. The levels of two types of coliphage, somatic and F-specific phage, were determined during the monthly samplings. The numbers did not generally correlate with the levels of E. coli or enterococci. Polymerase Chain Reaction, targeting pathogenic genes of E. coli, was used to screen E. coli colonies from wet sand, dry sand and sewage. Because of the surprisingly high numbers of positive results in the dry sand, further investigation will be needed to determine the significance of the results