Byappanahalli, Muruleedhara N., Richard L. Whitman, Dawn A. Shively, W. T. Evert Ting, Charles C. Tseng, and Meredith B. Nevers. 2006. Seasonal persistence and population characteristics of Escherichia coli and enterococci in deep backshore sand of two freshwater beaches. Journal of Water and Health 4 (3): 313-320.
Contribution # 1318
We studied the shoreward and seasonal distribution of E. coli and enterococci in sand (at the
water table) at two southern Lake Michigan beaches桪unbar and West Beach (in Indiana). Deep,
backshore sand (~20m inland) was regularly sampled for 15 months during 2002�03. E. coli
counts were not significantly different in samples taken at 5-m intervals from 0�m inland
(P = 0.25). Neither E. coli nor enterococci mean counts showed any correlation or differences
between the two beaches studied. In laboratory experiments, E. coli readily grew in sand
supplemented with lake plankton, suggesting that in situ E. coli growth may occur when
temperature and natural organic sources are adequate. Of the 114 sand enterococci isolates
tested, positive species identification was obtained for only 52 (46%), with E. faecium
representing the most dominant species (92%). Genetic characterization by ribotyping revealed no
distinct genotypic pattern (s) for E. coli, suggesting that the sand population was rather a mixture
of numerous strains (genotypes). These findings indicate that E. coli and enterococci can occur
and persist for extended periods in backshore sand at the groundwater table. Although this study
was limited to two beaches of southern Lake Michigan, similar findings can be expected at other
temperate freshwater beaches. The long-term persistence of these bacteria, perhaps
independent of pollution events, complicates their use as indicator organisms. Further, backshore
sand at the water table may act as a reservoir for these bacteria and potentially for human
pathogens.