Recent investigations found high abundances of the fecal indicator Escherichia coli in shoreline sand at freshwater beaches, but it is not known whether these high numbers are due to passive filtration/trapping of the bacteria, or to colonization and growth. This study was initiated to test the hypothesis that high abundance can be explained, at least in part, by the ability of E. coli to persist and grow in beach sand. A combination of laboratory and field studies was used to monitor the densities of environmental isolates of E. coli in beach sand. In controlled laboratory microcosm studies using autoclaved beach sand inoculated with E. coli strains previously isolated from ambient beach sand, E. coli densities increased from 2 CFU/g to more than 2 × 105 CFU/g sand after 2 days of incubation at 19°C, and remained above 2 × 105 CFU/g for at least 35 days. In field studies utilizing similarly inoculated beach sand in diffusion chambers incubated at a Lake Huron beach, E. coli also grew rapidly, reaching high densities (approximately 7.5 × 105 CFU/g), and persisting in a cultivable state at high density for at least 48 days. In comparison, E. coli levels in ambient beach sand adjacent to the chambers always had densities <100 CFU/g. Lake Huron beach sand clearly provides nutrients, temperatures, and other conditions needed to support growth of E. coli. The growth of E. coli in sterile sand diffusion chambers to higher levels than occurs in ambient beach sand may indicate the presence in ambient sand of biological controls on bacterial growth, such as predation or competition.
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Vol. 32 • No. 2