Quantitative evaluation of bacteria released by bathers in a marine water

Abstract

Enterococci, a common fecal indicator, and Staphylococcus aureus, a common skin pathogen, can be shed by bathers affecting the quality of recreational waters and resulting in possible human health impacts. Due to limited information available concerning human shedding of these microbes, this study focused on estimating the amounts of enterococci and S. aureus shed by bathers directly off their skin and indirectly via sand adhered to skin. Two sets of experiments were conducted at a marine beach located in Miami-Dade County, Florida. The first study, referred to as the "large pool" study, involved 10 volunteers who immersed their bodies in 4700L during four 15min cycles with exposure to beach sand in cycles 3 and 4. The "small pool" study involved 10 volunteers who were exposed to beach sand for 30min before they individually entered a small tub. After each individual was rinsed with off-shore marine water, sand and rinse water were collected and analyzed for enterococci. Results from the "large pool" study showed that bathers shed concentrations of enterococci and S. aureus on the order of 6x10(5) and 6x10(6) colony forming units (CFU) per person in the first 15min exposure period, respectively. Significant reductions in the bacteria shed per bather (50% reductions for S. aureus and 40% for enterococci) were observed in the subsequent bathing cycles. The "small pool" study results indicated that the enterococci contribution from sand adhered to skin was small (about 2% of the total) in comparison with the amount shed directly from the bodies of the volunteers. Results indicated that bathers transport significant amounts of enterococci and S. aureus to the water column, and thus human microbial bathing load should be considered as a non-point source when designing recreational water quality models.

Fig. 1

Total average enterococci and S. aureus densities (CFU/100mL) in the water column per cycle as observed during the large pool experiment. Error bars correspond to the standard deviation of replicate analyses.

Fig. 2

Mean total bacteria shed per person per 15 min exposure. Error bars correspond to the standard deviation of three replicate samples collected per cycle. Considering all 4 cycles, the overall average shedding of microbes shed per bather were 3 × 10⁶ CFU for S. aureus and 3 × 10⁵ CFU for enterococci.

Fig. 3

Results for group I and group II for the “Small Pool” experiment. (A) Average amount of sand released per subject and, average enterococci density in sand. (B) Average total enterococci from sand, CFU per subject.