Relationship between enterococcal levels and sediment biofilms at recreational beaches in South Florida

Abstract

Enterococci, recommended at the U.S. federal level for monitoring water quality at marine recreational beaches, have been found to reside and grow within beach sands. However, the environmental and ecological factors affecting enterococcal persistence remain poorly understood, making it difficult to determine levels of fecal pollution and assess human health risks. Here we document the presence of enterococci associated with beach sediment biofilms at eight south Florida recreational beaches. Enterococcal levels were highest in supratidal sands, where they displayed a nonlinear, unimodal relationship with extracellular polymeric secretions (EPS), the primary component of biofilms. Enterococcal levels peaked at intermediate levels of EPS, suggesting that biofilms may promote the survival of enterococci but also inhibit enterococci as the biofilm develops within beach sands. Analysis of bacterial community profiles determined by terminal restriction fragment length polymorphisms showed the bacterial communities of supratidal sediments to be significantly different from intertidal and subtidal communities; however, no differences were observed in bacterial community compositions associated with different EPS concentrations. Our results suggest that supratidal sands are a microbiologically unique environment favorable for the incorporation and persistence of enterococci within beach sediment biofilms.

Fig 1

Eight beaches sampled for the south Florida sediment characterization are shown.

Fig 2

Concentration of enterococci in beach sediments from the 10-day study conducted at Hobie Beach (A), the September multiple-beach study (B), and the February multiple-beach study (C).

Fig 3

South Florida beach sand under plane light stereoscope showing a typical mixture of quartz and calcium carbonate grains (A). Biofilms on sand grains were stained with the fluorescent probe wheat germ agglutinin, which binds to EPS (red), and imaged using a confocal laser scanning microscope to examine the EPS coatings on quartz (B) and calcium carbonate (C) grains.

Fig 4

Concentration of EPS extracted from supratidal, intertidal, and subtidal sediments from the 10-day study (A), the September multiple-beach study (B), and the February multiple-beach study (C).

Fig 5

A cross-plot of the log-transformed enterococcal levels and EPS extracted from supratidal sediments from the 10-day study, the September multiple-beach study, and the February multiple-beach study.

Fig 6

Nonmetric multidimensional scaling plot of bacterial communities inhabiting south Florida beach sediments. Plots show the relative similarity of bacterial communities from the supratidal, intertidal, and subtidal zones from the 10-day study (A), the September multiple-beach study (B), and the February multiple-beach study (C).

Fig 7

Bacterial clone libraries of the 16S rRNA gene from the supratidal sediments collected at Hobie Beach during the 10-day study and the multiple-beach study.