Enterococci Densities Research Articles

Environmental Growth of Enterococci and Escherichia coli in Feedlot Catch Basins and a Constructed Wetland in the Absence of Fecal Input.

Population structures of fecal indicator bacteria (FIB) isolated from catch basins, a constructed wetland, and feces from a beef cattle feedlot were compared over a two-year period. Enterococcus hirae accounted for 92% of the fecal isolates, whereas secondary environments were characterized by greater relative abundance of environmentally adapted species including Enterococcus casseliflavus. While enterococci densities in the catch basins and wetland were similar under wet and drought conditions, E. hirae predominated during rainy periods, while E. casseliflavus predominated during drought conditions. Environmentally adapted species accounted for almost half of the erythromycin resistant enterococci isolated from the wetland. Densities of Escherichia coli were also comparable during wet versus drought conditions, and the relative abundance of strains from environmentally adapted clades was greater in secondary environments compared to feces. Unlike enterococci, fewer environmentally adapted E. coli strains were isolated on selective media containing ceftriaxone from the wetland compared to feces, suggesting resistance to this antibiotic may not be well maintained in the absence of selective pressure. Overall, these findings suggest that secondary environments select for environmentally adapted FIB. While these species and clades tend to be of limited clinical relevance, they could potentially serve as reservoirs of antimicrobial resistance.

Microbiological monitoring of seawater and sand of beaches Matinhos, Caiobá e Guaratuba-PR, Brazil

This research aimed to verify the health conditions of seawater and beach sand in the beaches of Matinhos, Caiobá and Guaratuba in Paraná, summer 2004/2005. Total Coliforms (TC), Escherichia coli (EC) and Enterococci (ENT) parameters were evaluated. TC, EC, ENT densities were determined by the Multiple Tube Method and estimated by the most probable number (MPN) using, in that order, chromogenic substrate (X-GAL), fluorogenic substrate MUG, and chromogenic substrate (X-GLU). Only the beaches of Matinhos (sampling point MS5) and Guaratuba (sampling point GA6) have shown bathing water quality. Dry sand presented higher contamination than wet sand, 69.4% and 54.2% respectively. Among the indicators, Enterococci was the most restrictive where 51.4% of the water and 100% of the sand (wet and dry) samples were disqualifieda.

Human health risk implications of multiple sources of faecal indicator bacteria in a recreational waterbody

We simulate the influence of multiple sources of enterococci (ENT) as faecal indicator bacteria (FIB) in recreational water bodies on potential human health risk by considering waters impacted by human and animal sources, human and non-pathogenic sources, and animal and non-pathogenic sources. We illustrate that risks vary with the proportion of culturable ENT in water bodies derived from these sources and estimate corresponding ENT densities that yield the same level of health protection that the recreational water quality criteria in the United States seeks (benchmark risk). The benchmark risk is based on epidemiological studies conducted in water bodies predominantly impacted by human faecal sources. The key result is that the risks from mixed sources are driven predominantly by the proportion of the contamination source with the greatest ability to cause human infection (potency), not necessarily the greatest source(s) of FIB. Predicted risks from exposures to mixtures comprised of approximately 30% ENT from human sources were up to 50% lower than the risks expected from purely human sources when contamination is recent and ENT levels are at the current water quality criteria levels (35 CFU 100 mL-1). For human/non-pathogenic, human/gull, human/pig, and human/chicken faecal mixtures with relatively low human contribution, the predicted culturable enterococci densities that correspond to the benchmark risk are substantially greater than the current water quality criteria values. These findings are important because they highlight the potential applicability of site specific water quality criteria for waters that are predominantly un-impacted by human sources.

Standardization of enterococci density estimates by EPA qPCR methods and comparison of beach action value exceedances in river waters with culture methods

The U.S. EPA has published recommendations for calibrator cell equivalent (CCE) densities of enterococci in recreational waters determined by a qPCR method in its 2012 recreational water quality criteria (RWQC). The CCE quantification unit stems from the calibration model used to estimate enterococci densities in recreational beach waters in the EPA National Epidemiological and Environmental Assessment of Recreational (NEEAR) Water Study and directly informed the derivation of the RWQC recommendations. Recent studies have demonstrated that CCE estimates from the method can vary when using different cultured Enterococcus cell preparations in calibrator samples. These differences have been attributed to differences in the quantities of targeted gene copies (target sequences) that are recovered per nominal calibrator cell by DNA extraction. Standardization of results from the calibration model will require the estimation of target sequence recoveries from the calibrator and water samples. In addition, comparisons of water sample results with the RWQC values will require a knowledge of target sequence recoveries from the NEEAR study calibrator samples. In this study recoveries of target sequences and the mean target sequence/cell ratio for the NEEAR study calibrator samples were retrospectively estimated with a corroborated standard curve. A modification of the calibration model was then used to estimate enterococci target sequence quantities in water samples from eight midwestern U.S. rivers. CCE estimates were obtained by dividing these target sequence quantities by the mean NEEAR study target sequence/cell ratio. This target sequence-based quantification approach resulted in a high degree of agreement in beach action decisions (determinations of whether bacterial fecal indicator densities are above or below RWQC-recommended values) from CCE results of the qPCR method and from culture dependent enumeration of both enterococci and Eschericia coli in the corresponding water samples.

Comparison of Enterococcus quantitative polymerase chain reaction analysis results from fresh and marine waters on two real-time instruments

The U.S. Environmental Protection Agency will be recommending a quantitative polymerase chain reaction (qPCR) method targeting Enterococcus spp. as an option for monitoring recreational beach water quality. A practical consideration for implementation of this and other qPCR methods is whether the results are comparable on different PCR instruments. In this study, quantitative estimates of Enterococcus densities from marine and freshwater samples were determined by the qPCR method from cycle threshold (Ct) measurements obtained on Applied Biosystems StepOnePlus and Cepheid SmartCycler instruments. Three variations of a comparative Ct model, differing in their sources of calibration data, were used in the estimations. Both traditional and Bayesian statistical modeling approaches were examined in the instrument comparisons. The traditional analysis of variance (ANOVA) approach indicated no significant differences (p>0.05) between mean density estimates from the instruments in two of the three model variations. The Bayesian approach indicated that the 95% Bayesian credible intervals of density estimates from the instruments overlapped in all models; however, the uncertainty of the estimates varied depending on the model. These results support the interchangeable use of the two instruments in the method and also illustrate the importance of defining the source of calibration data used in the comparative Ct model.

MPN estimation of qPCR target sequence recoveries from whole cell calibrator samples

DNA extracts from enumerated target organism cells (calibrator samples) have been used for estimating Enterococcus cell equivalent densities in surface waters by a comparative cycle threshold (Ct) qPCR analysis method. To compare surface water Enterococcus density estimates from different studies by this approach, either a consistent source of calibrator cells must be used or the estimates must account for any differences in target sequence recoveries from different sources of calibrator cells. In this report we describe two methods for estimating target sequence recoveries from whole cell calibrator samples based on qPCR analyses of their serially diluted DNA extracts and most probable number (MPN) calculation. The first method employed a traditional MPN calculation approach. The second method employed a Bayesian hierarchical statistical modeling approach and a Monte Carlo Markov Chain (MCMC) simulation method to account for the uncertainty in these estimates associated with different individual samples of the cell preparations, different dilutions of the DNA extracts and different qPCR analytical runs. The two methods were applied to estimate mean target sequence recoveries per cell from two different lots of a commercially available source of enumerated Enterococcus cell preparations. The mean target sequence recovery estimates (and standard errors) per cell from Lot A and B cell preparations by the Bayesian method were 22.73 (3.4) and 11.76 (2.4), respectively, when the data were adjusted for potential false positive results. Means were similar for the traditional MPN approach which cannot comparably assess uncertainty in the estimates. Cell numbers and estimates of recoverable target sequences in calibrator samples prepared from the two cell sources were also used to estimate cell equivalent and target sequence quantities recovered from surface water samples in a comparative Ct method. Our results illustrate the utility of the Bayesian method in accounting for uncertainty, the high degree of precision attainable by the MPN approach and the need to account for the differences in target sequence recoveries from different calibrator sample cell sources when they are used in the comparative Ct method.

Comparison of Enterococcus density estimates in marine beach and bay samples by real-time polymerase chain reaction, membrane filtration and defined substrate testing

Currently, densities of Enterococcus in marine bathing beach samples are performed using conventional methods which require 24h to obtain results. Real-time PCR methods are available which can measure results in as little as 3h. The purpose of this study was to evaluate a more rapid test method for the determination of bacterial contamination in marine bathing beaches to better protect human health. The geometric mean of Enterococcus densities using Enterolert® defined substrate testing and membrane filtration ranged from 5.2 to 150MPN or CFU/100mL and corresponding qPCR results ranged from 6.6 to 1785CCE/100mL. The regression analysis of these results showed a positive correlation between qPCR and conventional tests with an overall correlation (r) of 0.71. qPCR was found to provide accurate and sensitive estimate of Enterococcus densities and has the potential to be used as a rapid test method for the quantification of Enterococcus in marine waters.

Microbial Fecal Indicator Concentrations in Water and Their Correlation to Environmental Parameters in Nine Geographically Diverse Estuaries

Fecal contamination of coastal recreational water is a public health concern, which coastal managers monitor using fecal indicator bacteria (FIB). In this study, 20 stations in nine United States (US) estuaries were monitored over three years for FIB (enterococci, fecal coliforms, Escherichia coli, and Clostridium perfringens) and fecal indicator virus (F+ coliphage and somatic coliphage) concentrations in water. Fecal indicator concentrations were significantly correlated with some environmental variables (rainfall, water temperature, salinity, and sample collection time) across all estuaries. Multilinear models utilizing environmental parameters predicted anywhere from 69% (C. perfringens) to 19% (F+ coliphages) of fecal indicator variability. Coliphage equivalence to US Environmental Protection Agency marine water enterococci criteria was estimated with 95% confidence intervals, though these findings should be interpreted with caution because enterococci densities were weakly positively correlated to coliphage densities (R=0.435 somatic coliphages, R=0.367 F+ coliphages). In a comparison of coliphage methods, enrichment (EPA Method 1601) outperformed single agar layer (EPA Method 1602) and membrane filtration. In summary, the relationships among coliphage methods, between fecal indicators, and their interactions with environmental variables represent findings from a geographically diverse set of US estuaries with different fecal waste sources and loads, which suggests these findings are highly generalizable.

Comparison of Fecal Indicator Bacteria Densities in Marine Recreational Waters by QPCR

The US EPA is currently investigating the use of quantitative PCR (qPCR) analysis techniques to estimate densities of fecal indicator bacteria (FIB) in recreational waters. Present water quality guidelines, based on culturable FIB, prevent same day water quality determination, whereas results from qPCR-based approaches are available within several hours. Epidemiological studies at Publicly-Owned Treatment Works (POTW)-impacted freshwater beaches have also indicated correlations between qPCR determined Enterococcus densities and swimming-related illness rates. Similar qPCR assays are now available for several other accepted or emerging FIB groups. This study provides an initial assessment of qPCR estimated Enterococcus, Bacteroidales, E. coli and Clostridium spp. densities in marine water and sand samples collected over one summer from two POTW-impacted recreational beaches. Relative target sequence densities of these organisms in the samples did not correspond with their relative estimated cell densities. These observations were attributable to differences in target sequences recovered from the calibrator cells of the different types of organisms. Comparative cycle threshold (CT) qPCR analyses of whole cell calibrator samples provide a simple and standardizable approach for estimating both total cell and target sequence densities of different types of FIB in water. Cell density estimates obtained by this approach are subject to uncertainty due to potential variability in absolute numbers of target sequences in the target organisms under different physiological or environmental conditions, but still may allow for informative comparisons with the target sequence estimates.

Modeling Enterococcus densities measured by quantitative polymerase chain reaction and membrane filtration using environmental conditions at four Great Lakes beaches

Data collected by the US Environmental Protection Agency (EPA) during the summer months of 2003 and 2004 at four US Great Lakes beaches were analyzed using linear regression analysis to identify relationships between meteorological, physical water characteristics, and beach characteristics data and the fecal indicator bacteria, Enterococcus. Water samples were analyzed for Enterococcus densities by quantitative polymerase chain reaction (qPCR) and membrane filtration (MF). This paper investigates the ability of regression models to accurately predict Enterococcus densities above or below a threshold value, using environmental data on a beach-by-beach basis for both methods. The ability to create statistical models for real-time water quality analysis would allow beach managers to make more accurate decisions regarding beach safety. Results from linear regression models indicate that environmental factors explain more of the variability in Enterococcus densities measured by MF than Enterococcus densities measured by qPCR. Results also show that models for both methods did not perform well at predicting occurrences in which water quality levels exceeded a threshold.

Mining and urban impacts on semi-arid freshwater aquatic systems: the example of Mount Isa, Queensland

This paper examines the environmental risk and impact of trace metals affecting river water and sediment in and around Mount Isa, Queensland, Australia. Bacterial indicator densities are also analysed throughout the catchment to assess the impacts and the potential hazards arising from agricultural activities, sewage treatment plant releases and urban runoff. The area is drained by the ephemeral Leichhardt River, which bisects Mount Isa City and the major Pb, Zn, Cu and Ag Mount Isa Mine. Runoff is captured downstream in Lake Moondarra, with the water being used following natural filtration via a lagoon-reed bed system for potable purposes by the residents of Mount Isa City. During the dry season, the channel is characterised by numerous pools that act as storage zones for sediment and water-soluble metals as well as urban and agriculturally derived nutrients and pathogens. Our results show that sediment and water quality within the Leichhardt River adjacent to and downstream of the mine frequently exceed Australian government sediment guidelines with average values of Cu, Pb and Zn found adjacent to the footprint of the mine being 1550, 510 and 470 mg kg(-1), respectively. Dry season analysis of water-soluble Cu, Pb and Zn concentrations within pools showed that Australian government low trigger guidelines are exceeded in 100, 46 and 100% cases, respectively. The densities of bacterial indicators in remnant pools throughout the Leichhardt River also exceeded acceptable guidelines. Maximum dry season faecal coliform densities of 2000 colony forming units (CFU) per 100 mL and Enterococcus counts of 900 organisms per 100 mL were recorded in dry season remnant pools compared to wet season maximum faecal coliform and Enterococcus densities of 119 000 CFU per 100 mL and 95 000 organisms per 100 mL, respectively. The impacts on biota were also examined by assessing the metal content of the tissue of seven fish from Lake Moondarra for their Cd, Cu, Hg, Pb and Zn concentrations. Tissue metal values were generally low with only a few samples having Cu and Zn values in excess of the recommended Australian retail guideline values for fish suitable for human consumption. The effectiveness of the City's natural reed bed potable water processing facility for removing metals held within the water body (soluble and particulate) was also assessed. The results show that the treated water generally meets Australian drinking water guidelines. Overall it is clear that sediment and water quality in the area upstream of the potable water storage zone is seriously impaired. The combined effects of industrial, agricultural and urban activities present a considerable risk to the consistency and reliability of water quality, especially during highly turbid wet season flows, which transfer highly elevated concentrations of water-soluble metal and microbial contaminants.

Quantification of enterococci and bifidobacteria in Georgia estuaries using conventional and molecular methods

Fecal pollution is a serious threat to the estuarine environment along the Georgia coast. Culture-dependant and molecular methodologies were utilized to compare and evaluate the abundance of fecal indicator bacteria in four Georgia estuaries (Darien River, Frederica River, Gulley Hole Creek, and St. Marys River). The functionality of enterococci and bifidobacteria as indicator organisms in marine environments was assessed, as well as Bifidobacterium adolescentis densities. At each study site, enterococci were enumerated as colony forming units (CFU) on mEI agar. For quantitative polymerase chain reaction (qPCR), genus- and species-specific primer sets were used to quantify bifidobacteria and B. adolescentis as 16S rRNA gene copies and enterococci as tuf gene copies. A high correlation ( r = 0.925) was observed between CFU and qPCR enumeration of enterococci. Enterococci densities in the estuarine rivers ranged from 3–449 CFU/100 ml on mEI plates and 4.58–5.39 Log 10 gene copies/100 ml by qPCR. Bifidobacteria densities ranged from 3.62–4.14 Log 10 gene copies/100 ml and suggested the Frederica River as least affected by fecal bacteria and the Darien River as most affected by fecal pollution. A correlation of 0.46 was observed among qPCR densities of enterococci and bifidobacteria at all sample sites. Quantitative polymerase chain reaction detection of B. adolescentis was a rapid (i.e., less than 2 h) indicator of presumptive human fecal pollution and suggested that Gulley Hole Creek, the Darien River, and the St. Marys River were affected by fecal bacteria derived from a human source. Gulley Hole Creek and the Darien River had the highest levels of fecal pollution detected in the studied estuaries. Molecular quantification of bifidobacteria may be a more accurate method of determining immediate health risks associated with fecal pollution in estuarine water than traditional and contemporary assessments of enterococci.

Effect of the South Bay Ocean Outfall (SBOO) on ocean beach water quality near the USA–Mexico border

In early 1999, primary treatment and discharge of sewage from Tijuana, Mexico (approximately 95 million liters per day) began through South Bay Ocean Outfall (SBOO) into the ocean 4.3 km offshore. In this study, statistical comparisons were made of the bacterial water quality (total and fecal coliforms and enterococci densities) of the ocean, both before and after discharge of sewage to the SBOO began, so that the effect of this ocean discharge on nearshore ocean water quality could be quantitatively assessed. The frequency of exceedence of bacterial indicator thresholds was statistically analyzed for 11 shore (surfzone) stations throughout US and Mexico using the Fisher's exact test, for the years before (1995–1998) as compared to after the SBOO discharge began (1999–2003). Only four of the 11 shoreline stations (S2, S3, S11, and S12) showed significant improvement (decreased frequency of exceedence of bacterial indicator thresholds) after SBOO discharge began.

Intensive water quality monitoring in a Taiwan bathing beach.

A total of 357 water samples were collected from a public beach in northern Taiwan during beach season, and the densities of enterococci were analyzed by Enterolert methods. The mean enterococci level was 356 MPN/100 ml and ranged from <10 to 2,005 MPN/100 ml, which was classified as high contamination level according to the WHO water quality guideline (95 percentile >1,000 MPN/100 ml). Most of the deteriorated water quality conditions occurred during rainfall. By excluding data from the rain days, the overall beach water quality would be considered in the moderate contamination level (95 percentile 200-1,000 MPN/100 ml). Among the selected microbiological parameters tested, the densities of total coliforms and enterococci exhibited the highest correlation (r = 0.449, p = 0.009), followed by the concentrations of total coliforms and fecal coliforms (r = 0.403, p = 0.02). Nonetheless, no significant correlation was found between enterococci and fecal coliform levels (r = 0.197, p = 0.271).

Prevalence and distribution of fecal indicator organisms in South Florida beach sand and preliminary assessment of health effects associated with beach sand exposure

Fecal indicator levels in nearshore waters of South Florida are routinely monitored to assess microbial contamination at recreational beaches. However, samples of sand from the surf zone and upper beach are not monitored which is surprising since sand may accumulate and harbor fecal-derived organisms. This study examined the prevalence of fecal indicator organisms in tidally-affected beach sand and in upper beach sand and compared these counts to levels in the water. Since indicator organisms were statistically elevated in sand relative to water, the study also considered the potential health risks associated with beach use and exposure to sand. Fecal coliforms, Escherichia coli, enterococci, somatic coliphages, and F +-specific coliphages were enumerated from sand and water at three South Florida beaches (Ft. Lauderdale Beach, Hollywood Beach, and Hobie Beach) over a 2-year period. Bacteria were consistently more concentrated in 100 g samples of beach sand (2–23 fold in wet sand and 30–460 fold in dry sand) compared to 100 ml samples of water. Somatic coliphages were commonly recovered from both sand and water while F +-specific coliphages were less commonly detected. Seeding experiments revealed that a single specimen of gull feces significantly influenced enterococci levels in some 3.1 m 2 of beach sand. Examination of beach sand on a micro-spatial scale demonstrated that the variation in enterococci density over short distances was considerable. Results of multiple linear regression analysis showed that the physical and chemical parameters monitored in this study could only minimally account for the variation observed in indicator densities. A pilot epidemiological study was conducted to examine whether the length of exposure to beach water and sand could be correlated with health risk. Logistic regression analysis results provided preliminary evidence that time spent in the wet sand and time spent in the water were associated with a dose-dependent increase in gastrointestinal illness.

Beach Sands along the California Coast Are Diffuse Sources of Fecal Bacteria to Coastal Waters

Fecal indicator bacteria (FIB) are nearly ubiquitous in California (CA) beach sands. Sands were collected from 55 beaches along the CA coast. Ninety-one percent of the beaches had detectable enterococci (ENT) while 62% had detectable E. coli (EC) in their sands. The presence of a putative bacterial source (such as a river), the degree of wave shelter, and surrounding land use explained a significant (p < 0.05) fraction of the variation in both ENT and EC densities between beaches. Sand characteristics including moisture content, organic carbon, and percentfines, significantly (p < 0.05) influenced only EC densities in beach sand. We assayed 34 of 163 sand samples for salmonellae, but did not detect this bacterial pathogen. The potential for FIB to be transported from the sand to sea was investigated at a single wave-sheltered beach with high densities of ENT in beach sand: Lovers Point, CA (LP). We collected samples of exposed and submerged sands as well as water over a 24 h period in order to compare the disappearance or appearance of ENT in sand and the water column. Exposed sands had significantly higher densities of ENT than submerged sands with the highest densities located near the high tide line. Water column ENT densities began low, increased sharply during the first flood tide and slowly decreased over the remainder of the study. During the first flood tide, the number of ENT that entered the water column was nearly equivalent to the number of ENT lost from exposed sands when they were submerged by seawater. The decrease in nearshore ENT concentrations after the initial influx can be explained by ENT die-off and dilution with clean ocean water. While some ENT in the water and sand at LP might be of human origin because they were positive for the esp gene, others lacked the esp gene and were therefore equivocal with respect to their origin. Follow-up sampling at LP revealed the presence of the human specific Bacteroides marker in water and sand.

Enterococci Predictions from Partial Least Squares Regression Models in Conjunction with a Single-Sample Standard Improve the Efficacy of Beach Management Advisories

Beach health advisories are issued if enterococci (ENT) densities exceed the 30-d geometric mean or single-sample water quality criteria. Current ENT enumeration procedures require 1 day of incubation; therefore, beach managers make policy decisions using 1-day-old data. This is tantamount to using a model that assumes ENT density on day t is equal to ENT density on day t-1. Research has shown that ENT densities vary over time scales shorterthan a day, calling into question the usefulness of the current model for decision-making. We created Dynamic Partial Least Square Regression (DPLSR) models for ENT at water quality monitoring stations within two adjacent marine recreational sites, Huntington State Beach (HSB) and Huntington City (HCB) Beach, California, using publicly available environmental data and tested whether these models overcome the drawbacks of the current model. The DPLSR models provide a better prediction of ENT than the current models based on comparisons of root-mean-square errors of prediction and the numbers of type 1 and 2 errors. We compared outcomes in terms of predicted illness, swimmers deterred from entering the water, and net benefits to swimmers for hypothetical management scenarios where beach advisories were issued based on (a) the previously collected sample's ENT density in conjunction with the two water quality criteria, and (b) predictions from DPLSR models in conjunction with the single-sample standard. At both HSB and HCB the DPLSR scenario produced a more favorable balance between illness prevention and recreational access. The results call into question the current method of beach management and show that model-informed decision-making and elimination of the geometric mean standard will aid beach managers in achieving more favorable outcomes in terms of illness and access than are presently achieved using 1-day-old measurements, especially at beaches where water quality problems are chronic.

Spatial and rainfall related patterns of bacterial contamination in Sydney Harbour estuary

Water quality in recreational areas in Sydney Harbour, Australia, was analysed first to identify spatial patterns in faecal coliform and enterococci densities, and then to determine the relationship between bacterial densities and catchment rainfall. Non-metric multidimensional scaling separated sites closest to the mouth of the harbour from those further up the harbour's west and north-west arms. Sites closest to the harbour mouth generally had lower frequencies of high bacterial densities that exceeded median water quality guideline values. We attribute this to greater tidal flushing at sites closer to the harbour mouth. Eight site groups were identified within the harbour. Within each group, multiple regression analyses indicated rainfall accounted for between 15 and 66% of the variability in the bacterial densities. Variation in bacterial densities explained by rainfall was lower for sites closer to the harbour mouth where tidal flushing is greatest. Thus, our findings indicate that simple rainfall-based regression models are appropriate for predicting bacterial concentrations when flushing at a site is limited. More complex models incorporating a suite of environmental variables may improve the ability to predict bacterial concentrations at well-flushed sites, but even then, their predictive ability may be low.

Comment on “Derivation of numerical values for the World Health Organization guidelines for recreational waters”

The subject paper describes a procedure for adjusting a risk model based upon a measure of personal exposure (the “UK personal exposure model”) in order to attribute an expected rate of gastroenteritis among a group of swimmers to a mean recreational water quality value (enterococci per 100 mL). We term the resulting model for group risk the “UK ecologic exposure model.” The distinction is essential to establishing recreational water quality guidelines because exposures of individual bathers are not known from a water monitoring program, the only assessment available being some form of ecologic exposure such as a mean log indicator density. While the authors of the subject paper solved the UK ecologic exposure model for only a single point (that value of mean log 10 enterococcus density which is expected to result in five extra cases of gastroenteritis per 100 swimmers), we extend their model to show the entire curve over a relevant range of densities. The resulting exposure–response curve is seen to not differ substantially from the existing USEPA model for “highly credible gastrointestinal illness” in marine waters. However, particularly since such correspondence is not guaranteed for future studies or for other existing epidemiological studies, we recommend the direct approach to evaluating ecologic exposure, such as used in the USEPA studies, rather than the indirect approach of the UK ecologic exposure model, given the number of untested assumptions that are necessary for accomplishing the latter.

Evaluation of recreational health risk in coastal waters based on enterococcus densities and bathing patterns.

We constructed a simulation model to compute the incidences of highly credible gastrointestinal illness (HCGI) in recreational bathers at two intermittently contaminated beaches of Orange County, California. Assumptions regarding spatial and temporal bathing patterns were used to determine exposure levels over a 31-month study period. Illness rates were calculated by applying previously reported relationships between enterococcus density and HCGI risk to the exposure data. Peak enterococcus concentrations occurred in late winter and early spring, but model results showed that most HCGI cases occurred during summer, attributable to elevated number of exposures. Approximately 99% of the 95,010 illness cases occurred when beaches were open. Model runs were insensitive to 0-10% swimming activity assumed during beach closure days. Comparable illness rates resulted under clustered and uniform bather distribution scenarios. HCGI attack rates were within federal guidelines of tolerable risk when averaged over the study period. However, tolerable risk thresholds were exceeded for 27 total days and periods of at least 6 consecutive days. Illness estimates were sensitive to the functional form and magnitude of the enterococcus density-HCGI relationships. The results of this study contribute to an understanding of recreational health risk in coastal waters.