Indicator bacteria

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Indicator bacteria are types of bacteria used to detect and estimate the level of fecal contamination of water. They are not dangerous to human health but are used to indicate the presence of a health risk.

Contents

Each gram of human feces contains approximately ~100 billion (1×1011) bacteria. [1] These bacteria may include species of pathogenic bacteria, such as Salmonella or Campylobacter , associated with gastroenteritis. In addition, feces may contain pathogenic viruses, protozoa and parasites. Fecal material can enter the environment from many sources including waste water treatment plants, livestock or poultry manure, sanitary landfills, septic systems, sewage sludge, pets and wildlife. If sufficient quantities are ingested, fecal pathogens can cause disease. The variety and often low concentrations of pathogens in environmental waters makes them difficult to test for individually. Public agencies therefore use the presence of other more abundant and more easily detected fecal bacteria as indicators of the presence of fecal contamination. Aside from bacteria being found in fecal matter, it can also be found in oral and gut contents. [2]

Criteria for indicator organisms

The US Environmental Protection Agency (EPA) lists the following criteria for an organism to be an ideal indicator of fecal contamination:[ citation needed ]

  1. The organism should be present whenever enteric pathogens are present
  2. The organism should be useful for all types of water
  3. The organism should have a longer survival time than the hardiest enteric pathogen
  4. The organism should not grow in water
  5. The organism should be found in warm-blooded animals' intestines.

None of the types of indicator organisms that are currently in use fit all of these criteria perfectly, however, when cost is considered, use of indicators becomes necessary.

Types of indicator organisms

Commonly used indicator bacteria include total coliforms, or a subset of this group, fecal coliforms, which are found in the intestinal tracts of warm blooded animals. Total coliforms were used as fecal indicators by public agencies in the US as early as the 1920s. These organisms can be identified based on the fact that they all metabolize the sugar lactose, producing both acid and gas as byproducts. Fecal coliforms are more useful as indicators in recreational waters than total coliforms which include species that are naturally found in plants and soil; however, there are even some species of fecal coliforms that do not have a fecal origin, such as Klebsiella pneumoniae. Perhaps the biggest drawback to using coliforms as indicators is that they can grow in water under certain conditions.

Escherichia coli (E. coli) and enterococci are also used as indicators.

Current methods of detection

Membrane filtration and culture on selective media

Enterococci colonies growing on a selective agar after membrane filtration. Enterococci colonies.JPG
Enterococci colonies growing on a selective agar after membrane filtration.

Indicator bacteria can be cultured on media which are specifically formulated to allow the growth of the species of interest and inhibit growth of other organisms. Typically, environmental water samples are filtered through membranes with small pore sizes and then the membrane is placed onto a selective agar. It is often necessary to vary the volume of water sample filtered in order to prevent too few or too many colonies from forming on a plate. Bacterial colonies can be counted after 24 to 48 hours depending on the type of bacteria. Counts are reported as colony forming units per 100 mL (cfu/100 mL).

Fast detections using chromogenic substances

One technique for detecting indicator organisms is the use of chromogenic compounds, which are added to conventional or newly devised media used for isolation of the indicator bacteria. These chromogenic compounds are modified to change color or fluorescence by the addition of either enzymes or specific bacterial metabolites. This enables for easy detection and avoids the need for isolation of pure cultures and confirmatory tests. [3]

Application of antibodies

Immunological methods using monoclonal antibodies can be used to detect indicator bacteria in water samples. Precultivation in select medium must preface detection to avoid detection of dead cells. ELISA antibody technology has been developed to allow for readable detection by the naked eye for rapid identification of coliform microcolonies. Other uses of antibodies in detection use magnetic beads coated with antibodies for the concentration and separation of the oocysts and cysts as described below for immunomagnetic separation (IMS) methods. [3]

IMS/culture and other rapid culture-based methods

Immunomagnetic separation involves purified antigens biotinylated and bound to streptoavidin-coated paramagnetic particles. The raw sample is mixed with the beads, then a specific magnet is used to hold the target organisms against the vial wall and the non-bound material is poured off. This method can be used to recover specific indicator bacteria. [3]

Gene sequence-based methods

Gene sequence-based methods depend on the recognition of exclusive gene sequences particular to specific strains of organisms. Polymerase chain reaction (PCR) and fluorescence in situ hybridization (FISH) are gene sequence-based methods currently being used to detect specific strains of indicator bacteria. [3]

Water quality standards for bacteria

Drinking water standards

World Health Organization Guidelines for Drinking Water Quality state that as an indicator organism Escherichia coli provides conclusive evidence of recent fecal pollution and should not be present in water meant for human consumption. [4] In the U.S., the EPA Total Coliform Rule states that a public water system is out of compliance if more than 5 percent of its monthly water samples contain coliforms. [5]

Recreational standards

Early studies showed that individuals who swam in waters with geometric mean coliform densities above 2300/100 mL for three days had higher illness rates. [6] In the 1960s, these numbers were converted to fecal coliform concentrations assuming 18 percent of total coliforms were fecal. Consequently, the National Technical Advisory Committee in the US recommended the following standard for recreational waters in 1968: 10 percent of total samples during any 30-day period should not exceed 400 fecal coliforms/100 mL or a log mean of 200/100 mL (based on a minimum of 5 samples taken over not more than a 30-day period). [7]

Despite criticism, EPA recommended this criterion again in 1976, however, the Agency initiated numerous studies in the 1970s and 1980s to overcome the weaknesses of the earlier studies. In 1986, EPA revised its bacteriological ambient water quality criteria recommendations to include E. coli and enterococci.

Single Sample Maximum Allowable Density per 100 mL
Water TypeIndicatorAcceptable Swimming-Associated Gastroenteritis Rate per 1000 SwimmersSteady State Geometric Mean Indicator Density per 100 mLDesignated Beach Area (upper 75% C.L.)Moderate Full Body Contact Recreation (upper 82% C.L.)Lightly Used Full Body Contact Recreation (upper 90% C.L.)Infrequently Used Full Body Contact Recreation (upper 95% C.L.)
FreshwaterE. coli8126235298409575
enterococci8336178107151
Marine WaterE. coli1935104158276501

[7]

Canada's National Agri-Environmental Standards Initiative's approach to characterizing risks associated with fecal water pollution bacterial water quality at agricultural sites is to compare these sites with those at reference sites away from human or livestock sources. This approach generally results in lower levels if E. coli being used as a standard or “benchmark” based on a study that indicated pathogens were detected in 80% of water samples with less than 100 cfu E. coli per 100 mL. [8]

Risk assessment for exposure to pathogens in recreational waters

Most cases of bacterial gastroenteritis are caused by food-borne enteric microorganisms, such as Salmonella and Campylobacter ; however, it is also important to understand the risk of exposure to pathogens via recreational waters. This is especially the case in watersheds where human or animal wastes are discharged to streams and downstream waters are used for swimming or other recreational activities. Other important pathogens other than bacteria include viruses such as rotavirus, hepatitis A and hepatitis E and protozoa like giardia, cryptosporidium and Naegleria fowleri . [9] Due to the difficulties associated with monitoring pathogens in the environment, risk assessments often rely on the use of indicator bacteria.

The New River as it enters California is dark green, white (foam), and milky brown/green. Fecal coliforms and fecal streptococci have been consistently detected in the New River at the Mexico-US border. Nrborderborderentrythreecolorsmay05-1-.JPG
The New River as it enters California is dark green, white (foam), and milky brown/green. Fecal coliforms and fecal streptococci have been consistently detected in the New River at the Mexico-US border.

Epidemiological studies

In the 1950s, a series of epidemiological studies were done in the US to determine the relationship between water quality of natural waters and the health of bathers. The results indicated that swimmers were more likely to have gastrointestinal symptoms, eye infections, skin complaints, ear, nose, and throat infections and respiratory illness than non-swimmers and in some cases, higher coliform levels correlated to higher incidence of gastrointestinal illness, although the sample sizes in these studies were small. Since then, studies have been done to confirm causative relations between swimming and certain health outcomes. A review of 22 studies in 1998 [10] confirmed that the health risks for swimmers increased as the number of indicator bacteria increased in recreational waters and that E. coli and enterococci concentrations correlated best with health outcomes among all the indicators studied. The relative risk (RR) of illness for swimmers in polluted freshwater versus swimmers in unpolluted water was between 1–2 for the majority of the data sets reviewed. The same study concluded that bacterial indicators were not well correlated to virus concentrations. [10]

Fate and transport of pathogens

Survival of pathogens in waste materials, soil, or water, depends on many environmental factors including temperature, pH, organic matter content, moisture, exposure to light, and the presence of other organisms. [11] Fecal material can be directly deposited, washed into waters by overland runoff, transported through the ground, or discharged to surface waters via sewer lines, pipes, or drainage tiles. Risk of exposure to humans requires:

  1. Pathogens to survive and be present;
  2. Pathogens to recreate in surface waters;
  3. Individuals to come in contact with water for sufficient time, or ingest sufficient volumes of water to receive an infectious dose.

Die-off rates of bacteria in the environment are often exponential, therefore, direct deposition of fecal material into waters generally contribute higher concentrations of pathogens than material that must be transported overland or through the subsurface.

Human exposure

In general, children, the elderly, and immunocompromised individuals require a lower dose of a pathogenic organism in order to contract an infection. Presently there are very few studies which are able to quantify the amount of time people are likely to spend in recreational waters and how much water they are likely to ingest. In general, children swim more often, stay in the water longer, submerge their heads more often, and swallow more water. This makes people more fearful of water in the sea as more bacteria will be growing on and around them.

Quantitative microbiological risk assessment

Quantitative microbiological risk assessments (QMRAs) combine pathogen concentrations in water with dose-response relationships and data reflecting potential exposure to estimate the risk of infection.

Data on water exposure are generally collected using questionnaires, but may also be determined from actual measurements of water ingested, or estimated from previously published data. Respondents are asked to report the frequency and timing and location of exposures, detailed information about the amount of water swallowed and head submersion, and basic demographic characteristics such as age, gender, socioeconomic status and family composition. Once sufficient data are collected and determined to be representative of the general population, they are usually fit with distributions, and these distribution parameters are then used in the risk assessment equations. Monitoring data representing occurrence of pathogens, direct measurement of pathogen concentrations, or estimations deriving pathogen concentrations from indicator bacteria concentrations, are also fit with distributions. Dose is calculated by multiplying the concentration of pathogens per volume by volume. Dose-responses can also be fit with a distribution. [12]

Risk management and policy implications

The more assumptions that are made, the more uncertain estimates of risk related to pathogens will be. However, even with considerable uncertainty, QMRAs are a good way to compare different risk scenarios. In a study comparing estimated health risks from exposures to recreational waters impacted by human and non-human sources of fecal contamination, QMRA determined that the risk of gastrointestinal illness from exposure to waters impacted by cattle were similar to those impacted by human waste, and these were higher than for waters impacted by gull, chicken, or pig faeces. [13] Such studies could be useful to risk managers for determining how best to focus their limited resources, however, risk managers must be aware of the limitations of data used in these calculations. For example, this study used data describing concentrations of Salmonella in chicken feces published in 1969. [14] Methods for quantifying bacteria, changes in animal housing practices and sanitation, and many other factors may have changed the prevalence of Salmonella since that time. Also, such an approach often ignores the complicated fate and transport processes that determine bacteria concentrations from the source to the point of exposure.

Addressing bacterial water quality problems

In the US, individual states are allowed to develop their own water quality standards based on EPA's recommendations under the Clean Water Act of 1977. Once water quality standards are approved, states are tasked with monitoring their surface waters to determine where impairments occur, and watershed plans called Total Maximum Daily Loads (TMDLs) are developed to direct water quality improvement efforts including changes to allowable bacteria loading by point sources and recommendations for changes to practices that reduce nonpoint-source contributions to bacteria loads. Also, many states have beach monitoring programs to warn swimmers when high levels of indicator bacteria are detected. [15]

Related Research Articles

<i>Enterococcus</i> Genus of bacteria

Enterococcus is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are gram-positive cocci that often occur in pairs (diplococci) or short chains, and are difficult to distinguish from streptococci on physical characteristics alone. Two species are common commensal organisms in the intestines of humans: E. faecalis (90–95%) and E. faecium (5–10%). Rare clusters of infections occur with other species, including E. casseliflavus, E. gallinarum, and E. raffinosus.

<span class="mw-page-title-main">Drinking water</span> Water safe for consumption

Drinking water or potable water is water that is safe for ingestion, either when drunk directly in liquid form or consumed indirectly through food preparation. It is often supplied through taps in which case it is also called tap water. Typically in developed countries, tap water meets drinking water quality standards, even though only a small proportion is actually consumed or used in food preparation. Other typical uses for tap water include washing, toilets, and irrigation. Greywater may also be used for toilets or irrigation. Its use for irrigation however may be associated with risks.

<span class="mw-page-title-main">Water quality</span> Assessment against standards for use

Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through treatment of the water, can be assessed. The most common standards used to monitor and assess water quality convey the health of ecosystems, safety of human contact, extent of water pollution and condition of drinking water. Water quality has a significant impact on water supply and oftentimes determines supply options.

<span class="mw-page-title-main">Water pollution</span> Contamination of water bodies

Water pollution is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water pollution results when contaminants mix with these water bodies. Contaminants can come from one of four main sources: sewage discharges, industrial activities, agricultural activities, and urban runoff including stormwater. Water pollution is either surface water pollution or groundwater pollution. This form of pollution can lead to many problems, such as the degradation of aquatic ecosystems or spreading water-borne diseases when people use polluted water for drinking or irrigation. Another problem is that water pollution reduces the ecosystem services that the water resource would otherwise provide.

<span class="mw-page-title-main">Bacteriological water analysis</span>

Bacteriological water analysis is a method of analysing water to estimate the numbers of bacteria present and, if needed, to find out what sort of bacteria they are. It represents one aspect of water quality. It is a microbiological analytical procedure which uses samples of water and from these samples determines the concentration of bacteria. It is then possible to draw inferences about the suitability of the water for use from these concentrations. This process is used, for example, to routinely confirm that water is safe for human consumption or that bathing and recreational waters are safe to use.

<span class="mw-page-title-main">Coliform bacteria</span> Group of bacterial species

Coliform bacteria are defined as either motile or non-motile Gram-negative non-spore forming bacilli that possess β-galactosidase to produce acids and gases under their optimal growth temperature of 35–37 °C. They can be aerobes or facultative aerobes, and are a commonly used indicator of low sanitary quality of foods, milk, and water. Coliforms can be found in the aquatic environment, in soil and on vegetation; they are universally present in large numbers in the feces of warm-blooded animals as they are known to inhabit the gastrointestinal system. While coliform bacteria are not normally causes of serious illness, they are easy to culture, and their presence is used to infer that other pathogenic organisms of fecal origin may be present in a sample, or that said sample is not safe to consume. Such pathogens include disease-causing bacteria, viruses, or protozoa and many multicellular parasites.

The coliform index is a rating of the purity of water based on a count of fecal bacteria. It is one of many tests done to assure sufficient water quality. Coliform bacteria are microorganisms that primarily originate in the intestines of warm-blooded animals. By testing for coliforms, especially the well known Escherichia coli, which is a thermotolerant coliform, one can determine if the water has possibly been exposed to fecal contamination; that is, whether it has come in contact with human or animal feces. It is important to know this because many disease-causing organisms are transferred from human and animal feces to water, from where they can be ingested by people and infect them. Water that has been contaminated by feces usually contains pathogenic bacteria, which can cause disease. Some types of coliforms cause disease, but the coliform index is primarily used to judge if other types of pathogenic bacteria are likely to be present in the water.

A fecal coliform is a facultatively anaerobic, rod-shaped, gram-negative, non-sporulating bacterium. Coliform bacteria generally originate in the intestines of warm-blooded animals. Fecal coliforms are capable of growth in the presence of bile salts or similar surface agents, are oxidase negative, and produce acid and gas from lactose within 48 hours at 44 ± 0.5°C. The term thermotolerant coliform is more correct and is gaining acceptance over "fecal coliform".

An anaerobic lagoon or manure lagoon is a man-made outdoor earthen basin filled with animal waste that undergoes anaerobic respiration as part of a system designed to manage and treat refuse created by concentrated animal feeding operations (CAFOs). Anaerobic lagoons are created from a manure slurry, which is washed out from underneath the animal pens and then piped into the lagoon. Sometimes the slurry is placed in an intermediate holding tank under or next to the barns before it is deposited in a lagoon. Once in the lagoon, the manure settles into two layers: a solid or sludge layer and a liquid layer. The manure then undergoes the process of anaerobic respiration, whereby the volatile organic compounds are converted into carbon dioxide and methane. Anaerobic lagoons are usually used to pretreat high strength industrial wastewaters and municipal wastewaters. This allows for preliminary sedimentation of suspended solids as a pretreatment process.

Wilderness-acquired diarrhea is a variety of traveler's diarrhea in which backpackers and other outdoor enthusiasts are affected. Potential sources are contaminated food or water, or "hand-to-mouth", directly from another person who is infected. Cases generally resolve spontaneously, with or without treatment, and the cause is typically unknown. The National Outdoor Leadership School has recorded about one incident per 5,000 person-field days by following strict protocols on hygiene and water treatment. More limited, separate studies have presented highly varied estimated rates of affliction that range from 3 percent to 74 percent of wilderness visitors. One survey found that long-distance Appalachian Trail hikers reported diarrhea as their most common illness. Based on reviews of epidemiologic data and literature, some researchers believe that the risks have been over-stated and are poorly understood by the public.

<span class="mw-page-title-main">Simmons' citrate agar</span> Differential culture medium

Simmons' citrate agar is used for differentiating gram-negative bacteria on the basis of citrate utilization, especially for distinguishing Gammaproteobacteria of the family Enterobacteriaceae or even between species of the same genus. For example, Salmonella enteritidis would yield a positive (blue) result on Simmons’ agar and thus be distinguished from other Salmonella species like Salmonella typhi, Salmonella pullorum, and Salmonella gallinarum, which would yield a negative (green) result.

<span class="mw-page-title-main">Food microbiology</span> Study of the microorganisms that inhibit, create, or contaminate food

Food microbiology is the study of the microorganisms that inhabit, create, or contaminate food. This includes the study of microorganisms causing food spoilage; pathogens that may cause disease ; microbes used to produce fermented foods such as cheese, yogurt, bread, beer, and wine; and microbes with other useful roles, such as producing probiotics.

<span class="mw-page-title-main">Water testing</span> Procedures used to analyze water quality

Water testing is a broad description for various procedures used to analyze water quality. Millions of water quality tests are carried out daily to fulfill regulatory requirements and to maintain safety.

Indicator organisms are used as a proxy to monitor conditions in a particular environment, ecosystem, area, habitat, or consumer product. Certain bacteria, fungi and helminth eggs are being used for various purposes.

<span class="mw-page-title-main">Freshwater environmental quality parameters</span>

Freshwater environmental quality parameters are those chemical, physical or biological parameters that can be used to characterise a freshwater body. Because almost all water bodies are dynamic in their composition, the relevant quality parameters are typically expressed as a range of expected concentrations.

<span class="mw-page-title-main">Environmental monitoring</span> Monitoring of the quality of the environment

Environmental monitoring describes the processes and activities that need to take place to characterize and monitor the quality of the environment. Environmental monitoring is used in the preparation of environmental impact assessments, as well as in many circumstances in which human activities carry a risk of harmful effects on the natural environment. All monitoring strategies and programs have reasons and justifications which are often designed to establish the current status of an environment or to establish trends in environmental parameters. In all cases, the results of monitoring will be reviewed, analyzed statistically, and published. The design of a monitoring program must therefore have regard to the final use of the data before monitoring starts.

<span class="mw-page-title-main">Groundwater pollution</span> Ground released seep into groundwater

Groundwater pollution occurs when pollutants are released to the ground and make their way into groundwater. This type of water pollution can also occur naturally due to the presence of a minor and unwanted constituent, contaminant, or impurity in the groundwater, in which case it is more likely referred to as contamination rather than pollution. Groundwater pollution can occur from on-site sanitation systems, landfill leachate, effluent from wastewater treatment plants, leaking sewers, petrol filling stations, hydraulic fracturing (fracking) or from over application of fertilizers in agriculture. Pollution can also occur from naturally occurring contaminants, such as arsenic or fluoride. Using polluted groundwater causes hazards to public health through poisoning or the spread of disease.

The Beaches Environmental Assessment and Coastal Health Act of 2000, or BEACH Act, is a United States federal statute that sets national standards for recreational water testing and authorizes grants to pay for beach monitoring programs at state and federal levels. The Act was signed by President Bill Clinton on October 10, 2000. The law amends the Clean Water Act and requires the Environmental Protection Agency (EPA) to set standard criteria for testing, monitoring, and notifying the public of possible pollution within coastal recreational waters. Water pollution levels are required to be monitored regularly for bacteria, such as Escherichia coli (E.coli), and other pathogen indicators. Agencies at local, state, and federal levels report and monitor the levels and post warning signs as necessary.

Diving equipment may be exposed to contamination in use and when this happens it must be decontaminated. This is a particular issue for hazmat diving, but incidental contamination can occur in other environments. Personal diving equipment shared by more than one user requires disinfection before use. Shared use is common for expensive commercial diving equipment, and for rental recreational equipment, and some items such as demand valves, masks, helmets and snorkels which are worn over the face or held in the mouth are possible vectors for infection by a variety of pathogens. Diving suits are also likely to be contaminated, but less likely to transmit infection directly.

<span class="mw-page-title-main">Pollution in Door County, Wisconsin</span> Overview of pollution in Door County, Wisconsin, United States

Pollution in Door County, Wisconsin relates to the degree of pollution in the air, water, and land in Door County, Wisconsin. Pollution is defined as the addition of any substance or any form of energy to the environment at a faster rate than it can be dispersed, diluted, decomposed, recycled, or stored in some harmless form.

References

  1. ↑ Beactiviahealth. "Intestinal Microflora". Activia. Archived from the original on 2012-04-25.
  2. ↑ Isolation and characterization of bacteria residing in the oral, gut, and fecal samples of different pheasant species
  3. 1 2 3 4 Ashbolt, N., Snozzi, G. and M. (2001). Water Quality: Guidelines, Standards and Health. World Health Organization (WHO). Chapter 13: Indicators of Microbial Water Quality. p. 289-316
  4. ↑ Guidelines for drinking-water quality Chapter 7 Microbial Aspects (PDF) (Fourth ed.). Geneva: World Health Organization. 2017. p. 148. ISBN   978-92-4-154995-0 . Retrieved 12 December 2022.
  5. ↑ "Revised Total Coliform Rule And Total Coliform Rule". Drinking Water Requirements for States and Public Water Systems. Washington, DC: U.S. Environmental Protection Agency (EPA). 2022-03-30.
  6. ↑ Stevenson, A (1953). "Studies of Bathing Water Quality and Health". American Journal of Public Health and the Nation's Health. 43 (5): 529–538. doi:10.2105/ajph.43.5_pt_1.529. PMC   1620266 . PMID   13040559.
  7. 1 2 EPA (1986). "Ambient Water Quality Criteria for Bacteria – 1986" (PDF). Document no. EPA-440/5-84-002.
  8. ↑ Edge, TA; El-Shaarawi, A.; Gannon, V.; Jokinen, C.; Kent, R.; Khan, I.U.H.; Koning, W.; Lapen, D.; Miller, J.; Neumann, N.; Phillips, R.; Robertson, W.; Schreier, H.; Scott, A.; Shtepani, I.; Topp, E.; Wilkes, G.; van Bochove, E. (2011). "Investigation of an esherischi coli Environmental Benchmark for Waterborne Pathogens in Agricultural Watersheds in Canada". Journal of Environmental Quality. 40: x.
  9. ↑ "Waterborne Pathogens". Montana State. Retrieved 14 August 2016.
  10. 1 2 Pruss, A (1998). "Review of epidemiological studies on health effects from exposure to recreational water". International Journal of Epidemiology. 27 (1): 1–9. doi:10.1093/ije/27.1.1. PMID   9563686.
  11. ↑ Guan, Tat Yee; R. A. Holley (2003). "Pathogen Survival in Swine Manure Environments and Transmission of Human Enteric Illness – A Review". Journal of Environmental Quality. 32 (2): 383–392. doi:10.2134/jeq2003.0383.
  12. ↑ Schets, Franciska M.; Schijven, Jack F.; de Roda Husman, Ana Maria (2011). "Exposure assessment for swimmers in bathing waters and swimming pools". Water Research. 45 (7): 2392–2400. Bibcode:2011WatRe..45.2392S. doi:10.1016/j.watres.2011.01.025. PMID   21371734.
  13. ↑ Soller, Jeffrey A.; Mary E. Schoen; Timothy Bartrand; John E. Ravenscroft; Nicholas J. Ashbolt (2010). "Estimated human health risks from exposure to recreational waters impacted by human and non-human sources of faecal contamination". Water Research. 44 (16): 4674–4691. Bibcode:2010WatRe..44.4674S. doi:10.1016/j.watres.2010.06.049. PMID   20656314.
  14. ↑ Kraft, D. J.; Carolyn Olechowski-Gerhardt; J. Berkowitz; and M.S. Finstein (1969). "Salmonella in Wastes Produced at Commercial Poultry Farms". Applied Microbiology. 18 (5): 703–707. doi: 10.1128/AEM.18.5.703-707.1969 . PMC   378074 . PMID   5370457.
  15. ↑ EPA (18 January 2013). "Beach Monitoring & Notification" . Retrieved 2014-05-31.
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