Microbial Contamination of Water Resources in the
Chattahoochee National Recreation Area, Georgia

Microbial Indicators of Water Quality

Bacterial populations are a natural component of lakes, rivers, and streams. These bacteria are a numerous and diverse assemblage of organisms that form an invisible link at the base of aquatic food chains. Over 60 genera of bacteria are present in aquatic systems and numbers can range from forty thousand to over twelve million bacterial cells in an amount of water that will barely cover the bottom of an average-sized coffee cup. The immense numbers of these small organisms can have an enormous impact on processes that occur in aquatic ecosystems such as carbon, nitrogen, and sulfur transformations. They can also have an impact on the quality of water by controlling the amount of oxygen in the water and causing diseases in aquatic organisms as well as in humans. Although relatively rare in the United States, harmful diseases such as cholera and shigellosis are caused by bacteria which can often be traced to sources along rivers and lakes. The majority of microorganisms that cause diseases in humans originate directly from human sources. Human contamination can occur during recreational activities, such as canoeing or swimming, when defecation may occur along a river or stream or directly in the water. However, in densely populated areas such as Metropolitan Atlanta, much of the contamination can be traced to leaking and overflowing sanitary sewer systems, wastewater treatment facilities, leachate from septic tanks, and fecal matter associated with storm runoff from areas with high densities of wildlife, pets, or livestock. Because of the seriousness of the diseases caused by water-born bacteria and the importance of water as a natural and recreational resource, levels of "indicator" bacteria are often monitored. These indicator bacteria usually are harmless, present in higher numbers and originate from the same potential sources as the dangerous disease-causing organisms. Although the presence of indicator bacteria does not prove the presence of pathogenic bacteria, monitoring for indicator bacteria is less expensive and easier than monitoring for pathogenic bacteria and provides a useful indicator of the relative safety for recreational use of a water body. Long before modern methods for testing water and isolating bacteria had been developed, sewage was recognized as a source of pathogenic bacteria. It has been known since the middle ages that water contaminated with sewage could cause sickness and disease, but it wasn't until 1885 that it was discovered that a particular type of bacteria called coliform bacteria were numerous and could always be detected in animal feces and sewage. It is now known that an average of 1.5 billion coliform bacteria are present in one ounce of human feces. In 1899, scientists found that even after sewage had been diluted more than 10,000 times, coliform bacteria could still be detected even though no other known water-quality tests would have indicated the presence of sewage. Although coliform bacteria were not known to cause any illnesses, their presence was thought to be a predictor of other disease-causing agents, and this finding established total coliform bacteria as the first "microbial indicator" of sewage contamination. While no scientific proof relates elevated levels of total coliform bacteria to an increased risk to swimmers, some states adopted standards to inform the public of an increased risk of sewage contamination. The State of Connecticut, for example, in 1932 used a concentration of >1,000 coliforms per 100 total milliliters to warn swimmers of an increased risk of sewage contamination. In 1953 the first study to actually relate the concentration of total coliform bacteria to an increased risk of sickness to swimmers was conducted. The study found an increase in the rate of sickness at concentrations of >2,000 coliforms per 100 ml. This finding established a scientific basis for water-quality standards using total coliform bacteria and led to the general acceptance of total coliform bacteria as an indicator of water pollution with most standards being set at 1,000 total coliforms per 100 ml. Prior to the passage of the Clean Water Act (PL 92-500) and the establishment of the U.S.Environmental Protection Agency (USEPA) in 1972, most water-quality standards were set at 1,000 total coliforms per 100 ml. Additional research on the use of coliform bacteria as a water-quality indicator bacteria, as well as other types of indicator bacteria led to the development of National water-quality standards. The USEPA lowered the total coliform standard of 1,000 total coliform bacteria per 100 ml to 200 total coliform bacteria per 100 ml, to reflect the finding that natural environmental sources contributed total coliform bacteria to surface water and that a type of total coliform called fecal coliforms were more specifically related to human and warm- blooded animal sources. The USEPA conducted extensive studies of water quality as related to the microbial components of a stream and found the other microbes such as E. coli, enterococci, and Clostridium perfringens also could be used as indicators of water quality and developed specific tests and water-quality standards. Early water-quality standards were set based solely on the notion that once it had been proven that indicator bacteria were associated with illness, the water-quality standards would prevent the illness from occurring. In 1986, the USEPA changed its basis for setting water-quality standards that used bacteria as an indicator of water pollution, shifting from the detectable risks to an acceptable risks model. The acceptable risks model recognizes that every activity entails a certain risk and most people are willing to accept a low probability of becoming sick or ill from participating in water-related recreation activities. For example, the current water-quality standard set at 35 enterococci per 100 ml correlates to an expected rate of 19 diarrheal diseases per 1,000 swimmers. In other words if water quality of a particular waterbody is maintained at or below this standard, then swimmers have a 1.9 percent or less chance of becoming ill. The main reason for impairment of the Chattahoochee River and tributaries in Metropolitan Atlanta is consistently high levels of fecal indicator bacteria, especially during the summer when human water contact is highest. This study has been designed to investigate temporal and spatial trends of microbial water-quality indicators in Metropolitan Atlanta streams and rivers using E. coli, fecal-coliform bacteria, enterocci and Clostridium perfringens. Although some of these indicators, such as fecal coliform bacteria, have been used extensively and incorporated into specific water-quality standards, others, such as Clostridium perfringens, have been used only recently. The following is a brief description of each of these indicators and what the presence of high numbers may indicate. Fecal coliform bacteria are a subgroup of coliform bacteria that were used to establish the first microbial water quality standards. The ability to grow at an elevated temperature (44.5 C) separate this bacteria from the total coliforms and make it a more accurate indicator of fecal contamination by warm-blooded animals. Fecal- coliform bacteria are detected by counting the dark-blue to blue-grey colonies that grow on a 0.65 micron filters placed on mFC agar incubated in a 44.5 C oven for 22-24 hours. The presence of fecal coliforms in water indicates that fecal contamination of the water by a warm-blooded animal has occurred, however, recent studies have found no statistical relationship between fecal coliform concentrations and swimmer-associated sickness. Escherichia coli (E. coli) is a rod-shaped bacteria commonly found in the gastrointestinal tract and feces of warm-blooded animals. It is a member of the fecal coliform group of bacteria and is distinguished by its inability to break down urease. E. coli numbers in freshwater are determined by counting the number of yellow and yellow brown colonies growing on a 0.45 micron filter placed on m-TEC media and incubated at 35.0 C for 22-24 hours. The addition of urea substrate confirms that colonies are E. coli. This bacteria is a preferred indicator for freshwater recreation and its presence provides direct evidence of fecal contamination from warm-blooded animals. Although usually harmless, E. coli can cause illnesses such as meningitis, septicemia, urinary tract, and intestinal infections. A recently discovered strain of E. coli (E. coli 0157:H7) can cause severe disease and may be fatal in small children and the elderly. Enterococci are a subgroup of the more general group of fecal streptococci bacteria. These bacteria are common inhabitants of the human intestinal tract. Numbers of enterococci are determined by counting pink to red colonies that have a black or reddish brown precipitate on the underside of a 0.45 micron filter placed on m-E media for at 41 C for 48-50 hours, then on EIA media for 20 minutes. Enterococci also is a preferred indicator organism for freshwaters and marine waters used for recreation. These bacteria have been found to be superior to fecal streptococci as a predictor of swimming-associated gastroenteritis. Numbers of enterococci in fresh or marine surface waters have been correlated with an increased rate of swimming-associated gastroenteritis. Clostridium perfringens is an anaerobic, spore-forming, rod-shaped bacteria that is consistently found at high concentrations in human and animal feces as well as sewage. Clostridium perfringens must be analyzed under anaerobic conditions by highly trained technicians. These bacteria are useful as a water-quality indicator because it forms spores that are more resistant to disinfection and environmental stresses than other indicator bacteria. This makes it indicative of remote, intermittent, and possibly point-source pollution and may be used as a surrogate for stress-resistant organisms. Information used in this article was taken from the following sources: Lynch, J.M., and J.E. Hobbie. 1988. Micro-organisms in Action: Concepts and Applications in Microbial Ecology. Blackwell Scientific Publications. Pala Alto, California. 363 p. Craun, G.F. 1986. Waterborne Diseases in the United States. CRC Press, Inc. Boca Raton, Florida. 295 p. Hurst, C. J. (Chief Editor) et al. 1997. Manual of Environmental Microbiology. American Society for Microbiology. Washington, D.C. 894 p.

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Last Modified: Tuesday, 03-Mar-2009 08:41:26 EST