What are coliforms? Coliforms are bacteria that are always present in the digestive tracts of animals, including humans, and are found in their wastes. They are also found in plant and soil materials. The most basic test for bacterial contamination of a water supply is the test for total coliform bacteria. Total coliform counts give a general indication of the sanitary condition of a water supply.
Here's a look at coliforms in general:
- Total coliforms include bacteria that are found in the soil, in water that has been influenced by surface water, and inhuman or animal waste.
- Fecal coliforms are the group of the total coliforms that are considered to be present specifically in the gut and feces of warm-blooded animals. Because the origins of fecal coliforms are more specific than the origins of the more general total coliform group of bacteria, fecal coliforms are considered a more accurate indication of animal or human waste than the total coliforms.
- Escherichia coli (E. coli) is the major species in the fecal coliform group. Of the five general groups of bacteria that comprise the total coliforms, only E. coli is generally not found growing and reproducing in the environment. Consequently, E. coli is considered to be the species of coliform bacteria that is the best indicator of fecal pollution and the possible presence of pathogens.
Are Coliform Bacteria Harmful?
Most coliform bacteria do not cause disease. However, some rare strains of E. coli, particularly the strain 0157:H7, can cause serious illness. Recent outbreaks of disease caused by E. coli 0157:H7 have generated much public concern about this organism. E. coli 0157:H7 has been found in cattle, chickens, pigs, and sheep. Most of the reported human cases have been due to eating undercooked hamburgers. Cases of E. coli 0157:H7 caused by contaminated drinking water supplies are rare. Water pollution caused by fecal contamination is a serious problem due to the potential for contracting diseases from pathogens (disease-causing organisms). Frequently, concentrations of pathogens from fecal contamination are small, and the number of different possible pathogens is large. As a result, it is not practical to test for pathogens in every water sample collected. Instead, the presence of pathogens is determined with indirect evidence by testing for an "indicator" organism such as coliform bacteria.
Coliforms come from the same sources as pathogenic organisms. Coliforms are relatively easy to identify, are usually present in larger numbers than more dangerous pathogens, and respond to the environment, wastewater treatment, and water treatment similarly to many pathogens. As a result, testing for coliform bacteria can be a reasonable indication of whether other pathogenic bacteria are present.
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A number of bacteria occur naturally in freshwater streams. Some are found living in the water and sediments as photosynthetic autotrophs or saprophytes living on dead matter. Others exist in or on other organisms as mutual symbiotes (providing some benefit to the host organisms in exchange for a place to live), commensals (neither helping nor harming the host), or parasites (utilizing the host in a way that causes harm). Certain bacteria that live in the intestinal tracts of animals are essential for the recovery of nutrients from digested food. Millions of these naturally occurring organisms are passed out of the body with fecal wastes.
If pathogenic (disease-causing) organisms are present, they may be passed as well. When a stream is polluted by fecal material, pathogenic bacteria, viruses, and parasites may be introduced, posing a health hazard to those who come in contact with the water. Municipal and rural water supplies can transmit human diseases such as cholera (Vibrio cholera), typhoid fever (Salmonella typhi), shigellosis (Shigella), salmonellosis (Salmonella), and gastroenteritis (Campylobacter jejuni, Escherichia coli, Giardia lamblia). The threat of such disease transmission becomes more serious as the population density increases and more sewage pollutes public water supplies, carrying with it human intestinal pathogens. Rather than test water directly for pathogens, which can be difficult, expensive, and even hazardous, researchers use indicator organisms to assess the possibility of fecal contamination. Fecal coliform bacteria, members of the family Enterobacteriaceae, which include Escherichia coli, Citrobacter, Enterobacter, and Klebsiella species, are often used as indicators.
These gram-negative bacilli (rod-shaped bacteria) are found in the digestive tracts of all warm-blooded animals. Most are not pathogenic. However, because they are eliminated with feces, they are sometimes associated with pathogens such as Vibrio cholera bacteria or a form of Hepatitis virus that is found in the digestive tract. Total coliform bacteria counts are sometimes used to test for water contamination also. These organisms are less precise as fecal contamination indicators because many can live and reproduce in soil and water, without having a human host. If high numbers of fecal coliform bacteria are found in a sample of stream water, one may conclude that there has been recent fecal contamination, although not necessarily human in origin. Other intestinal bacteria, such as streptococci or enterococci, may have a stronger correlation to human sewage, but no indicator has been identified that is exclusive to humans.
The ratio of streptococci to fecal coliform was once thought to determine human versus animal fecal contamination. But, this is no longer thought to be reliable because streptococci do not persist long in an open water environment, making it difficult to assess true concentrations. Enterococcal bacteria seem to be consistently associated with human sewage and subsequent diseases, but testing for these organisms involves a lengthy and complicated procedure. Despite the fact that they can not be linked directly to contamination by human sewage, fecal coliform bacteria counts are often used to regulate surface waters for recreational use, shellfishing, and portability (ability to be safely consumed). Federal regulations stipulate maximum allowable numbers of these bacteria for various uses.
If fecal coliform counts are high (over 200 colonies per 100 ml of water sample) in the river or stream, there is a greater chance that pathogenic organisms are also present. A person swimming in such water has a greater chance of getting sick from swallowing disease-causing organisms, or from pathogens entering the body through cuts in the skin, the nose, mouth, or ears. Diseases and illnesses such as typhoid fever, hepatitis, gastroenteritis, dysentery, and ear infections can be contracted in waters with high fecal coliform counts.
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