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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.
Certain bacteria can be used as indicator organisms in particular situations, such as when present in bodies of water. The presence of bacteria commonly found in human feces, termed coliform bacteria (e.g. E. coli), in surface water is a common indicator of faecal contamination. For this reason, sanitation programs often test water for the presence of these organisms to ensure that drinking water systems are not contaminated with feces. This testing can be done using several methods which generally involve taking samples of water, or passing large amounts of water through a filter to sample bacteria, then testing to see if bacteria from that water grow on selective media such as MacConkey agar. Alternatively, the sample can be tested to see if it utilizes various nutrients in ways characteristic of coliform bacteria.[1]
Introduction: Certain bacteria can be used as indicator organisms in particular situation, such as when present in bodies of water. Indicator bacteria themselves may not be pathogenic but their presence in waste may indicate the presence of other pathogens[2]. Similar to how there are various types of indicator organisms, there are also various types of indicator bacteria. The most common indicators are total coliforms, fecal coliforms, E. coli, and enterococci[3]. The presence of bacteria commonly found in human feces, termed coliform bacteria (e.g. E. coli), in surface water is a common indicator of fecal contamination. The means by which pathogens found in fecal matter can enter recreational bodies of water include, but are not limited to, sewage, septic systems, urban runoff, coastal recreational waste, and livestock waste[3].
Testing: For this reason, sanitation programs often test water for the presence of these organisms to ensure that drinking water systems are not contaminated with feces. This testing can be done using several methods which generally involve taking samples of water, or passing large amounts of water through a filter to sample bacteria, then testing to see if bacteria from that water grow on selective media such as MacConkey agar. MacConkey agar will only allow the growth of gram-negative bacteria and the bacteria will grow differently according to how it metabolizes lactose or its lack of ability to metabolize it[4]. Alternatively, the sample can be tested to see if it utilizes various nutrients in ways characteristic of coliform bacteria.
Coliform bacteria selected as indicators of fecal contamination must not persist in the environment for long periods of time following efflux from the intestine, and their presence must be closely correlated with contamination by other fecal organisms. Indicator organisms need not be pathogenic[5].
Non-coliform bacteria, such as Streptococcus bovis and certain clostridia may also be used as an index of fecal contamination[6].
The presence of indicator bacteria is measured in a variety of ecosystems and sometimes alongside other measurements. In the Great Lakes, a study was conducted testing for both fecal indicator bacteria (FIB) concentrations and pathogen gene markers[7]. The FIB measured in this study included fecal coliform bacteria, E. coli, and enterococci[7]. FIB were collected via membrane filtration and serial dilution methods, producing samples which could be cultured and used to run PCR and amplify the pathogenic genes in question[7]. Among the 22 sampling locations, 165 samples were analyzed and E. coli concentrations were found to range from less than 2 to 26,000 CFU/100mL, enterococci ranged from less than 2 to 31,000 CFU/100mL, and fecal coliform bacteria ranged from less than 2 to 950 CFU/100mL[7].
Another example of indicator bacteria being measured for safety purposes is in Malibu, CA. The state of California requires that beaches with greater than 50,000 visitors a year be monitored for FIB[8]. High FIB concentrations, exceeding what is considered acceptable by the EPA were observed in Malibu Lagoon and other Malibu beaches[8]. Measurement of high levels of FIB leads to a search to determine what the source(s) is/are. Potential sources of FIB in the Malibu area include waste from sewage treatment systems, runoff from local developments, and wildlife waste[8]. Common FIB were measured including enterococci which presented itself in levels as high as 242,000 MPN/100mL within onsite wastewater treatment systems[8]. The measurement of FIB is widespread and used for the purpose of providing safe waters.
In Texas, the occurrence and distribution of FIB, in particular fecal coliforms and E. coli, were measured in streams that receive discharge from the Dallas Fort Worth International Airport and the surrounding area[9]. These streams receiving the waste are home to aquatic life, used for recreational purposes, and as fishing sites[9]. Various standards exist in order to ensure the safety of all organisms present in the ecosystem, including humans. E. coli is used as an indicator of unsafe or below standard water quality for recreational use in Texas[10]. The standards for E. coli levels that declare contact recreation unsafe are a geometric mean of over 126 cfu/100mL or over a fourth of the samples measuring levels greater than 394cfu/100mL[10]. Various sites were tested, some found to exceed acceptable levels of E. coli and therefore did not support recreational use[9]. This is yet another example of how testing for indicator bacteria is used to determine whether bodies of water are safe for various uses, particularly recreational use.