Coliform Bacteria: An Indicator of Water Contamination

Coliform commonly known as ” indicator organisms” , refers to a wide groups of bacteria ( about 16 species) that found throughout the environment and used to evaluate the water quality.  These organisms have a wide distribution in soil, water surface, vegetations as well as on the skin or intestinal tract of worm-blooded organisms including humans.

Except some species ,most of the coliforms are harmless. Coliform groups are relatively easy to culture in lab. Therefore these are used as a primary indicators for the presence of potential disease causing bacteria in drinking water as well as foods. Some representatives are  Citrobacter,  Enterobacter,  Hafnia, Klebsiella, Escherichia  etc.

Coliforms are characteristically

  • rod shaped
  • gram negative 
  • motile or non motile 
  • facultative anaerobic
  • lactose fermentor 

Coliform bacteria ferment lactose suger producing gas (CO2) and acid when incubated at 35°-37°C. This unique feature of coliform helps in their identification during coliform testing.

Coliforms are divided into three main groups. Each  group is an indicator of drinking water quality with a different level of risk. Groups  are including:

  1. Totall Coliform Bacteria
  2. Fecal  Coliform Bacteria
  3. E. coli Coliform Bacteria

Total Coliform Bacteria

This group’s bacteria are found  in environment as water, soil, vegetations where they live freely apart from human or animal waste.

These are generally harmless because most species have no impact on human health instead these are a warning sign of risk of water contamination. High total Coliform level in water has a high probability of water contamination by protozoa ,virus ,disease causing bacteria and other pathogen. Testing the  presence of total Coliform Bacteria basically involves growing them in lactose media at about 35°C.


Fecal Coliform Bacteria

Fecal coliform E.coli bacteria

Fecal coliforms are a sub group of total Coliform Bacteria. They  are found in intestine and feces of warm-blooded animals including humans.

Fecal Coliform comprises both pathogenic and non pathogenic bacteria. Fecal Coliform detection in a  sample drinking water indicates the sewage contamination of the water. Increased level of fecal coliform indicates a warning of failure of water treatment. Fecal Coliforms  are also known as “Thermotolerant Coliform ” which are capable of fermenting lactose suger at 45°C.


E. coli Coliform

The  sub group of fecal Coliform ,E.coli Coliform is the strongest indicator of contamination of serious waterborne disease.

E. coli bacteria

These are a more specific indicator of fecal pollution. Although these are mostly harmless in host’s intestine , some strains (E.coli 0157:H7)j cause serious illness like bloody diarrhea, stomach cramp , vomiting ,fever as well as pneumonia, respiratory illness, urinary tract infection. E. coli cannot live long outside the host, for this their presence evidence the recent contamination of water.

Eosin Methylene Blue agar plate( partially inhibitory to gram positive bacteria ) is an effective medium to differentiate between different types of Coliforms. Based on lactose fermenting ability coliforms will produce a color change in the bacterial colony. Strong lactose fermenters appear as dark /blue/purple/black while weak fermenters are pink. E. coli colonies will be dark with metallic green sheen. Other Coliform appear as thick ,slimy colony. Non fermenter Coliform  appear colorless .

Coliform have been used to evaluate the general quality of water as they are often found alongside dangerous pathogenic bacteria. Since coliforms are not such a pathogenic but  only mildly infectious , these are relatively safe in work within lab.  To test the individual pathogen specifically, testing for coliform is faster and cheaper to have an  idea about drinking water pollution. There are some approved techniques to determine the presence of Coliform in water and get idea of contamination level. These are :

  • Membrane filter technique
  • Multiple tube fermentation technique
  • Most Probable Number (MPN) technique
  • MPN Index

Disinfection and filtration are the effective  method to remove bacteria. Coliform contamination can easily be treated by boiling water, steam sterilization, treating with disinfectant  chlorine, iodine, ozone, UV light etc.

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