Coliform Bacteria as Indices

Directly detecting harmful pathogens in water can be expensive, unreliable and incredibly complicated. Fortunately, certain organisms are known to consistently coexist with these harmful microbes which are substantially easier to detect and culture: coliform bacteria. These generally non-toxic organisms are frequently used as “indicator species,” or organisms whose presence demonstrates a particular feature of its surrounding environment.

Scientists test water for the presence of coliform bacteria as a proxy for more complex, easily hidden and dangerous pathogens. Both thrive under very similar conditions. When significant numbers of coliform bacteria are present, it is generally assumed their illness-inducing counterparts exist as well.

Testing for coliform bacteria can be accomplished in a number of ways, including membrane filtration and the most probable number (MPN) method. Filtration captures bacteria, which are then exposed to a medium in which they can grow further for easier detection. Coliform bacteria are known for their characteristic gold-green pigment, allowing them to be easily counted. The MPN method relies on growing bacteria within the water sample in a test tube, then collecting the carbon dioxide emitted by the organisms. Those which produce the gas at 35 degrees Celsius are considered to be coliform bacteria and counted.  

Coliform Bacteria as Contaminants

These bacteria themselves are generally not considered harmful to human health; rather, their presence generally indicates the simultaneous existence of more dangerous and difficult-to-detect pathogenic organisms. However, E. Coli is an example of coliform bacteria and some of its strains can cause major illness. This species exclusively inhabits the intestines of humans and other warm-blooded animals, so if it is present in the water supply, so is animal or human waste. 

Killing and/or removing these bacteria can be accomplished by chlorination, iodination, UV light, ozonation, reverse osmosis, distillation, or filtration. Addition of chlorine is relatively inexpensive and will kill most pathogens present, but its byproducts could cause unwanted side effects and the water is given an odd taste. 

Iodine also effectively kills harmful bacteria and viruses, however over time and in large concentrations could be physiologically damaging. 

Use of UV light destroys microorganisms, but only in the exact area it is being applied.

Ozone is an unstable molecule of oxygen atoms that kills microorganisms with greater success than chlorine and without iodine’s health dangers, while also reducing levels of iron, manganese, and sulfur. Due to ozone’s instability, its usefulness is short-lived and consequently localized to a specific area.

Reverse osmosis requires that the water be pressurized and filtered to remove pathogens.

Distillation is a form of demineralization that requires boiling the water, catching its steam and re-condensing it back to liquid form.