Coliforms are rod-shaped, gram negative bacteria which can be found in the environment, such as in soils, and in mammal faeces.
Coliform bacteria encompass a wide variety of bacteria, most of which will not cause harm to humans, however their presence is indicative of other, more harmful bacteria also being present.
For this reason, coliform bacteria are known as an indicator of faecal pollution in water.
In the context of water quality, Total Coliforms (TCs) are all the coliforms found in the soil, waters and in mammal waste.
Within the TC group, Faecal Coliforms (FCs) are a subset classified by their presence in the gut and faeces of warm-blooded mammals; therefore, seen as a general indicator of faecal pollution in water.
Sometime FCs are used interchangeably with Thermotolerant Coliforms (TTCs), a term used to classify bacteria that can tolerate temperatures above 44 °C. Previously it was thought that the higher temperature tolerance eradicated non-faecal bacteria however this is no longer the case meaning TTCs are not necessarily only faecal in origin and the term is less popular.
E. coli are a species of bacteria that are found in the FC group, set apart from other bacterial species in the group by the fact they are the only species that generally cannot originate naturally in the environment and therefore must have a faecal origin. As such, E. coli are a strong faecal indicator bacteria (FIB) for detecting faecal contamination.
Coliforms are important as they are a strong indicator of faecal pollution. E. coli in particular is a prolific parameter in drinking water quality surveys globally.
Not just applicable to drinking water, as waters for recreational use such as water sports and wild swimming also present a risk if suffering from faecal contamination.
E. coli itself has only a few pathogenic (disease causing) strains however its presence denotes a serious risk of other, pathogenic bacteria such as Enterococcus, Salmonella, Shigella, Campylobacter etc., which can cause harm if ingested.
Unsafe water sources are responsible for 1.2 million deaths every year and FIB/E. coli detection can play a major part in reducing this number.
There are many methods for bacterial enumeration, however the most common method is membrane filtration.
Membrane filtration uses a filter to capture the bacteria on cellulose membrane which is then transferred to agar and incubated for a set time, usually overnight, allowing cultures to grow and then be counted; this gives a count of colony forming units (CFUs).
Different agar can be used depending on the target bacteria i.e. just E. coli or a count for total coliforms and E. coli using a chromogenic media.
Other methods such as Most Probable Number, a statistical method based on the dispersion of bacteria in different dilutions, are also commonly found in water quality testing guidelines.
Most of the more ‘traditional’ methods require a laboratory set-up and some specialist equipment such as incubators, autoclaves and a trained technician as well as an incubation period usually in excess of 18 hours.
Fluorescence for E. coli, FCs and TCs.
The Proteus provides a coliform reading based on the detection of Tryptophan-like fluorescence (TLF). The link between TLF and coliforms/E. coli has been proven in scientific literature as E. coli cells emit TLF and other associated compounds and therefore, using paired sampling, a strong relationship which allows TLF to be a proxy for bacteria in water.
As TLF is ‘like fluorescence’ this includes other faecal matter associated compounds, such as indole, and some that are not whose influence is reduced during the calibration phase as the fluorescence readings are analysed with the sampling data, providing a robust site calibration for each water source.
Unlike more culture-based methods, fluorescence can detect viable but not culturable bacteria where fluorescent matter is still being produced such as in the case of nutrient stress.
