E-coli: harmful? why and how tested

What are E. coli and coliform?

Coliform bacteria are Gram-negative, non-spore-forming, rod-shaped bacteria that ferment lactose with gas and acid at 35 to 37 degrees C within 48 hours. They live in soil, vegetation and in the intestines of warm-blooded animals.

Escherichia coli (E. coli) is the main faecal coliform. It lives only in the intestines of humans and warm-blooded animals and is passed in large numbers (about $10^{8}$ to $10^{9}$ per gram) in faeces. It can ferment lactose at 44.5 degrees C and produces indole.

Are they harmful?

• Most E. coli strains are harmless and even help digestion in the gut.
• Some strains (for example E. coli O157:H7, enterotoxigenic and enteropathogenic strains) cause diarrhoea, bloody dysentery, kidney failure (haemolytic uremic syndrome) and can be fatal, especially to children.
• Coliform bacteria in general are usually harmless, but their presence shows that water may have been polluted.
• The main danger is what they stand for: faecal contamination, which means pathogens such as Salmonella, Vibrio cholerae, Shigella, viruses and protozoa may also be in the water.

Why they are tested (indicator organisms)

• Testing for each pathogen is difficult, costly and slow, as many pathogens exist only in small numbers.
• E. coli and coliforms are always present in faeces, easy to detect, survive longer than most pathogens in water and do not multiply in clean water.
• Their absence therefore shows that the water is bacteriologically safe. The standard is zero E. coli per 100 ml (Nepal Drinking Water Quality Standards, WHO).

How they are tested

1. Multiple tube fermentation (MPN) technique: presumptive, confirmed and completed tests in lactose broth, BGLB and EC broth; the result is given as MPN per 100 ml.
2. Membrane filter technique: 100 ml of water is passed through a 0.45 micron filter, which is placed on selective media (m-Endo, m-FC) and incubated; colonies are counted as CFU per 100 ml.
3. Presence-absence test: a simple test that shows only whether coliforms are present or absent in 100 ml.
4. Chromogenic substrate tests (for example Colilert): a colour change (and fluorescence under UV light) shows coliforms and E. coli. Field kits such as the H2S strip test turn black when faecal contamination is present.