Wednesday, July 18, 2007

Coliforms and Faecal Contamination

I recently read a newspaper article about the potential contamination of ground water from failing septic tanks. Sanitarians had found septic tank systems close to a restaurant that might be feeding human faecal matter to the ground water. However, wells in the surrounding area, both up and downstream of the potentially contaminated site tested negative for Escherichia coli. The significance of this finding is that the sanitarians could not demonstrate that the wells were in fact contaminated by human faeces or the outflow from the septic tanks.

Escherichia coli is a bacterium found in the intestines of man and animals, but it doesn’t grow in water and doesn’t grow well in the environment. So if it is found in water, the implication is that the water has been contaminated with faecal material. Some strains of E. coli, such as O157:H7, cause serious disease, though most do not. The micro-organism is used as a Faecal Indicator. The importance of finding any E. coli in water or food is that other enteric pathogens, such as Salmonella or Norovirus, might be present.

The article went on to say that 28% of the wells tested did contain “coliforms” and this indicated that the water system was at risk of more serious contamination. This is probably correct, because a properly constructed well should draw water from deep in the ground; such water has often been in the ground for hundreds of years and has been filtered through the soil and should have a low bacterial count. Though the article was correct, it may have caused some confusion in the mind of the reader.

The finding of coliforms in water is not necessarily indicative of danger. The coliform group is defined by the tests* that we use, i.e. “coliform” is not a species, it simply says that these organisms give us a positive result in the tests we apply. This means that some bacteria are included in the group that have no faecal connotation at all. These bacteria may be found in plant material that has never come in contact with faeces of man or animals and the bacteria may even be transmitted in the seeds of the plants. So if we find coliforms in the water or in food, it is not a cause for immediate panic. Obviously, if we find large numbers present, we should look to find out why. The well may be affected by inward leakage of surface water; fresh salad vegetables may have these micro-organisms present at low numbers on their tissues, but high numbers suggest poor kitchen hygiene or temperature abuse.

If we wish to demonstrate that contamination by faeces has occurred, we need to do further tests on the bacteria to see if they are actually E. coli. The tests involve different culture media, different incubation temperatures and biochemical tests.

*The tests that we use to detect coliforms vary somewhat between countries. The tests are based on the ability of coliforms to grow in the gut, so they must be able to grow in the presence of bile salts. Bile salts are secreted into the intestine to aid in digestion of fats; they are natural detergents. So some media formulations include bile salts, while others employ synthetic detergents. The media also contain lactose as a carbon and energy source for the bacteria and the tests are incubated at the optimum temperature for the growth of the target organisms. So the coliform group is defined by their ability to produce a positive result:
Acid and gas production in 24 to 48 hours from lactose at 37C in the presence of bile salts.

If we wish to demonstrate that Escherichia coli is present, then we may subculture into a similar medium containing selective agents (which prevent other similar micro-organisms from growing) and raise the temperature to 44.5C and again look for gas production. At the same time we may look for the ability of the micro-organisms to convert tryptophan to indole; produce sufficient acid to change the colour of methyl red pH indicator; produce acetoin from glucose and grow on citrate as the sole carbon and energy source. This series of tests is known as the IMViC tests. E. coli normally gives a "++--" profile. Finally, we may grow the culture on Eosin Methylene Blue agar and look for the formation of a metallic green sheen. If all these tests give the correct result, we can be pretty sure that E. coli is present and faecal contamination has occurred.


Anonymous said...

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zaienab /EGYPT

Anonymous said...

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AUT microbiology student (ID# 0795374)

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