7/27/2019 Escherichia Coli Test

http://slidepdf.com/reader/full/escherichia-coli-test 1/1

Testing for Escherichia coli  in water

If the pollutant is of faecal matter then there is a chancethat pathogenic bacteria will be present in the water.Several human diseases such as cholera and typhoid feverare transmitted by faecal contamination of drinking water.

Under crowded living conditions, with poor sanitaryarrangements. outbreaks of either disease are likely tooccur. The pathogenic bacteria of cholera, Vibrio. and of typhoid fever, Salmonella, are often present in faecal-contaminated water in numbers too low to enable them tobe readily identified. However. the common and relativelyharmless intestinal bacterium Escherichia coli will bepresent in very large numbers. If this can be recognizedand confirmed by laboratory tests such as the presumptive

coliform test, then faecal pollution of the water is proved.

The presumptive coliform test This test is carried out on water samples to

test for the presence of coliform bacteria. These are found naturally in the mammaliangut, and are present in large numbers infaeces. If a water sample is found to containcoliform organisms, it follows that the water iscontaminated with sewage. Not all coliformsare harmful; many are found as harmlesscommensals in the human gut. Nevertheless,their presence in water used as a source fordrinking supplies causes concern, since theyhave come from the same source as manypotentially dangerous pathogens. 

Faecal coliforms (FCs

) This is determined by means of the presumptive coliform test. This gives an indication of the bacteriological quality of the effluent. Coliforms are motile,Gram negative bacteria, present in large numbers in excreta. They should die during thesewage treatment process. Coliforms are relatively safe but, as they have come from thesame source as potential pathogens, they are; considered to be good indicator organisms toshow the likelihood of pathogens being present. In good quality effluent, FCs should be below100 per cm3.

Equipmento MacConkey bottles of red broth

o sterile loop

o incubator set at 37ºC

o bottle of sterile water (control)o collected samples of water to be tested

Procedure You are provided with a series of numbered water samples which you are going to test forthe presence of sewage bacteria. The bottle marked control contains sterile water. Broth tes t  - the broth turns yellow if sewage bacteria are present. 1. Label one bottle of MacConkey red broth as control.2. Sterilise the inoculation loop in a Bunsen flame.3. Dip the loop into the water sample marked 'control' and then into the bottle of broth.

Repeat steps 2 and 3 twice more.4. Replace the caps on the bottles.

5. Store the bottle of broth in an incubator set at 37°C for 24 hours.6. Repeat steps 1 to 5 for each water sample.