Testing of water potability by MPN


Water sample can be tested for its potability by using microorganisms as an indicator of fecal pollution. These indicators are usually common intestinal commensal bacteria which are universally present in and excreted in large numbers by man and animals. One such indicator organism is coliform. These coliforms are generally detected by presumptive coliform counts (PCC) using multiple tube tests.

In multiple tube test, measured volumes of water are added to a series of tubes containing a liquid indicator growth medium. The medium receiving one or more of the indicator bacteria show growth and characteristic color change. From the number and distribution of positive and negative reactions, the most probable number (MPN) of indicator organisms in the sample may be estimated by referring statistical tables.


  1. Water sample
  2. Double and single strength Mac Conkey broth
  3. Pipettes
  4. Test tubes


Arrange the given media in three sets with each tube containing inverted Durham’s tube to check gas production. Arrange the tubes as follows:

  1. 5 tubes containing 10ml double strength media (DSM)
  2. 5 tubes containing 5ml single strength media (SSM)
  3. 5 tubes containing 10ml single strength media (SSM)
  4. Add water sample to the tubes as follows:
  • 10ml of water to set (a)
  • 1ml of water to set (b)
  • 0.1ml of water to set (c)
  1. Incubate the tubes at 37°C for 48 hrs.
  2. During and after the incubation observe the tubes for acid and gas production indicated by colour change and gas bubble in Durham’s tube.
  3. Interpret the results as most probable number by comparing with statistical table.
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