Question

2. You are given an unlabelled water sample with the below MPN tube results presented as volume of raw sample followed by number of positive tubes out of 5 total tubes incubated. 1 mL : 2/5 positive, 0.1 mL : 1/5 positive, 0.01 mL : 0/5 positive

   1. Report the estimated titre and 95% confidence interval.

   2. Indicate a potential source based on the magnitude of coliform contamination and

      provide your reasoning.

Answer 1

In the multiple-tube method, a series of tubes containing a suitable selective broth culture medium is inoculated with test portions of a water sample. After a specified incubation time at a given temperature, each tube showing gas formation is regarded as “presumptive positive” since the gas indicates the possible presence of coliforms. However, gas may also be produced by other organisms, and so a subsequent confirmatory test is essential. The two tests are known respectively as the presumptive test and the confirmatory test. For the confirmatory test, a more selective culture medium is inoculated with material taken from the positive tubes. After an appropriate incubation time, the tubes are examined for gas formation as before. The most probable number (MPN) of bacteria present can then be estimated from the number of tubes inoculated and the number of positive tubes obtained in the confirmatory test, using specially devised statistical tables. This technique is known as the MPN method.