Clean Water
Materials:
3 Durham tubes of double strength lactose broth (DSLB)
6 Durham tubes of single strength lactose broth (SSLB)
1 10 ml pipette
1 1 ml pipette
1. Set up three DSLB and six SSLB tubes as illustrated in the water testing
figure. Label each tube according to the amount of water that is to be
dispensed to it: 10 ML, 1.0 ML, and 0.1 ML, respectively.
2. Mix the bottle of water to be tested by shaking 25 times.
3. With a 10 ml pipette, transfer 10 ml of water to each of the DSLB tubes.
4. With a 1.0 ml pipette, transfer 1 ml of water to each of the middle set
of tubes, and 0.1 to each of the last three SSLB tubes.
5. Incubate the tubes at 35oC for 24 hours.
6. Examine the tubes and record the number of tubes in each set that have
10% gas or more.
7. Determine the MPN by referring to MPN Determination Table.
Example: If you had gas in the first three tubes and gas only in one
tube of the second series, but none in the last three tubes, your test
would be read as 3-1-0. The MPN Determination Table indicates that the
MPN for this reading would be 43. This means that this particular sample
of water would have approximately 43 organisms per 100 ml with 95%
probability of there being between 7 and 210 organisms. Keep in mind
that the MPN figure of 43 is only a statistical probability figure.
Results: If your water sample produces gas then it is presumed that the sample
contains coliforms. It is necessary to go on to the Confirmed Test.
To see how to perform the Confirmed Test, click on the Back icon twice and clik on The Confirmed Test.