Turbid Water
Materials
3 Durham tubes of double strength lactose broth (DSLB)
9 Durham tubes of single strength lactose broth (SSLB)
1 10 ml pipette
2 1 ml pipettes
1 water blank (99 ml of sterile water)
1. Setup three DSLB and nine SSLB tubes in a test-tube rack, with the DSLB
tubes on the left.
2. Label the three DSLB tubes 10 ML; the next three SSLB tubes 1.0ML; the
next three SSLB tubes 0.1 ML; and the last three tubes 0.01ML.
3. Mix the bottle of water to be tested by shaking 25 times.
4. With a 10 ml pipette, transfer 10 ml of water to each of the DSLB tubes.
5. With a 1.0 ml pipette, transfer 1 ml to each of the next three tubes, and
0.1 to each of the third set of tubes.
6. With the same 1 ml pipette, transfer 1 ml of water to the 99 ml blank of
sterile water and shake 25 times.
7. With a fresh 1 ml pipette, transfer 1.0 ml of water from the blank to the
remaining tubes of SSLB. This is equivalent to adding 0.01 ml of full strength
water sample.
8. Incubate the tubes at 35oC for 24 hours.
9. Examine the tubes and record the number of tubes in each set that have
10% gas or more.
10. Determine the MPN by referring to the MPN Determination Table. This table
is set up for only nine tubes. To apply a twelve tube reading to it, do as
follows:
a. Select the three consecutive sets of tubes that have at least one
tube with no gas.
b. If the first set of tubes (10 ml tubes) are not used, multiply the
MPN by 10.
Example: Your tube reading was 3-3-3-1. What is the MPN? The first set
of tubes (10ml) is ignored and the figures 3-3-1 are applied to the
table. The MPN for this series is 460. Multiply this by 10, and the MPN
becomes 4600.
Example: Your tube reading was 3-2-2-0. What is the MPN? The first
three numbers are applied to the table. The MPN is 210. Since the
last set of tubes is ignored, 210 is the MPN.
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 click on The Confirmed Test.