In the presumptive test a series of nine or twelve tubes of lactose broth
are inoculated with measured amounts of water to see if the water contains
any lactose-fermenting bacteria that produce gas. If, after incubation, gas is
seen in any of the lactose broth, it is presumed that coliforms are present
in the water sample.
Clean Surface Water Test
If clear surface water is
being tested, nine tubes of lactose broth will be used.
Six tubes of the lactose broth are single strength lactose broth and
three tubes are double strength lactose broth. If turbid surface water is
tested three additional tubes of single strength lactose broth will be
inoculated.
In addition to determining the presence or absence of coliforms, we can also use this series of lactose broth tubes to determine th most probable number (MPN) of coliforms present in 100 ml of water. A table for determining this value from the number of positive lactose tubes is provided in MPN Determination Table.
Click here to see the instruction for clear surface water test.
Click here to see the instruction for
turbid surface water test.
Click
here to see the next step of the test which is the Confirmed Test.Turbid Surface Water Test
If the water sample appears to have considerable pollution (turbid surface
water), you should follow different instruction than clean water one.