Environmental Engineering Reference
In-Depth Information
6. Examine tubes for gas.
Gas present (positive test)—transfer.
No gas (negative test).
Note:
Keep in mind that the fecal coliform MPN confirming procedure
and the fecal coliform procedure using the A-1 broth test are used to
determine the most probable number per 100 mL. The MPN procedure
for fecal coliform determination requires a minimum of three dilutions
with five tubes per dilution.
Calculation of Most Probable Number per 100 mL
Most probable number test calculations require the selection of a valid series
of three consecutive dilutions. The number of positive tubes in each of the
three selected dilution inoculations is used to determine the MPN/100 mL.
When selecting the dilution inoculations to be used in the calculation, each
dilution is expressed as a ratio of positive tubes per tubes inoculated in the
dilution (e.g., three positive per five inoculated, or 3/5). Several rules must be
followed in determining the most valid series of dilutions. In the following
examples, four dilutions were used for the test.
1. Using the confirming test data, select the highest dilution showing
all positive results (no lower dilution showing less than all positive)
and the next two higher dilutions.
2. If a series shows all negative values (with the exception of one dilu-
tion), select the series that places the only positive dilution in the
middle of the selected series.
3. If a series shows a positive result in a dilution higher than the
selected series (using rule 1), it should be incorporated into the high-
est dilution of the selected series.
After selecting the valid series, the MPN/100 mL is determined by locat-
ing the selected series on the MPN reference chart. If the selected dilution
series matches the dilution series of the reference chart, the MPN value from
the chart is the reported value for the test. If the dilution series used for the
test does not match the dilution series of the chart, the test result must be
calculated.
lution
Sample volume in 1stdilution
Sample volume in 1stdi
MPN/100 mL
=
MPN
×
chart
(6.1)
chart
sample
