We transfer a sample of 1 ml from each of the serial dilutions to
five fermentation test tubes containing each a Durham tube and 10 ml
of single strength medium (peptone (20 g), sodium chloride (30 g)
and water (1000 ml)). The same procedure is repeated in the case of
a 10 ml sample with a double strength medium concentration. Then,
all tubes are incubated for 24 h at 35 °C.
After incubation, observation of positive and negative tubes is
noted, and a standardized MPN Table (Table of Mac Grady) is consulted
to determine the most probable number of organisms (causing
the positive results) per unit volume of the original sample.