Soil Microbial Ecology - Soil Molecular Ecology Laboratory

Page 47
The most probable number (MPN) technique is a statistical approach to quantification
and, as in the plate count technique, involves the preparation of a decimal dilution series.
The sample must be diluted to extinction. An aliquot of each dilution (usually 1.0 or 0.1
ml) is then added to a series of tubes containing a growth medium. The number of
replicate tubes employed dictates the degree of accuracy. Tables are available for 3, 5, 8,
and 10 tube determinations. The table below is a three-tube table. As an example,
consider the following: a sample is decimally diluted to extinction. Three 1-ml aliquots
from each dilution are dispensed to three tubes of nutrient broth. After incubation, growth
is determined by turbidity. Any turbidity is considered a positive test. At a dilution of 10 -
2 all three tubes are turbid, 10 -3 has two turbid tubes, 10 -4 has one turbid tube and 10 -5 has
none. The combination is 3, 2, 1, 0 or 2, 1, 0. Referring to the table, the combination 3, 2,
and 1 gives a MPN value of 1.50 for the center dilution and 2, 1, 0 gives a value of 0.15.
The MPN value is multiplied by the reciprocal of the dilution. In these examples, the
MPN is (1.5 x 10 3 ).
THREE-TUBES MOST PROBABLE NUMBER (MPN) TABLE
# OF POSITIVE TUBES # OF POSITIVE TUBES
SERIES SERIES SERIES MPN SERIES SERIES SERIES MPN
A B C
A B C
0 0 0 24.00
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