Air IDEAL
®
3P™ Traceability Solution
AeroBioCollector User's Manual
7-1
7 Appendix
U s i n g t h e r e a d i n g t a b l e
The following reading tables indicate the most probable number of micro-organisms
collected per plate (MPN collected) with respect to the number of agglomerates of colonies
counted on the agar (CFU read).
The
MPN
value is calculated from the
CFU
count, using FELLER’s law. This statistical
correction corresponds to the random passing of bacteria through the orifices of the grid.
It quantifies for each “visited” orifice, (i.e. for each cluster counted) and as a function of the
total number of clusters counted, the most probable number of bacteria which make up the
cluster concerned, i.e. the number of bacteria having passed through the same orifice.
The application of the statistical correction assumes that the CFU count on the agar
concerns the number of colony clusters, i.e. the number of orifices with a positive impact,
without distinguishing, within a given cluster, the number of confluent colonies of which it is
made up.
In order to determine the most probable number of micro-organisms collected per
cubic meter,
the most probable number of micro-organisms collected per plate (MPN
collected) must be multiplied by 1000 and divided by the volume sampled in liters.
Example:
Volume of air sampled = 50 liters
CFU count: 120
MPN value: 159, 380905
Result expressed as the MPN collected per cubic meter = 159 x 1000 / 50 = 3180
I n f o r m a t i o n o n F E L L E R ’ s l a w
The formula for FELLER’s law is the following:
MPN = N . (1/N + 1/N-1 + 1/N-2 + ………. + 1/N-CFU+1)
where:
MPN = most probable number of bacteria having passed through the orifices of the grid
N = number of orifices on the grid.
CFU = colony forming units, value obtained by the laboratory.
In the case of a sampling grid for which the passage of a particle through a given orifice of
the grid is purely random, there is a probability that, during a sampling cycle,
several
particles pass through the
same
orifice and are therefore counted as a
single and unique
CFU, while other orifices are not passed through by any particles.
The closer the CFU count obtained by the laboratory is to N, (total number of orifices on the
grid), the higher the probability is.
Moreover, the notion of probability density arises:
Example: for a given CFU value:
−
there is a probability p2 that the same orifice is passed through by 2 particles
−
there is a probability p3 that the same orifice is passed through by 3 particles
−
there is a probability pi that the same orifice is passed through by i particles etc…
