Table of Contents
Advertisement
Determining Results
Once the sample has been taken, the contact plate or Petri dish should be removed immediately
from the MicroBio and the lid replaced on the plate and sealed.
The plate should then be incubated for a period of time and temperature dependant on the
requirements of the media. Further information can be found in the Microbiology section of this
manual.
Once incubated, the colony growths are then counted, either manually or using an automatic colony
counter. Due to the statistical nature of the sampling method and the chance more than one colony
impinged at one point on the dish, a count correction needs to be performed. The table in Appendix
A gives the corresponding corrected value based on the sampling head used on the MicroBio.
Alternatively, the following equation can be used to determine a corrected count.
!
!
!
!
Where n
is the number of holes on the sampling head, n
h
the corrected count. If the counted colonies, (n
(n
), then the mathematics will fail and the results cannot be trusted. If this is the case, then the
h
sample dish can be considered as overloaded with organisms and the user should consider lower
sampling volumes.
The colony concentration is then the corrected count, above, per volume of air sampled. The results
are normally expressed in colony forming units per cubic metre.
To convert the corrected count to CFU/m
Where n
is the corrected number of colonies counted and V
c
Spreadsheets to automate the correction process and to collate and present results are available for
free download from:
Excel™:
Apple™ Numbers™:
June 2012
⎛
⎜
= n
⎜
!
n
c
f
⎜
⎜
⎝
)
f
3
use the equation:
CFU
3
m
http://www.cantiumscientific.com/downloads/PHCA.xls
http://www.cantiumscientific.com/downloads/PHCA.numbers
MicroBio Operating Manual [P0001W006]
0.483
⎞
⎟
1.075
⎟
n
⎟
f
1.052 -
⎟
⎠
n
h
is the number of counted colonies and n
f
exceeds the number of sampling head holes holes
n
= 1000.
c
V
s
is the sampled volume in litres.
s
is
c
10 / 16
Advertisement
Table of Contents
