Petrifilm Counts with the
Apogee BioCount System:
Minimum Incubation Time
Joy Elbert and J. D. Hendrix
Kennesaw State University
Background
BioCount:
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A software program that uses image
analysis technology to determine the
number of microbial colonies in a petri
dish.
Determines the number of colonies with
accuracy and precision comparable to a
manual count.
Previous Study
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Saadat and Hendrix determined that the
BioCount system accurately counted
total aerobic colonies on Petrifilm media
Parameters were optimized to allow the
program to detect an accurate number
of colonies on the Petrifilm media
Petrifilm

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
Developed By 3M
Rehydratable sample-ready media
Used for environmental and industrial
purposes
24 hour incubation required to count
colonies accurately
Selective and/or differential medium
Current Methods



Require a diluted sample of the
microorganism spread evenly over the
surface of a medium
Cells or groups of cells form colonies
(“Colony Forming Units”)
The initial number of microorganisms
can be calculated
Objectives
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Establish culture and dilution
parameters to give a reproducible count
of 20 - 200 colonies per film
Determine the minimum incubation time
needed to detect colonies on Petrifilm
Compare manual and automated counts
of colonies at different incubation times
Procedure
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Turbid broth cultures of Escherichia coli
(24 hr, 37°C in tryptic soy broth) were
diluted 10-5, 10-6, 10-7, 10-8, and 10-9
0.5 ml samples were plated on 3M
Petrifilm (“Total Aerobic” films) and
incubated at 37°C for time intervals
between 6 and 24 hr
Colony counts were determined by
manual and automated methods
Results
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Optimal dilution was determined on films
incubated for 24 hr
At 10-7 dilution the colonies remained in the
range of 50 to 80 (0.5 ml volume plated)
Results
E. coli Colony Counts on Petrifilm
6 to 24 hr Manual Counts
300
Total colonies counted (three films)
250
200
150
100
50
0
0
6
12
18
Incubation Time (hr)
24
30
Results
E. coli Colony Counts on Petrifilm
6 to 24 hr Automated Counts
300
Total colonies counted (three films)
250
200
150
100
50
0
0
6
12
18
Incubation Time (hr)
24
30
Results
E. coli Colony Counts on Petrifilm
6 to 12 hr Manual Counts
400
Total Colonies Counted (Three Films)
350
300
250
200
150
100
50
0
0
2
4
6
8
Incubation Time (hr)
10
12
14
Results
E. coli Colony Counts on Petrifilm
6 to 12 hr Automated Counts
400
Total Colonies Counted (Three Films)
350
300
250
200
150
100
50
0
0
2
4
6
8
Incubation Time (hr)
10
12
14
Results
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The t-test was used to determine if there was
a significant difference between the manual
and automated counts
For 29 films counted:
t = 0.38
There was no significant difference between
manual and automated counts
Conclusions
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To detect Escherichia coli colonies on
total aerobic Petrifilm, a minimum
incubation time of 12 hr is required
There was no significant difference
between manual and automated counts
Using automated counting is more rapid
than manual counting, but it does not
decrease the minimum incubation time
needed to form the bacterial colonies