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Reading of plates computation and
reporting of results
Microbiology Unit
Food Analytical Service Laboratory
FOOD AND NUTRITION RESEARCH INSTITUTE
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Rules in reporting the results
1. Round by raising the second digit to the next highest number
if the third digit is 6,7,8, and 9, and use zeroes for each
successive digits to the right from the second digit.
156, 210 147, 200 129, 080
160, 000
150, 000 130, 000
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Rules in reporting the results
2. Round down if the third digit is 1,2,3 or 4, and use zeroes
for each successive digits o the right from the second digit.
242, 780 543, 000 194, 240
240, 000
540, 000
190, 000
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Rules in reporting the results
3. When the third digit is 5:
round up if the second digit is odd,
round down if the second digit is even.
275, 000 345, 000 985, 000 325, 060 195, 033
280, 000
340, 000 980, 000
320, 000
200, 000
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Computation
N = _______C___________
(1 x n1) + (0.1 x n2) d
Where:
N = number of colonies per g or mL of product
C = sum of all the colonies on all plates counted
n1 = number of plates in first dilution counted
n2 = number of plates in second dilution counted
d = dilution from which the first counts where obtained
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Reporting
1. Report APC as CFU/ g or mL
2. Report only the first two significant digits.
3. Round off to two significant digits only at the time of
conversion to APC.
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Normal plates : Count 25-250 colonies
Dilution
Colony Count
Plate 1 Plate 2
10-1 TNTC TNTC
10-2 225 210
10-3 30 21
10-4 1 2
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Normal plates : Count 25-250 colonies
CFU / gram =_______C___________
(1 x n1) + (0.1 x n2) d
= 225 + 210 + 30_____
(1 x 2) + (0.1 x 1) 10-1
= ______465________
2.1 x 10-1
= 2214
= 2,200
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Plates with no colonies
Report as
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Plates with more than 250 colonies
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Plates with more than 250 colonies
Square Count
1 5
2 5
3 7
4 8
5 6
6 7
7 5
8 6
9 4
10 8
11 9
12 5
Total
Average
Total = 75 12
Estimated Colony Count per plate = AVE colonies/cm2 x AREA of
the plate
= 6 x 65
= 390
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Plates with more than 250 colonies
DILUTION COUNT
A B
10-1 TNTC TNTC
10-2 TNTC TNTC
10-3 TNTC TNTC
10-4 390 360
= 390 + 360 2 x 104
= 375 x 104
= 3,750,000 = 3,800,000 EAPC
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Plates with more than 250 colonies > 10 colonies /
cm2
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Plates with more than 250 colonies
SQUARE COUNT
1 19
2 15
3 16
4 13
TOTAL
AVERAGE
Estimated Colony Count per plate = AVE colonies/cm2 x AREA of
the plate
= 16 x 65
=1040
=1000
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Plates with more than 250 colonies
DILUTION COUNT
A B
10-1 TNTC TNTC
10-2 TNTC TNTC
10-3 TNTC TNTC
10-4 1000 1100
CFU / g = 1000 +1200 2 x 104
= 1100 x 104
= 11,000,000 EAPC
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Spreaders
Types of Spreaders
1. Chain of colonies not too distinctly separated.
2. Developed in film of water between bottom of plate and
agar
3. Developed in film of water at the edge or surface of
agar.
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Spreaders
Report plate as spreaders (SPR) if:
1. Area covered by the spreader, including areas of represses
growth, exceeds 50% of plate area.
2. Area of repressed growth exceeds 25% of plate area
DILUTION COUNT
A B
10-1 225 230
10-2 25 SPR
10-3 2 1
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Spreaders
When it is necessary to count plates with spreader:
Count each of the 3 distinct types of spreaders as one.
If only one chain exists, count as one. Count each source as
one.
Combine the spreader count and colony count per plate to compute
for APC.
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Plates with less than 25 colonies
DILUTION COUNT
A B
10-1 20 19
10-2 1 2
10-3 0 0
Report as less than 25 x lowest dilution used estimated
count.
< 250* * Estimated Aerobic Plate Count
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Exercises
DILUTION COUNTS
A B
10-1 255 245
10-2 30 26
10-3 0 1
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Exercises
DILUTION COUNTS
A B
10-1 TNTC TNTC
10-2 TNTC TNTC
10-3 239 243
10-4 24 23
10-5 1 0
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EXERCISES
DILUTION COUNTS
A B
10-0 226 221
10-1 30 31
10-2 3 2
10-3 0 0
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Exercises Square
Plate A
Plate B
Count
1 6 9
2 7 8
3 8 6
4 5 7
5 6 7
6 5 7
7 6 8
8 8 9
9 8 5
10 9 7
11 4 8
12 7 9
Dilution Count*
10-1 TNTC TNTC
10-2 TNTC TNTC
10-3
* Using glass plates
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Exercises
DILUTION COUNTS
A B
10-1 TNTC TNTC
10-2 TNTC TNTC
10-3 239 243
10-4 24 23
10-5 1 0
