GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 068 of 500
Confirmation count worksheet. Enter the presumptive colonies on MYP agar and the confirmation result to get the B. cereus count in CFU per gram.
Try sample values — fills the form with the worked example below.
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CFU/g
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Log₁₀ CFU/g
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Confirmed (%)
Equation: CFU/g = presumptive colonies × (confirmed ÷ tested) ÷ (V × d), where V = volume plated (mL) and d = total dilution factor. Presumptive CFU/g uses the count before confirmation. Log₁₀ CFU/g = log₁₀(CFU/g). Detection limit = 1 ÷ (V × d).
Method: B. cereus enumeration, ISO 7932. Mannitol egg yolk polymyxin (MYP) agar, 30 °C for 18 to 24 hours; confirm presumptive colonies by the tests in the method. Countable range 10 to 150 colonies per plate.
Assumptions: The tested colonies represent all presumptive colonies on the plate; each colony grew from one viable unit; the sample was mixed evenly.
Limitations: Counts below 10 or above 150 colonies are estimates; only confirmed colonies are counted; spores survive cooking, so a low count does not rule out later growth.
Rounding: CFU/g shown as a whole number; log₁₀ to 2 decimal places; confirmed percentage as a whole number; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: B. cereus enumeration, ISO 7932. Mannitol egg yolk polymyxin (MYP) agar, 30 °C for 18 to 24 hours; confirm presumptive colonies by the tests in the method. Countable range 10 to 150 colonies per plate.
Assumptions: The tested colonies represent all presumptive colonies on the plate; each colony grew from one viable unit; the sample was mixed evenly.
Limitations: Counts below 10 or above 150 colonies are estimates; only confirmed colonies are counted; spores survive cooking, so a low count does not rule out later growth.
Rounding: CFU/g shown as a whole number; log₁₀ to 2 decimal places; confirmed percentage as a whole number; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: 86 presumptive colonies, 5 tested and 5 confirmed, 1 mL plated at a total dilution of 10⁻². Confirmed fraction 1.0; CFU/g = 86 × 1.0 ÷ 0.01 = 8,600 CFU/g, log₁₀ = 3.93.
Enter values above to see results.
B. cereus limits are strict for cooked rice and similar foods: many specifications sit under 1,000 CFU/g, and counts above 100,000 CFU/g raise a toxin concern — check the specification for the product. Tool 068 of 500 — GIFSQ
