GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 067 of 500
Confirmation count. Enter the typical colonies on Baird-Parker agar and the coagulase confirmation result to get the S. aureus 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 = typical colonies × (confirmed ÷ tested) ÷ (V × d), where V = volume plated (mL) and d = total dilution factor. Log₁₀ CFU/g = log₁₀(CFU/g). Detection limit = 1 ÷ (V × d).
Method: S. aureus enumeration, ISO 6888-1. Baird-Parker agar, 37 °C for 24 to 48 hours; confirm typical colonies by the coagulase test. Countable range 20 to 200 colonies per plate.
Assumptions: The tested colonies represent all typical colonies on the plate; each colony grew from one viable unit; the sample was mixed evenly.
Limitations: Counts below 20 or above 200 colonies are estimates; only coagulase-positive colonies are counted; enterotoxin can be present even when the count is low.
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: S. aureus enumeration, ISO 6888-1. Baird-Parker agar, 37 °C for 24 to 48 hours; confirm typical colonies by the coagulase test. Countable range 20 to 200 colonies per plate.
Assumptions: The tested colonies represent all typical colonies on the plate; each colony grew from one viable unit; the sample was mixed evenly.
Limitations: Counts below 20 or above 200 colonies are estimates; only coagulase-positive colonies are counted; enterotoxin can be present even when the count is low.
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: 120 typical colonies, 5 tested for coagulase and 4 confirmed, 1 mL plated at a total dilution of 10⁻². Confirmed fraction 0.8; CFU/g = 120 × 0.8 ÷ 0.01 = 9,600 CFU/g, log₁₀ = 3.98.
Enter values above to see results.
S. aureus limits are strict: many ready-to-eat foods are specified under 100 CFU/g, and counts above 100,000 CFU/g raise an enterotoxin concern — check the specification for the product. Tool 067 of 500 — GIFSQ
