GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 070 of 500
Plate-count worksheet. Enter the sample, dilution and duplicate plate counts to get the Enterobacteriaceae 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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Detection limit (CFU/g)
Equation: CFU/g = mean count ÷ (V × d), where d = [sample ÷ (sample + diluent)] × further dilution; V = volume plated (mL); d = total dilution factor. Log₁₀ CFU/g = log₁₀(CFU/g). Detection limit = 1 ÷ (V × d).
Method: Enterobacteriaceae enumeration, ISO 21528-2. Violet red bile glucose (VRBG) agar, 37 °C for 24 hours; count pink-red colonies with a precipitation zone. Countable range 10 to 300 colonies per plate.
Assumptions: Each colony grew from one viable Enterobacteriaceae; the sample was mixed evenly; the dilution series was prepared correctly; typical colonies were identified correctly.
Limitations: Counts below 10 or above 300 colonies are estimates; Enterobacteriaceae are hygiene indicators, not proof of pathogens; atypical colonies need confirmation.
Rounding: CFU/g shown as a whole number; log₁₀ to 2 decimal places; intermediates unrounded. In a laboratory report, give CFU/g to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Enterobacteriaceae enumeration, ISO 21528-2. Violet red bile glucose (VRBG) agar, 37 °C for 24 hours; count pink-red colonies with a precipitation zone. Countable range 10 to 300 colonies per plate.
Assumptions: Each colony grew from one viable Enterobacteriaceae; the sample was mixed evenly; the dilution series was prepared correctly; typical colonies were identified correctly.
Limitations: Counts below 10 or above 300 colonies are estimates; Enterobacteriaceae are hygiene indicators, not proof of pathogens; atypical colonies need confirmation.
Rounding: CFU/g shown as a whole number; log₁₀ to 2 decimal places; intermediates unrounded. In a laboratory report, give CFU/g to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: 10 g sample in 90 mL diluent with a further 10⁻³ step, 1 mL plated, duplicate counts 210 and 190. Total dilution = 0.1 × 0.001 = 0.0001; mean count 200; CFU/g = 200 ÷ 0.0001 = 2,000,000 CFU/g, log₁₀ = 6.30. Both counts are inside the countable range.
Enter values above to see results.
Enterobacteriaceae limits vary by product: pasteurised milk is often specified under 10 CFU/mL, dried infant formula at not detected — check the specification for the product. Tool 070 of 500 — GIFSQ
