GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 063 of 500
Plate-count worksheet. Enter the sample, dilution and duplicate plate counts to get the coliform 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: Coliform enumeration, ISO 4832. Violet red bile agar (VRBA), 30 °C or 37 °C for 24 hours; count red colonies with a precipitation zone. Countable range 10 to 150 colonies per plate.
Assumptions: Each colony grew from one viable coliform; the sample was mixed evenly; the dilution series was prepared correctly; typical colonies were identified correctly.
Limitations: Counts below 10 or above 150 colonies are estimates; coliforms are indicator organisms, 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: Coliform enumeration, ISO 4832. Violet red bile agar (VRBA), 30 °C or 37 °C for 24 hours; count red colonies with a precipitation zone. Countable range 10 to 150 colonies per plate.
Assumptions: Each colony grew from one viable coliform; the sample was mixed evenly; the dilution series was prepared correctly; typical colonies were identified correctly.
Limitations: Counts below 10 or above 150 colonies are estimates; coliforms are indicator organisms, 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 10⁻² step, 1 mL plated, duplicate counts 120 and 132. Total dilution = 0.1 × 0.01 = 0.001; mean count 126; CFU/g = 126 ÷ 0.001 = 126,000 CFU/g, log₁₀ = 5.10. Both counts are inside the countable range.
Enter values above to see results.
Coliform limits are strict for many foods: pasteurised milk is often specified under 10 CFU/mL — check the specification for the product. Tool 063 of 500 — GIFSQ
