GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 064 of 500
Enumeration tracker. Enter duplicate plate counts and the specification limit to get the E. coli result in CFU per gram, check it against the limit, and save it to the on-device log.
Try sample values — fills the form with the worked example below.
Saved results
Saved on this device only — entries stay in this browser and are not sent anywhere.
| Date | Sample | Result |
|---|
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CFU/g
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Log₁₀ CFU/g
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Limit (CFU/g)
Equation: CFU/g = mean count ÷ (V × d), where V = volume plated (mL) and d = total dilution factor. Log₁₀ CFU/g = log₁₀(CFU/g). Detection limit = 1 ÷ (V × d). Pass when CFU/g is at or below the specification limit.
Method: E. coli enumeration, ISO 16649-2. Tryptone bile X-glucuronide (TBX) agar, 44 °C for 24 hours; count blue-green colonies. Countable range 10 to 150 colonies per plate.
Assumptions: Each colony grew from one viable E. coli; the sample was mixed evenly; the dilution series was prepared correctly; the limit entered is the one from the product specification.
Limitations: Counts below 10 or above 150 colonies are estimates; TBX counts glucuronidase-positive E. coli only; a low count does not prove the absence of pathogens.
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: E. coli enumeration, ISO 16649-2. Tryptone bile X-glucuronide (TBX) agar, 44 °C for 24 hours; count blue-green colonies. Countable range 10 to 150 colonies per plate.
Assumptions: Each colony grew from one viable E. coli; the sample was mixed evenly; the dilution series was prepared correctly; the limit entered is the one from the product specification.
Limitations: Counts below 10 or above 150 colonies are estimates; TBX counts glucuronidase-positive E. coli only; a low count does not prove the absence of pathogens.
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: Duplicate counts 45 and 55, 1 mL plated at a total dilution of 10⁻², limit 100 CFU/g. Mean count 50; CFU/g = 50 ÷ 0.01 = 5,000 CFU/g, log₁₀ = 3.70 — above the limit, so the result fails.
Enter values above to see results.
E. coli limits are strict: many ready-to-eat foods are specified at under 100 CFU/g, and some at not detected — check the specification for the product. Tool 064 of 500 — GIFSQ
