GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 066 of 500
Detection and enumeration workflow. Follow the ISO 11290 stages, then enter the ALOA plate count and the detection results to get the L. monocytogenes count in CFU per gram and save it to the on-device log.
Try sample values — fills the form with the worked example below.
Workflow stages
- Detection: primary and secondary enrichment in half-Fraser then Fraser broth (ISO 11290-1).
- Plating: ALOA agar and a second selective agar, 37 °C for 24 to 48 h.
- Enumeration: direct plating on ALOA agar (ISO 11290-2) for the count below.
- Confirmation: haemolysis, rhamnose and xylose tests on suspect colonies.
Saved results
Saved on this device only — entries stay in this browser and are not sent anywhere.
| Date | Sample | Result |
|---|
–
CFU/g
–
Log₁₀ CFU/g
–
Samples positive
Equation: CFU/g = colonies ÷ (V × d), where V = volume plated (mL) and d = total dilution factor. Log₁₀ CFU/g = log₁₀(CFU/g). Detection limit = 1 ÷ (V × d). Enumeration passes when CFU/g is at or below the limit.
Method: Listeria detection ISO 11290-1 and enumeration ISO 11290-2. ALOA agar, 37 °C for 24 to 48 hours; count blue-green colonies with a halo. Countable range 10 to 150 colonies per plate. A common ready-to-eat criterion is 100 CFU/g — check the product specification.
Assumptions: Each colony grew from one viable L. monocytogenes; the sample was mixed evenly; the dilution series was prepared correctly; suspect colonies were confirmed.
Limitations: Counts below 10 or above 150 colonies are estimates; a negative detection result means not detected in the portions tested, not proof the lot is free of Listeria.
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: Listeria detection ISO 11290-1 and enumeration ISO 11290-2. ALOA agar, 37 °C for 24 to 48 hours; count blue-green colonies with a halo. Countable range 10 to 150 colonies per plate. A common ready-to-eat criterion is 100 CFU/g — check the product specification.
Assumptions: Each colony grew from one viable L. monocytogenes; the sample was mixed evenly; the dilution series was prepared correctly; suspect colonies were confirmed.
Limitations: Counts below 10 or above 150 colonies are estimates; a negative detection result means not detected in the portions tested, not proof the lot is free of Listeria.
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: 85 colonies on ALOA, 1 mL plated at a total dilution of 10⁻², 1 of 5 detection samples positive, limit 100 CFU/g. CFU/g = 85 ÷ 0.01 = 8,500 CFU/g, log₁₀ = 3.93 — detected and above the limit, so the result fails.
Enter values above to see results.
Ready-to-eat foods that support Listeria growth are often specified at not detected in 25 g, or under 100 CFU/g where growth cannot occur — check the specification for the product. Tool 066 of 500 — GIFSQ
