CCT Calculator for Accurate Microbial Enumeration as per ISO 7218:2024

Introduction

Microbiological testing plays a critical role in food safety, ensuring compliance with international standards and regulatory requirements. ISO 7218:2024 is a globally recognized standard that provides general requirements and guidance for microbiological examinations within the food chain. One of the key aspects covered in this standard is the Colony Count Technique (CCT) and the use of the CCT Calculator to enhance accuracy and consistency in microbial enumeration.

What is ISO 7218:2024?

ISO 7218:2024 is an updated version of the international standard for microbiological examinations in food and animal feed testing. It outlines essential guidelines for:

• Laboratory design and biosafety considerations.

• Personnel competence and hygiene.

• Equipment and consumables used in microbiological testing.

• Methods for microbial enumeration and detection.

• Calculation and expression of microbiological test results.

Clause 11.2.6.1 of the standard specifically focuses on plate counts and the correct interpretation of colony counts on solid media.

Understanding Clause 11.2.6.1: Plate Count Calculations

1. Counting Colonies Correctly

The standard specifies that colonies should be counted from the lowest dilution plate that contains:

• No more than 300 total colonies.

• No more than 150 typical or presumptive colonies on a 90 mm Petri dish.

If a specific method prescribes different count limits, those should be followed accordingly.

2. Handling Overloaded or Underloaded Plates

• If the colony count slightly exceeds the upper limit (e.g., 301 colonies on a 90 mm plate), it may still be considered if it falls within the upper confidence interval.

• When counts are too high to differentiate individual colonies, results should be reported as Too Numerous to Count (TNTC) or as an estimate based on observable areas.

3. Managing Spreading Colonies

• If spreading colonies cover less than 25% of the plate, count the unaffected part, extrapolate, and report as an estimate.

• If spreading colonies cover more than 25% of the plate, the count should be discarded and another dilution should be used.

4. Low Microbial Counts and Duplicate Plates

• If multiple plates from the same dilution are counted, they should be averaged.

• For low microbial counts, all plates from a sample should be considered as a single count for accuracy.

The Role of the CCT Calculator

The Colony Count Technique (CCT) Calculator is a digital tool designed to facilitate accurate microbial enumeration by automating colony count calculations based on ISO 7218:2024 guidelines. It ensures consistency by applying:

• Dilution factor adjustments.

• Validation of duplicate plates.

• Correction for spreading colonies.

• Acceptability limits for differences in duplicate counts based on ISO 14461-2:2005 formulas.

This calculator is especially beneficial in food microbiology laboratories where accurate colony enumeration is essential for regulatory compliance and quality control.

Benefits of Using the CCT Calculator

• Reduces human error in colony counting.

• Ensures compliance with ISO 7218:2024.

• Standardizes calculations, making results more reproducible.

• Saves time by automating dilution factor applications and statistical checks.

How to Access the CCT Calculator

To help food microbiology laboratories implement ISO 7218:2024 efficiently, we are providing access to the CCT Calculator (Excel Tool). You can download the tool using the link below:

Download the CCT Calculator (Excel)

How to Use the CCT Calculator (Excel Tool) for Plate Count Calculations

Step 1: Open the Excel File

• Download and open the CCT Calculator Excel File.

• Ensure that macros (if any) are enabled for full functionality.

Step 2: Navigate to the Correct Sheet

The Excel file contains multiple sheets. Choose the relevant one:

• “plate count tool” – For standard colony counting calculations.

• “spiral plate” – If using a spiral plating method.

• “swabs” – If working with microbial enumeration from swab samples.

Step 3: Enter Sample Information

In the plate count tool sheet, locate the data input section:

• Dilution Factor – Enter the dilution factor of your sample (e.g., 1/10, 1/100).

• Volume per Petri Dish (mL) – Specify how much sample was plated per dish.

• Colony Counts for Duplicate Plates – Enter colony counts from multiple plates (if applicable).

Step 4: Automatic Calculations

The Excel sheet will automatically apply ISO 7218:2024 rules, including:

• Selecting the valid plate count range (≤300 total colonies or ≤150 typical colonies for 90 mm dishes).

• Applying correction factors for dilution and plate volume.

• Averaging duplicate counts and checking for acceptability differences using ISO 14461-2:2005 formulas.

Step 5: Interpreting Results

The sheet will display:

• Final microbial concentration (CFU/mL or CFU/g).

• Validation checks for duplicate plate variation (highlighting inconsistencies if present).

• Warnings if plate counts exceed acceptable limits

Step 6: Adjust and Verify Data

• If results indicate invalid counts, check for higher dilutions or repeat plating.

• For spreading colonies, use the adjusted count feature (if available).

• Save the file and generate a report if required.