Microbial Limit Test Using Plate Count Method: Controlling Errors in Colony Enumeration

Microbial Limit Testing (MLT) is essential in pharmaceutical quality control to ensure products meet microbiological safety standards. The plate count method—including pour plate and spread plate techniques—is widely used but can be affected by errors during colony counting.

What Is the Plate Count Method?

This method estimates microbial load by culturing diluted samples on agar plates:

• Pour Plate: Mix sample with molten agar before pouring.

• Spread Plate: Spread diluted sample on solidified agar.

Ideal range: 30–300 CFU per plate for accurate results.

Common Sources of Error

• Dilution Inaccuracy: Use calibrated pipettes and fresh diluents.

• Colony Overlap: Avoid overloading plates (>300 CFU).

• Media Variability: Maintain consistent agar thickness and pH.

• Incubation Issues: Control temperature and humidity.

• Manual Counting Errors: Use digital or automated colony counters.

Best Practices

✔ Choose appropriate dilutions (target 30–300 CFU).
✔ Use validated equipment and standardize media preparation.
✔ Monitor incubation conditions.
✔ Consider digital counters for improved accuracy.
✔ Perform duplicate tests to confirm results.

Compliance Tips

Follow pharmacopeial standards:

• USP <61>/<62>

• EP 2.6.12 / 2.6.13

• JP 4.05

Maintain full documentation and include controls in each test batch.

Conclusion

The plate count method is reliable for microbial testing when properly executed. By minimizing errors through good lab practices and validation, pharmaceutical labs can ensure accurate, reproducible, and regulatory-compliant results.