Microbial Limits Testing (MLT) for Batch Release

Confirming the Microbiological Quality of Your Finished Product

Microbial Limits Testing (MLT) is a critical quality control procedure performed on finished batches of non-sterile pharmaceuticals, cosmetics, and personal care products. It verifies that the total count of bacteria and fungi in the product remains within acceptable limits and that it is free from specific objectionable microorganisms that could compromise product safety or consumer health. 

While sometimes referred to in relation to bioburden assessments, MLT specifically evaluates the microbiological quality of the final product prior to release, ensuring compliance with Good Manufacturing Practices (GMP) and regulatory requirements.

Understanding the Two-Part Test

The Microbial Limits Test is governed by two distinct but related chapters in the United States Pharmacopoeia (USP), which work together to provide a complete picture of a product's microbiological quality.

USP: Microbial Enumeration Tests

This is the quantitative component of the antimicrobial efficacy test. It is performed to determine the total number of viable aerobic microorganisms present in a sample. The test provides two key values:

• Total Aerobic Microbial Count (TAMC): Quantifies the total number of aerobic bacteria.
• Total Yeast and Mould Count (TYMC): Quantifies the total number of fungi (yeasts and moulds). These counts are compared against established acceptance criteria (e.g., not more than 1000 CFU/g for a non-eye-area cosmetic) to determine if the product's overall bioburden is within safe limits.

USP: Tests for Specified Microorganisms

This is the qualitative component of the preservative efficacy test method. While a product may have a low overall microbial count, it is still considered unsafe and adulterated if it contains certain high-virulence or objectionable microorganisms. The USP test is performed to ensure the complete

absence of these specific pathogens. The specified organisms tested for can vary based on product type and intended use, but commonly include:

• Staphylococcus aureus
• Pseudomonas aeruginosa
• Escherichia coli
• Salmonella species
• Candida albicans
• Bile-Tolerant Gram-Negative Bacteria
• Clostridia species
  
  

The Critical First Step: Method Suitability Testing

Before routine Microbial Limits Testing (MLT) can be performed, a one-time Method Suitability Test must be conducted according to USP guidelines. This critical validation step ensures the accuracy and reliability of the quantitative antimicrobial efficacy test results.

During the test, the product is spiked with a low, known number of specified microorganisms to demonstrate that the preservative efficacy test method, including dilution steps and neutralising agents, can effectively recover and detect microbes within that specific product matrix. Without this validation, the antimicrobial properties of the product’s own ingredients, such as preservatives, could inhibit microbial growth on the test plates, potentially causing a false negative result.

Successful completion of the Method Suitability Test confirms that the antimicrobial efficacy test is scientifically valid for your product, ensuring compliance with USP standards and supporting both quantitative and qualitative preservative efficacy testing.

FAQs 

Q1: What is Microbial Limits Testing (MLT)?

MLT, or "Bioburden" testing, is a quantitative antimicrobial efficacy test that measures total aerobic microbial and yeast/mould counts in finished products. It ensures products meet microbiological quality standards before batch release.

Q2: What is the purpose of Preservative Efficacy Testing UK?

Preservative efficacy testing UK evaluates whether a product’s preservatives effectively prevent microbial contamination over its shelf life. It supports regulatory compliance for cosmetic and pharmaceutical products.

Q3: What are the quantitative and qualitative components of MLT?

The quantitative component measures total microbial counts (TAMC & TYMC), while the qualitative component ensures the absence of objectionable pathogens. Together, they provide a complete assessment of microbiological safety.

Q4: What is a Preservative Efficacy Test Method?

A preservative efficacy test method validates that the product maintains its antimicrobial performance, even in the presence of water and other ingredients. It confirms the product’s safety and shelf-life stability.

Q5: Why is a Method Suitability Test important in MLT?

A Method Suitability Test validates that the MLT procedure can accurately detect and recover microorganisms from the product matrix. It prevents false negatives caused by the inhibitory effects of the product’s preservatives.

Release Batches with Confidence. Request Microbial Limits & Preservative Testing Today.