Antimicrobial Effectiveness Testing (AET), also called Preservative Efficacy Testing (PET), is a microbial challenge test performed to determine if the preservative in a product, or the antimicrobial effect created by the properties of a product, is sufficient to prevent the growth of microorganisms. In other words AET demonstrates the effectiveness of a substance— when used as a preservative or additive— to stop the growth of microorganisms. Generally 3 bacteria and 2 fungi are used. A controlled inoculum of the challenge organism(s) is placed in suspension with the sample to be tested, and then the number of survivors determined at different time points and the log reduction determined. Usually this is done at 7, 14, 21 and 28 days. A typical test may include gram-positive cocci, gram-negative fermentative bacilli, gram-negative non-fermentative bacilli, yeast, and mold. For example the following organisms— Aspergillus niger, Candida albicans, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus are used.

Antimicrobial preservatives are added to non-sterile products to protect them from microbiological growth or from microorganisms that are introduced inadvertently during or subsequent to the manufacturing process. In the case of sterile products packaged in multiple-use containers, antimicrobial preservatives are added to inhibit the growth of microorganisms that may be introduced from repeatedly opening the container and withdrawing small amounts of the product.

Mold & Bacteria Consulting Laboratories (MBL) can perform antimicrobial effectiveness testing using various standard methods including, USP 51 – Antimicrobial Effectiveness Testing, ISO 11930 – Evaluation of the Antimicrobial Protection of a Cosmetic Product, and client specific methods.