42 PRESERVATIVES
Antimicrobial testing: what do the results mean?
n Kevin Roden – Independent Consultant, Australia
Antimicrobial active substances may be included in finished goods to provide internal protection from contamination by microorganisms, or to provide an external effect to kill microorganisms on surfaces. The antimicrobial actives that may be used are usually called preservatives when included in personal care products, whilst those used in other applications are referred to as referred to as biocides. This paper will concentrate on preservatives but the principles outlines are also valid for testing biocides. When any product is formulated with a
preservative, there may be interactions between the active substance and the product or container. The active may be lost due to partitioning into the oil phase or micelles, interaction with other ingredients, absorption onto the closure or container or adsorption onto solid particles. This may be further compromised by the preservative converting to an inactive form due to a pH effect or it may be degraded by the storage temperature or other factors. These factors may reduce the level of
preservative available leaving only the aqueous phase residual concentration to be effective to control microbial growth.
Abstract
Tests designed to determine the antimicrobial efficacy of a product, such as a preservative in a cosmetic, a sanitizer in a household cleaner, or a disinfectant, have set pass requirements. These often specify minimum log reductions or % kill to meet the requirements of the test. This article will discuss how these tests are conducted, what these terms mean, how they are calculated and how changes in the method used for the testing may lead to variations in the results achieved. We will also cover changes to the Laboratory Accreditation Standard, ISO 17025
with regards to Measurement of Uncertainty and Statements of Conformity and what impact these changes may have on certificates of analysis issued for Testing.
It is difficult to determine the aqueous
phase residual concentration by chemical analysis as extraction techniques may recover bound, absorbed or adsorbed active and give false high results. Microbiological testing is a convenient way to determine the amount of active material available in the water phase, as this is where microorganisms are found and where the preservative needs to be to provide good antimicrobial efficacy. Testing for disinfectant/sanitizer
efficacy requires microbiological testing to ensure that in use concentrations are effective as biocides may experience the same loss of activity issues outlined above
Laboratory population % of population Wild population
Table 1: Steps in Antimicrobial testing Step
Procedure
1 2 3
4 5 6 7
Prepare the test organisms
Add test organisms to the product under test
Allow a desired contact period Neutralise the antimicrobial Determine survivors
Calculate Log reduction or % kill Report the results
and may also be affected by the environment that they are used in where organic or other soiling may reduce their effectiveness. Microbiological testing for antimicrobial
activity follows a simple pattern as shown in Table 1. Once an appropriate test method is selected all tests follow the same basic format. Although the basic procedure is fairly
simple to conduct, variations in the procedure can result in significant changes to the results achieved. Published standard test methods usually specify the steps to be taken to ensure reproducibility of the results. Possible variations in all steps from 1 – 6 may affect the results obtained and what they actually mean.
MIC Value Figure 1: Variation in MIC values for lab vs wild strain organisms. PERSONAL CARE NORTH AMERICA
Step 1: Preparation of the test organism The test organism plays an obvious and critical role in the test outcome. The species of microorganisms used in the test must be selected on the basis that they are realistic,
May 2020
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