PRESERVATIVES 27
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 governed by two main regulations; for personal care products antimicrobial actives, known as preservatives, must be included in Annex V of the Cosmetic Regulations1
whilst those used in other
applications, referred to as biocides are covered by the Biocidal Products Regulation.2
Both will be referred to as
biocides in this paper. When any product is formulated with a biocide, 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 biocide 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 biocide available leaving only the aqueous
Abstract
Tests designed to determine the antimicrobial efficacy of a product, such as a preservative in a cosmetic, a sanitiser 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.
phase residual concentration to be effective to control microbial growth. 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 biocide needs to be to provide good antimicrobial efficacy. Testing for disinfectant/sanitiser
efficacy requires microbiological testing to ensure that in use concentrations are effective as biocides may experience the
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
same loss of activity issues outlined above 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. April 2020
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
PERSONAL CARE EUROPE
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