Antimicrobial Activity Testing Conditions: Why Should You Care?

Microbes are those pesky little microscopic creatures that cause havoc, creating unwanted odors and degradation of products. Antimicrobial fabric and other finished products will keep these little odor and stain causing beasts at bay. When you apply an antimicrobial finish to a textile to produce antimicrobial fabric, how do you know it will work? This is where standardized industry test methods and testing conditions come into play.

Over the years, a number of industry standard tests have been established to evaluate antimicrobial treatment performance on textiles. ISO20743, JISL1902, AATCC147 and AATCC100 are just of few of the methods that are commonly used today. With so many methods available, it can be extremely challenging for anyone not familiar with these standards to pick the most suitable one to test a new antimicrobial finish. Although there is no definitive “best” test method for antimicrobial activity testing, most industry standards are designed to test under conditions that simulate the environment where the final product will be used. To ensure antimicrobial performance, under real world conditions, it is not only important to select the correct industry standard test method, it is also important to run that standard using the correct testing conditions.  

Testing Conditions for Antimicrobial Products

The type of organisms and level of nutrient provided to those organisms has the biggest impact. First, the type of organism used in antimicrobial activity testing should be found in the environment the textile encounters. For instance, if we are testing antimicrobial clothing, you should select organisms that would normally transfer to the garment while wearing it. Staphylococcus, for example, is odorless bacteria that breaks down human sweat into offensive body funk. Verifying that antimicrobial fabric is effective against bacteria like Staphylococcus helps ensure the clothing is odor-free.

Nutrient available to the bacteria during antimicrobial activity testing is just as important as the types of microbes you test against. As we all know, food is essential for life. This holds true for both humans and microbes. If food is available, microbes are happy and they multiply. When they multiply, they will produce odor. On the other hand, when microbes are starved, they die off quickly and are unable to produce body odor. Using the appropriate amount of nutrient during industry standard antimicrobial activity testing is essential to simulating the nutrients normally found on textiles such as sweat, oils, dead skin, soils, etc.

Antimicrobial Treatment

Some antimicrobial suppliers will suggest that you test their antimicrobial treatment under low or very low nutrient conditions. Testing under no or low nutrient conditions make that supplier’s antimicrobial treatment appear to work well, but it does not guarantee their treatment will hold up in the real world. Take the following example where the amount of odor produced by bacteria on a fabric was measured when antimicrobial activity testing was run with no nutrient available to the bacteria. In the chart below, when bacteria were fed no nutrient (saline), they produced no odor on a textile. As we know from smelling a sweaty garment, there is plenty of odor produced by bacteria. So clearly testing an antimicrobial treatment with no or low nutrient available to the bacteria is not representative of actual conditions. On the other hand, when the test is rerun, this time giving the bacteria a nutrient level representative of real world conditions, the bacteria thrives and produces odor, as we would expect in the real world. 

Antimicrobial Fabric

This observation of odor is not readily apparent simply by conducting antimicrobial testing, because antimicrobial activity testing standards only look at how much bacteria dies off when that bacteria are placed on a fabric. Under artificially low nutrient conditions, bacteria will starve and die off regardless of whether an antimicrobial treatment is present.

Take this second example where a fabric treated with a silver antimicrobial is testing via the JISL1902 antimicrobial test standard. In the first case, the testing standard was run using no nutrient (note that the recommended amount of nutrient in the JISL1902 standard is 5%). In the test where no nutrient is provided to the bacteria, the antimicrobial treatment appears to have amazing effect, reducing bacteria levels on the fabric by more than 99.9%. Not so fast! The 99.9% reduction largely resulted from bacterial starvation, not the silver antimicrobial treatment. When the more real world level of nutrient is used (5%), we see that the silver antimicrobial treatment is not very effective at reducing bacteria on the fabric. Under these testing conditions, there is still enough bacteria on the fabric to create a big stink. Thus, we see the antimicrobial is actually not doing its job.

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Do Your Homework Before Testing Antimicrobial Fabric

Bottom line, when it comes to testing antimicrobial fabric, it pays to do your homework. Research and understand the types of organisms your textile product is likely to encounter. Discuss these with your antimicrobial supplier and/or microbiological testing facility before conducting performance testing. Also, understand the importance of nutrient level used in the antimicrobial test. Do not test your fabrics under no or low nutrient levels, as this is not typically a fair challenge of your antimicrobial treatment. Stick with trusted, industry developed test standards like ISO20743 and JISL1902, which prescribe real world levels of nutrient (5% nutrient broth).

To learn more about antimicrobial testing and conditions from Microban, contact us today!