Antimicrobial Textiles Testing
Humans have been weaving textiles and fashioning clothing from fabrics for thousands of years, but one of the great challenges since the dawn of civilization has been the degradation and fouling of clothing from microorganisms. Bacteria and fungi from the wearer’s skin and those picked up from the environment can settle into the spaces between textile fibers, and overtime form biofilms which are difficult to eradicate. With the advent of synthetic fibers such as polyester, which are more resistant to degradation due to the insolubility of the polymer in water and low moisture retention, there has been improvement on some of these issues. However, microbial soiling continues to be a pressing concern for the textile industry, particularly for textiles used in hospital and health care settings.
The modern application of antimicrobial to textile was pioneered by the British surgeon, John Lister, in the 1860s [1]. Bandages soaked in phenol were used to prevent infections in surgical sites, but prolonged contact between the phenol saturated dressing and the patient’s skin led to chemical burns. Antimicrobial fabrics have come a long way since the 19th century. Modern antimicrobial fabrics utilize nanoparticles (e.g., silver), natural fiber and polymer (e.g., lignin, chitosan, etc.), as well as treatments to recharge and prolong the efficacy of antimicrobial agents (e.g., N-halamine, plasma treatment, etc.) [2].
To assess the efficacy of new antimicrobial treatments, standardized evaluation of efficacy is key. To that end, the American Association of Textile Chemists and Colorists (AATCC) offer guidelines on different test methods that can be used to assess a fabric’s antimicrobial potency. Three test methodologies are highlighted below, with a description of their setup as well as a brief overview of some of their strengths and limitations.
AATCC – TM147: Parallel Streak Method
The parallel streak method is a qualitative method for antimicrobial activity. A suspension of a bacteria test strain is prepared, and five horizontal streaks are made across an agar plate—each approximately 60 cm in length—with an inoculation loop. A swatch of treated fabric, an untreated control fabric, or a control reference fabric with known antimicrobial activity is then placed over the streaks. After overnight incubation the clear zone of inhibition can be measured with a ruler on either side of the test swatch.
Figure 1. Photo of parallel streak test agar plates containing an untreated control (left) and antimicrobial treated fabric (right).
The advantage of this method is that it allows for quick and relatively easy assessment of antimicrobial activity and whether a fabric is potentially bacteriostatic. The test criterion can be determined by the sponsor and other interest holders.
While this method is flexible in terms of antimicrobial criteria, the results cannot be construed as qualitative evaluation, for which other test methods are better suited for quantitative endpoint (see AATCC—TM100). In addition, the method may not be suitable for antimicrobial treatments or agents which do not readily diffuse from the fabric and could lead to a false negative result.
AATCC – TM030, Test III: Aspergillus niger
This test specifically assesses the ability of antimicrobial textile to inhibit the growth of Aspergillus niger, a common environmental fungus. A liquid suspension of A. niger spores are dispensed over the surface of an agar plate and allowed to air dry. A pre-wet text swatch or negative control (either filter paper or untreated cotton swatch) is laid atop the agar, and a small volume of A. niger suspension is distributed atop the fabric. The agar is incubated for up to 7 days, and the resulting zone of inhibition can be measured with a ruler, and fungal growth on top of the test swatch can also be visually assessed.
Figure 2. Photos of untreated (left) and antimicrobial treated (right) fabric after incubation with a lawn of A. niger spores.
Similar to TM147, this assay can provide qualitative assessment on the antifungal capacity of the test swatch for spore forming fungi. As such, it also shares some of the potential limitations as TM147, as non-diffusible antimicrobial may produce false negative results. In addition, thick or non-porous material may not allow fungal spores inoculated on top of the swatch to reach the nutrients in the agar, thereby producing a false negative result.
AATCC – TM100: Quantitative CFU Reduction
This method allows for quantitative assessment of antimicrobial activity, which can inform presence and degree of bactericidal activity in a treated fabric. A test swatch is placed in a specimen container with a screw cap lid and inoculated with a liquid suspension of the test bacteria. After a desired contact time (typically 24 hours), a neutralizing solution is added to the container to quinch the antimicrobial activity from the test swatch. The contents of the container are vigorously shaken for a minute, then serial diluted and plated on agar for CFU enumeration. The bacterial colonies recovered is compared to a control untreated swatch, or to the initial inoculum density.
Figure 3. Test swatches submerged in bacteria inoculum and neutralizing solution (the source of the purple color) after the 24 hours contact time between fabric and bacteria.
Compared to the qualitative methods in TM147 and TM030, the quantitative data generated using the TM100 test methods allows for more meaningful assessment of antimicrobial treatments and comparisons between different conditions. The ability to assess the kinetic of bactericidal activity by testing different contact time between fabric and bacteria can also provide valuable information on how quickly an antimicrobial treatment can reduce viable bacteria. The enumeration of bacteria CFU does increase the cost and setup time when compared to other methods, and similar to the methods discussed above, there is no universally accepted antimicrobial activity criteria for this AATCC method. Instead, acceptance criterion should be agreed upon by the sponsor and other interest holders before the assay is conducted.
The scientists at Emery Pharma have extensive experience in antimicrobial testing and development and are well versed in the methods discussed above and can also adapt other methods as they suit our customer’s needs. For help with your novel antimicrobial textile, please inquire and speak to one of our scientists.
References
- https://pubmed.ncbi.nlm.nih.gov/5336181/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8275915/
