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How clean are the surfaces we touch?

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Have you ever wondered how dirty the objects we touch on a daily basis really are? With recent pandemics and outbreaks, there is often a question of how much a role the object surfaces we come into contact with every day are playing a role in transmission. Fomites, or inanimate objects, play an important role in transmission of various microbes and some microbes can last on surfaces for a long time. This can make highly traversed places like public transportation hotbeds for transmission of diseases. In fact, a study conducted in Mexico showed that train and station surfaces are covered in typical skin microbiota.

Prior to the SARS-COV-2 pandemic, public transportation use was on the rise as people were concerned about environmental costs and sustainability challenges of vehicle ownership. However, concern for minimizing infection risk has led to a decrease in the use of public and other shared transportation services. To combat this, equipment manufacturers have been developing new technologies to tackle vehicle sanitation through developing materials with antimicrobial functionalities.

To assess the antimicrobial activity of these surfaces, manufacturers often use protocols established by either the Japanese Industrial Standards (JIS) or International Standards Organization (ISO). Common shared standards include testing surfaces using a liquid bacterial inoculum which is then cultivated at 37℃ for 24 hours. The number of bacteria are then tallied and compared to counts on untreated surfaces, Unfortunately, neither of these standard protocols reflect the conditions the materials will be used, for instance, the dry surface exposure conditions. Researchers in this study developed a modified test protocol (based on ISO) to evaluate the antimicrobial efficacy of 20 materials.

To better simulate dry exposure conditions, researchers modified 5 parameters of the ISO 22196 and 21702 protocols as shown in Figure 1. These two protocols are related to measurement of antibacterial and antiviral activity on plastics and other non-porous surfaces. First, instead of a 400 µl inoculum volume, 10 aliquots of 5 µl were utilized. The test organisms were then allowed to dry on the test surface as opposed to being covered by a thin film which would give a constant state of moisture. In addition, the bacterial inoculum was prepared in distilled water differing from the nutrient broth and/or saline solution used previously. Furthermore, instead of a 37°C temperature being applied during contact, ambient temperature (avg temp of air in the given environment) and humidity were applied. Lastly, contact time was reduced from 24 hrs to 4 hrs.

A) ISO 22196 Standard Method. B) Proposed modified method. | Source: https://doi.org/10.1128/aem.01150-24

The 20 samples tested in using this protocol showed significantly different antimicrobial efficacy when compared to the test results using the standard ISO methods. Only 2 samples achieved an ideal standard of a 3-log reduction in microorganisms grown. 8 of the samples were between 1-3-log reduction and the remaining 10 had no effect at all. This variation highlights the importance of matching laboratory and testing conditions to conditions encountered under real-life circumstances. In an ideal scenario testing methods should be tailored to the specific requirements, applications and characteristics of the product as well taking into account real-world scenarios.

Link to the original post: Cheong YE, Weyandt R, Dewald W, Tolksdorf T, Müller L, Braun A.0.A realistic approach for evaluating antimicrobial surfaces for dry surface exposure scenarios. Appl Environ Microbiol0:e01150-24.https://doi.org/10.1128/aem.01150-24

Featured image: Germs on public transportation. | Source: DeepAI