More thorough washing protocols reveal a new mechanism for antimicrobial materials

In the world of preventing both infection and microbial resistance, devices and materials that incorporate antimicrobial compounds could provide novel solutions. Materials whose properties originate from antimicrobials covalently grafted to their surface are gaining traction. The precise mechanisms of how these new devices prevent the growth of microbes remain elusive however, and can sometimes lead to confusion. One group has found a surprisingly straightforward cause of some of these discrepancies.

By using more thorough washing protocols than are present in many studies of antimicrobial coatings, and examining thoroughness of washing with spectrometry, Naderi et al. report that the mechanism for one antifungal material’s properties lies in the drug eluting from the surface of the material.

The misinterpretation of antimicrobial mechanism could be problematic when it comes to regulatory approval of novel devices, said author Bryan Coad. “This study is interesting in that we might have misinterpreted the mechanism of action had we not thoroughly investigated,” he said.

Coatings with polyenes seemed very effective, yielding a one-millionth reduction of Candida albicans colony units after washing protocols assumed to be sufficient to remove covalently attached molecules. But after additional washing with surfactants, X-ray photoelectron spectrometry and time-of-flight mass spectrometry experiments instead showed that more firmly adsorbed drug molecules were released from the surface.

Coad said the findings in this case point toward covalently bonded drugs lacking the contact-killing mechanism often reported for such materials, likely because they fail to engage with their membrane targets. Instead, the mechanism was consistent with desorption of physisorbed polyenes that caused the antifungal effect. The group recommends analytical verification using instrumental methods to avoid confirmation bias.

Source: “Surface-grafted antimicrobial drugs: Possible misinterpretation of mechanism of action,” by Javad Naderi, Carla Giles, Solmaz Saboohi, Hans J. Griesser, and Bryan R. Coad, Biointerphases (2018). The article can be accessed at https://doi.org/10.1116/1.5050043 .