Antimicrobial coatings – capabilities and challenges

Referencia Apresentador Autores
Hans J. Griesser Griesser, H.J.(University of South Australia); Michl, T.(University of South Australia); Mon, H.(University of South Australia); Giles, C.(University of South Australia); Jasieniak, K.(University of South Australia); Coad, B.R.(University of South Australia); There exists extensive literature on antimicrobial coatings, with results that at times are surprising or contradictory. For example, when coupling onto a biomaterials surface an antibacterial compound whose mode of action is interference with an intracellular target molecule, how can such a coating be active, unless one proposes that instead of the classical mechanism proposed by microbiologists, a transmembrane signalling process applies. Further confusion arises because of differences in media, particularly when assessing cytotoxicity of antimicrobial coatings. Our research groups have focused in a number of studies on questions surrounding covalently anchored graft coatings comprising antimicrobially active molecules (low molecular weight, 300-1200 Da) or polymers. In particular, we have focused on detailed multi-technique surface characterisation, to probe for the presence of bioactive molecules that may not be covalently grafted and can interfere in biological tests, leading to incorrect conclusions that a covalently attached layer of molecules is active. We have used a range of antimicrobial molecules, both commercial and experimental, for various designed experiments with various grafted layers. The molecules were immobilised onto plasma polymer interlayers with suitable reactive surface groups. Surface analyses were performed using XPS, ToF-SIMS, and ellipsometry before and after extensive various washing protocols. Antimicrobial activity of samples was evaluated via attachment and biofilm formation assays. Our results show that physisorption of antimicrobials occurs to substantial extents onto many substrates – even in the presence of reactive surface groups and catalysts, such as carbodiimide. However, much larger amounts of molecules can be absorbed into polymer substrates – again bypassing reactive surface groups. Surface analysis data before and after various washing procedures clearly demonstrate the presence of both covalently grafted and non-covalent molecules. When placed into bioassays, the latter molecules diffuse into solution and interfere with microbes. This can lead to erroneous interpretation that a covalently grafted layer has activity, as we demonstrated using a negative control, levofloxacin, which has an intracellular target and thus no activity when covalently grafted, yet diffusing molecules gave rise to activity. Our results also demonstrate that detailed surface analytical studies in combination with various washing protocols – including severe conditions such as SDS and high temperature – are needed to ascertain that a putative antimicrobial coating exerts its bioactivity only through covalently grafted molecules rather than through solubilising molecules. It is easy for molecules to avoid reacting while physisorbing and absorbing onto/into the substrate (or spacer hydrogel layers), and if not recognised and accounted for, such contributions can confuse interpretations. Some of the surprising and contradictory literature data may have been caused by unrecognised contributions from systems more complex than the intended covalent graft layer.
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