Formulation and antibacterial properties of clay mineral-tetracycline and -doxycycline composites

Clay minerals have been used as adsorbents for decades but research into their use within healthcare as drug-carriers and modified drug release materials is an increasingly common area of interest. In current clinical practice the management of acute bacterial skin and skin-structure infections (ABSSSIs) requires patients to take systemic antibacterial treatment due to a lack of appropriate topical options. In this research tetracycline (TC) and doxycycline (DC) were adsorbed onto a range of clay minerals (kaolinite, montmorillonite, acid-activated montmorillonite, Laponite® RD and Laponite® XL21) to evaluate their potential as materials for the delivery of these antibiotics to infected wounds. A dispersion pH that favoured the zwitterionic form of the antibiotic molecules was shown to favour adsorption onto the clay minerals. FTIR and pXRD showed that positively charged groups on the antibiotic molecules interacted with the negatively charged clay mineral surface, whilst negatively charged groups on the antibiotic molecules could interact with the positively charged edge-sites of the clay minerals. Swelling clays such as the two Laponites® were able to adsorb much more TC and DC due to their structure and chemistry. The clay minerals alone did not have any antibacterial effects against Staphylococcus epidermidis, Cutibacterium acnes, and Pseudomonas aeruginosa. Antibiotic containing composites successfully released TC and DC, exhibiting activity against the three bacterial strains proportional to the antibiotic-loading on the composites. This research demonstrates the ability of these clay minerals to deliver TC and DC against common skin pathogens and their potential for future development towards clinical applications.