Mesoporous silica as a protective matrix for nisin

Currently, the treatment of certain antimicrobial infections is hindered by the increase in resistance acquired by pathogenic species of bacteria, fungi and viruses. Thus, there is a growing need for novel antimicrobial agents which are active against resistant bacterial strains. Bacteriocins are antimicrobial peptides, some of which have shown activity against resistant strains of bacteria, e.g. MRSA. However, as peptides, they are subject to proteolytic degradation in vivo following administration, particularly by the oral route due to the abundance of proteases in the GI tract. Therefore, suitable protective matrices are required to prevent the degradation of bacteriocins in vivo and allow their antimicrobial properties to be exploited. In this work, mesoporous silica matrices with different pore size and functionality were prepared and tested for their suitability in protecting the bacteriocin nisin from protease degradation in vitro. Reducing the pore size of these matrices appeared to improve the protection of nisin A from digestion by the broad-spectrum protease, proteinase K. MCM-41 (pore size 28 Å, unfunctionalised) improved protection of nisin A over SBA-15 (pore size 68 Å, unfunctionalised). Increasing the hydrophobicity of silica by functionalisation also improved the protection of nisin, with MSE (pore size 60 Å, ethylene-functionalised) showing improved protection of nisin against proteinase K compared to SBA-15. Under simulated gastric conditions, a burst release was observed from unfunctionalised MCM-41 and SBA-15, which present silanol groups at the surface. The ethylene-functionalised, hydrophobic matrix, MSE, provided a more gradual, extended release of nisin showing the impact of functionalisation on the release profile of nisin from the matrices. Under simulated intestinal conditions, release from MCM-41 and MSE was not detected, while SBA-15 only released 12% of the adsorbed nisin. Although the release of nisin from MCM-41 and MSE was not detected into simulated intestinal fluid, the release media, and the suspended particles, were still active against the indicator strain, Lactobacillus delbrueckii subsp. bulgaricus. According to these results, MCM-41 may prove to be a successful matrix in partially protecting nisin, and potentially other bacteriocins, following oral administration for infections of the colon, e.g. Clostridium difficile. However, an enteric coating would need to be applied, to prevent premature release in the gastric environment. The slow release provided by the ethylene-functionalised MSE may be beneficial for subcutaneous implants of therapeutic peptides to prolong their effect and reduce the need for frequent, painful injections.