An in vitro evaluation of the antibacterial and anticancer properties of the antimicrobial peptide nisin Z

Abstract

The rise in antibiotic resistance and the lack of the development of new antibiotics poses a considerable threat to human health. This is especially of concern in individuals who are immune-compromised (due to immunosuppressive diseases or chemotherapy). There is a desperate need for intervention aimed at strengthening our current arsenal of antibiotics or developing new antibiotics to prevent high death rates due to infections with antibiotic resistant superbugs. In spite of the higher susceptibility to bacterial infections in cancer patients undergoing chemotherapy, resistance to conventional chemotherapy agents also poses a threat to the successful treatment of cancer. Antimicrobial peptides (AMPs) are multifunctional and several peptides have both antibacterial and anticancer activities, as well as displaying immune-modulatory properties. Therefore, AMPs may be considered alternatives to antibiotics/chemotherapy agents or as adjuvants to conventional antibiotics/chemotherapy agents. In this thesis, the antibacterial as well as anticancer activities of the Generally Regarded as Safe (GRAS) status AMP, nisin Z, were evaluated. The antibacterial activity was assessed with regard to the interaction of nisin Z with conventional antibiotics on Staphylococcus aureus, S. epidermidis and Escherichia coli. Additionally, the use of biodegradable lipid nanoparticles has been shown to enhance the antibacterial activity of antibiotics and AMPs. Therefore, the effectiveness of nanostructured lipid carriers (NLCs) for the entrapment of nisin Z was also assessed. The anticancer activity of nisin Z was evaluated against cultured melanoma cells. Reprogramming of cellular metabolism is now considered one of the hallmarks of cancer. Most malignant cells present with altered energy metabolism which is associated with elevated reactive oxygen species (ROS) generation. This is also evident for melanoma, the leading cause of skin cancer related deaths. Altered mechanisms affecting mitochondrial bioenergetics pose attractive targets for novel anticancer therapies. In this study, the anti-melanoma potential of nisin Z was evaluated in vitro. The underlying anticancer mechanism of nisin Z with regard to the ability of this AMP to induce ROS production, apoptosis, disrupt the energy metabolism (glycolysis and mitochondrial respiration) and inhibit cell proliferation and invasion of melanoma cells was investigated. Likewise, the ability of nisin Z to enhance the cytotoxicity and selectivity of conventional chemotherapeutic agents was also investigated. Finally, synergistic interactions between nisin Z and conventional chemotherapeutic agents were examined. Results indicated that nisin Z exhibited additive interactions with numerous conventional antibiotics. Notable synergism was observed for novobiocin-nisin Z combinations. The addition of the non-antibiotics adjuvant ethylenediaminetetraacetic acid (EDTA) significantly improved the antimicrobial activity of free nisin Z towards E.coli. NLCs containing nisin Z were effective against Gram positive species at physiological pH, with an increase in effectiveness in the presence of EDTA. Results indicate that nisin Z may be advantageous as an adjuvant in antimicrobial chemotherapy, while contributing in the battle against antibiotic resistance. NLCs have the potential to enhance the antibacterial activity of nisin Z towards Gram-positive bacterial species associated with skin infections. The minimum inhibitory concentrations (MICs) and half maximal inhibitory concentrations (IC50) were used as a measure of the toxicity and selectivity of nisin Z to bacterial and mammalian cells, respectively. Based on the results from this study, nisin Z displays selective toxicity to bacterial and cancer cells, compared to non-malignant cells. Furthermore, nisin Z was shown to negatively affect the energy metabolism (glycolysis and mitochondrial respiration) of melanoma cells, increase ROS production and cause apoptosis. Results also indicate that nisin Z can decrease the invasion and proliferation of melanoma cells demonstrating its potential use against metastasis associated with melanoma. In the current study it was found that combinations of nisin Z with 5-fluoruracil, hydroxy urea and etoposide were able to enhance the cytotoxicity of these conventional chemotherapeutic agents to melanoma cells. The etoposide-nisin Z combination also displayed a synergistic interaction. In conclusion nisin Z with its GRAS status, in addition to displaying direct antibacterial and anticancer properties, shows great potential to be used as an adjuvant with conventional antibiotics and chemotherapy agents