Discovery and evaluation of novel antimicrobial agents against nosocomial pathogens

Abstract

[Abstract]: High risks and costs are associated with infections caused by antibiotic-resistant organisms. The continued development of antibiotic resistance in pathogenic organisms means that in order to maintain our current standards in health care and disease treatment, new antibiotics need to be sourced. Such was the ultimate goal of this project. Endophytic fungi, due to their broad diversity, unique habitats and ease of acquisition, have been shown to be an important source of bioactive natural products, including novel antibiotics. Thus, the effort of this project was concentrated on the isolation of endophytic fungi associated predominantly with native plants, and the evaluation of the secondary metabolites they produced. Tissues were harvested from 61 individual plant hosts representing 15 plant species. Tissue segments were prepared and incubated on two different media formulations. As fungal growths became apparent, they were subcultured in order to obtain pure fungal isolates. A total of 133 fungal cultures were progressively isolated, covering over 100 fungal species. Ultimately, 77 of these isolates were screened for antimicrobial activity against a panel of 7 bacterial strains. Three isolates subsequently demonstrated repeatable antimicrobial action. They were identified as Schizophyllum commune, a species of Penicillium and Sporormiella isomera, and their bioactivity further evaluated. During the course of the project, activity originally detected in the S. isomera became indiscernable. The Penicillium species demonstrated some activity, being able to inhibit the growth of Staphylococcus aureus at a dilution of 1:2. It was also found to be mildly toxic to murine fibroblast L929 cells at a dilution factor of between 1:16 and 1:32, depending on the cytotoxicity assay used. Of the three isolates S. commune possessed the highest level of activity. A 1:256 dilution inhibited the growth of staphylococci, including an MRSA strain. A 1:32 dilution was bactericidal against E. coli. However, while it demonstrated useful antimicrobial activity, its toxicity would prevent its immediate development as such – a dilution factor of 1:1024, the highest dilution tested in the cytotoxicity assay, was capable of killing over 50% of L929 cells. Further analysis of this isolate’s products may reveal multiple products, with varying levels of activity and toxicity, novel modes of action, or novel structures. Separation of the active products could allow for further investigations, and if combinatorial chemistry is utilised, their structures may be manipulated in order to reduce the levels of toxicity. As long as bacteria continue to develop resistance to the antibiotics we use to treat their infections, we will need to source new antibiotics. The continued isolation, screening and evaluation of endophytic fungi and their secondary metabolites can only be of benefit to all higher life forms on Earth.