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Title: The antibacterial activity of Humulus lupulus against mycobacteria
Author: Blaxland, James
ISNI:       0000 0004 5355 1869
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2015
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One third of the world’s population is estimated to be infected with M. tuberculosis, a pathogen which causes more human death and misery than any other bacterial disease. Whilst treatment is available, resistance to commonly used antimicrobials is a growing problem. Thus there is an urgent need to identify new compounds that can kill drug resistant isolates and are able to potentiate the activity of currently available antibiotics. The plant kingdom is a rich source of antibacterial compounds and a plant which has attracted particular interest is Humulus lupulus, more commonly known as the hop, which has been used as an antibacterial in beer for hundreds of years. Its antibacterial properties are thought to be due to the combined action of alpha and beta acids and polyphenols such as xanthohumol although the precise nature of their interactions and relative importance has yet to be determined. An optimised agar antimicrobial assay was developed and employed based on Mycobacterium smegmatis, to characterize the antibacterial activity of fifty commercially available hop varieties with a view to identifying novel antibacterial compounds. Surprisingly, no correlation was found between alpha and beta acid content and antibacterial activity. Chemical analysis of the most (Citra) and least (Galena) active hop variants using a combination of bioactivity based thin layer chromatography, mass spectrometry and HPLC revealed differences in the relative amounts of antimicrobial compounds such as humulone (alpha acid), lupulone (beta acid) and xanthohumol but failed to identify the presence of novel antibacterial compounds. Whilst no new antimicrobial compounds were identified, the Citra hop extract was able to potentiate the activity of the antibiotics imipenem and ciprofloxacin against clinical isolates of M. abscessus, a fast growing member of the mycobacterium family which infects individuals suffering from cystic fibrosis. The Citra hop extract also inhibited the growth drug resistant isolates of M. abscessus suggesting that it may have activity against other antibiotic resistant mycobacteria such as M. tuberculosis With regards to the mode of action, scanning electron microscopy revealed distinct changes to the outer cell structure of the bacteria, suggesting that hops contain compounds that interact with the bacterial cell membrane and/or cell wall. These changes were more profound in the presence of sub-inhibitory concentrations of imipenem, a compound which also targets the cell wall. Overall hops were shown to contain compounds which inhibited the growth of mycobacterium and were able to potentate the activity of antibiotics currently used to treat these pathogens. These findings suggest hops may be a fruitful source from which to isolate next generation compounds with which to treat increasingly drug resistant strains of mycobacteria.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QR Microbiology ; RS Pharmacy and materia medica