Holloway, Andrew Charles (2013) Antimicrobial effects of white tea extracts in combination with putative adjuncts against 'Staphylococcus aureus' and other microbes of importance. (PhD thesis), Kingston University, .
Abstract
Previous studies have investigated biological activities of plant extracts and their subcomponent compounds as antimicrobial treatments. The addition of adjunct substances in varying combinations to such agents has also been shown to enhance their antimicrobial activities - a desirable outcome in the development of novel treatment substances to combat pathogens. Whole extracts of tea leaves processed in different ways to produce black, green and white tea types (BT, GT, WT) show different activities. In addition, the effects on pathogens of highly active GT extract rich in flavan-3-ols which are catechins have also been investigated alone and with additives and these can compare favourably with the activities of conventional antibiotics such as oxacillin and ciprofloxacin. However, relatively little attention has been paid to enhancing the activity of whole WT or its weak antimicrobial subcomponents. The activities of GT and BT were compared to the less researched white tea (WT). Principal tea flavan-3-ols were also screened for activity alone, and combined with a known antimicrobial agent copper(II)sulphate against 'Staphylococcus aureus' NCTC 06751. WT, a sub-fraction, (WTF<lkDa) and the weak antimicrobial catechin were further tested with another antimicrobial agent vitamin C. A catechin-copper(II) combination was initially tested against a panel of 4 bacterial species and then investigated further against the least susceptible 'S. aureus' using the antimicrobial agents: caffeine, and iron(II), as well as vitamin C additions. 'Escherichia coli' NCTC 14441 was also tested for comparison. Reaction mechanisms of catechin enhanced with additives were investigated using stoichiometry, crystallisation, UV-vis, solubility, heat treatment, pH, EDTA, H2O2 generation, catalase, kinetics and the effects of storage on antimicrobial activity. In 30 minute exposures and within the other conditions tested, the following were seen: Copper(II) alone, and combined with WT produced similar antimicrobial activities. Other teas reduced the copper(II) activity indicating the presence of inhibitory components. Copper(II) combined with vitamin C or WTF increased activity. Iron(II) enhanced catechin against 'S. aureus' but not against 'E. coli'; subsequent addition of copper(II) or vitamin C did not raise activity further. Caffeine, and further antimicrobial agent zinc, and manganese had no effect against 'S. aureus' whereas heat treatment of catechin raised activity of catechin-copper(Il) against 'S. aureus'. A catechin-copper(II) solution, stored at 20 degrees C for 1 day, revealed enhanced activity against 'Proteus mirabilis' NCTC 7827 and E. coli. Vitamin C added to catechin-copper(II) did not enhance activity of stored solutions, but caused losses in activity. However, freeze-drying preserved mixture activities. WTF; freshly-made as well as stored catechin solution; and iron(II) combined with copper(II) generated bactericidal H2O2 whose formation was prevented by EDT A and removed by catalase. Addition of copper(II) to catechin caused a fall in pH, a rise in solubility and in UV-vis absorption indicating a complex which showed rapid effects against 'S. aureus'. Catechin heat treatment raised both speed and level of effect against 'S. aureus' probably due to the production of novel substances. This is the first time, we believe, an enhancement of a weak antimicrobial such as catechin by adjuncts and by heating has been reported and the findings suggest new ways to develop antimicrobials. An additional finding was that catechin was enhanced mist by copper(ll) in its activity 'Pseudomonas aeruginosa' NCTC 950, a finding which may find importance in agricultural well as other settings. The addition of catechin may facilitate a reduction in the use and footprint of copper(ll) in the environment.
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