Browsing by Author "Herbst, Willem Jacobus"
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- ItemKinetic evaluation of lipopeptide production by Bacillus amyloliquefaciens(Stellenbosch : Stellenbosch University, 2017-03) Herbst, Willem Jacobus; Clarke, Kim Gail; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH SUMMARY: Crop losses, caused by phytopathogens and pests, are estimated to be as high as 83% worldwide. These losses along with the world's growing population put additional strain on food production and security which emphasise the need for improved crop protection strategies. Chemically derived pesticides and fungicides are the preferred control strategy against postharvest diseases, however their detrimental effect on the environment and human life have directed research towards alternative strategies. Biocontrol have been identified as an alternative since they are environmentally safe, biodegradable and show antagonistic behaviour against fungi, bacteria and even viruses. Bacillus spp. have been shown to be effective as they produce the lipopeptides surfactin, iturin and fengycin. Direct application of the organism as cells and spores have been well documented and is the focus of commercially developed products. However, cell free lipopeptides have achieved greater inhibition against phytopathogens and are less sensitive to environmental factors. The study optimised upstream production of antifungal lipopeptides, iturin and fengycin by Bacillus amyloliquefaciens in controlled batch cultures. The effect of nitrogen source, concentration and dissolved oxygen availability were quantified through rigorous kinetic [NEXT THREE PARAGRAPHS DID NOT COPY. PLEASE REFER TO ORIGINAL DOCUMENT] production (158.15 mAU*min/g/l), highest maximum productivity (26.92 mAU*min/h) and competitively high selectivity (86.7%). A decrease in dissolved oxygen availability, decreased antifungal lipopeptide production kinetics. Low oxygen conditions forced nitrate to be used as an alternative electron acceptor, decreasing the amount of nitrate available for lipopeptide production. Optimum conditions cultured in the bioreactor performed better with respect to antifungal kinetics (maximum concentration, specific production and productivity) except µmax and CDW compared to the optimum reported in a previous study (8 g/l NH4NO3). Maximum concentration increased from 285.66 to 290.17 mAU*min, specific antifungal production from 51.85 to 58.1 mAU*min/g/l and productivity from 5.67 to 22.32 mAU*min/h. Culture supernatant, concentrated by acid precipitation, were used for antifungal efficacy tests. Fungal inhibition was observed against Botrytis cinerea, Alternaria brassicicola, Monilinia fructigena, Penicillium expansum and Rhizopus stolonifer while no inhibition was observed against Aspergillus sclerotiorum. The high effectiveness of antifungal lipopeptides in combination with kinetic data from this study indicate the potential to develop a standardised antifungal product for use against phytopathogens affecting post-harvest fruit. The effect of the process parameters on homologue production and ratio should be investigated, which could allow antifungal products to be tailored to contain specific homologues effective against specific phytopathogens. The use of continuous cultures for further kinetic evaluation and optimisation should be considered as it’s been shown, in this study, to be possible.