Optimisation of antibiotic production in actinomycetes
Colony development and secondary metabolite production of the streptomycete strain 1336 was studied on solid and in liquid media. During colony development on solid media specific rates of increase in branch number and total mycelial length were constant for a particular medium composition. When the nutrient status of the medium was improved these rates increased. Despite these effects of nutrient concentration the ratio between total branch number and total mycelial length, the hyphal growth unit, was constant, i.e. its length is an intrinsic species dependent characteristic. In liquid medium patterns of pellet development, their interrelationships with secondary metabolite production, specifically of avermectins and oligomycin A, and the nutrient status of media were studied. The distance between the tip of a hypha and its first subtended branch, the tip to first branch (TFB) distance, in the periphery of pellets was examined and a reproducible pattern of change observed. The TFB distance doubled during the first 10 h of the fermentation. This was followed by a 20 h period in which the distance decreased. Towards the end of this phase the rate of decrease fell until after a short period of stability the distance increased once more. During this second phase of increase, lasting 30 h, TFB distance increased 3-fold. Throughout these periods of change the TFB distances differed by up to 50 μm. In addition the frequency distribution of TFB distances showed that control of the positioning of a branch behind the tip of a parental hypha varied during a fermentation. In the initial stages of a fermentation on Merck Sharp Dohme Medium E (MSDE) no dominant class was observed. This changed during the later exponential phase of growth when over 70% of branches were subtended within a distance of 20 μm of the hyphal tip. During the stationary phase dominance by these classes was slowly released. Perturbations in the nutrient composition of MSDE either had no effect on this observed pattern, as with glucose concentration, or affected the rate at which the pattern developed. Reducing the concentration of yeast autolysate and peptone increased the rate of development whilst reducing the concentration of inorganic salts reduced the rate of development. Comparisons of fermentations of MSDE and a related medium, Micromonospora medium (MMSP), were made. In MMSP no pattern of pellet development was observed in terms of temporal changes in tip to first branch distance and frequency distribution of those distances. In addition very low levels of oligomycin A were produced during growth on this medium. However, when inorganic salts were added to MMSP, pellet development and secondary metabolite production were comparable to those observed in an MSDE fermentation. The final part of the thesis is devoted to a mathematical model which was developed to describe the fermentation of MSDE by strain 1336.