A theoretical model of streptomycete growth in submerged culture
A theoretical model based on finite elements has been formulated for the growth and development of submerged mycelial pellet populations. Models based on partial differential equations proved to be mathematically unstable. The model is tested experimentally by studying the growth of the filamentous bacterium Streptomyces coelicolor A3(2) in batch and continuous culture. The effects of shear, growth rate and inoculum concentration on population development are considered. All pellets are assumed to grow according to cube root kinetics. Variations in the amount of biomass occurring throughout the width of the peripheral growth zone are accounted for. An inability to accurately simulate variations in pellet strength meant that whilst growth in low shear environments could be simulated well, growth at high shear could only be simulated qualitatively. Biomass yield was directly related to agitation rate, a phenomenon, suggested by the model, to be caused by the influence of agitation on 'pro-pellet' formation. Pellet density was inversely related to pellet radius. Although gross pellet morphology was dependent on medium composition, it was the relative effects of agitation rate and growth rate which determined the form of the pellet size distribution. Antifoam, and pH had no effect on the morphology of pellets grown in shake flasks, though the pellet size distribution was affected. Increasing concentrations of antifoam emulsion above 1&'37 v/v caused a reduction in pellet number, suggesting inhibition of germination, whereas below 1&'37 v/v an increase led to an increase in pellet number suggesting an effect on pro-pellet formation. The effect of increasing pH over a range from 5 to 9 was to reduce pellet number. Modal pellet radius was inversely related to agitation rate and inoculum concentration. Agitation rate influenced the pellet size distribution in two ways (i) by affecting spore agglomeration and hence pro-pellet formation at low shear rates, and (ii) by causing mechanical damage to pellets at high shear rates.