BACKGROUND: Design principles of a fed-batch bioreactor system to fine-tune bioreactor performance by increasing production and productivity are investigated. To enhance the production and productivity in fed-batch fermentation of recombinant human growth hormone (rhGH) by Bacillus subtilis, this work develops a continuous feed stream (CFS) design method with the parameters, cultivation time to switch batch-to fed-batch operation, so called transition time, t(T), and specific growth rate, mu. RESULTS: Based on batch-bioreactor data for rhGH production by recombinant B. subtilis (scoC(-)) carrying pMK4::pre(subC)::hGH, three components of the defined medium were determined for continuous feeding. Theoretical analysis and modelling of the fed-batch reactor lead to CFS design, to be applied starting at t(T), which is a function of specific growth rate, mu. Transition time t(T) was optimised experimentally using the batch operation mu value, 0.100 h(-1); and the highest rhGH concentration was obtained at t(T)=4 h. rhGH concentration increased further to 127 mg dm(-3), when the CFS flow rate applied was calculated using mu = 0.125 h(-1). CONCLUSION:The performance of the fermentation process was enhanced by 36% and 8 h shortened cultivation time of t = 16 h, increased the rhGH productivity 2.4-fold compared with batch-fermentation. (C) 2015 Society of Chemical Industry