A two-compartment bioreactor system made of commercial parts for bioprocess scale-down studies: Impact of oscillations on Bacillus subtilis fed-batch cultivations

This study describes an advanced version of a two-compartment scale-down bioreactor that simulates inhomogeneities present in large-scale industrial bioreactors on the laboratory scale. The system is made of commercially available parts and is suitable for sterilization with steam. The scale-down bioreactor consists of a usual stirred tank bioreactor (STR) and a plug flow reactor (PFR) equipped with static mixer modules. The PFR module with a working volume of 1.2 L is equipped with five sample ports, and pH and dissolved oxygen (DO) sensors. The concept was applied using the non-sporulating Bacillus subtilis mutant strain AS3, characterized by a SpoIIGA gene knockout. In a fed-batch process with a constant feed rate, it is found that oscillating substrate and DO concentration led to diminished glucose uptake, ethanol formation and an altered amino acid synthesis. Sampling at the PFR module allowed the detection of dynamics at different concentrations of intermediates, such as pyruvic acid, lactic acid and amino acids. Results indicate that the carbon flux at excess glucose and low DO concentrations is shifted towards ethanol formation. As a result, the reduced carbon flux entering the tricarboxylic acid cycle is not sufficient to support amino acid synthesis following the oxaloacetic acid branch point.