Fermentation characteristics of l-tryptophan production by thermoacidophilic Bacillus acidocaldarius in a defined medium

A wild-type thermoacidophilic bacteria Bacillus acidocaldarius which excretes l-tryptophan (Trp) was used for the production of Trp. A fructose-based defined fermentation medium, i.e. containing 8 kg m−3 fructose, 5.0 kg m−3 (NH4)2HPO4, and a salt solution, was designed by investigating the influence of carbon- and nitrogen-sources and ions, to create the physiological conditions where the regulatory mechanisms in B. acidocaldarius weakly operate. The effects of oxygen transfer were investigated within limited-oxygen transfer (LimOT), low-oxygen transfer (LOT), and medium-oxygen transfer (MOT) conditions by using the parameters QO/VR = 0.2 and 0.5 vvm and N = 250 and 500 min−1 in V = 3 dm3 bioreactors, at initial pH 5.0 and T = 55 °C. The fructose, cell, Trp and the other amino acid and organic acid concentrations, the oxygen transfer rate (OTR), oxygen uptake rate (OUR), and oxygen transfer coefficients (KLa) were determined throughout the bioprocess; moreover, the yield and maintenance coefficients as well as the kinetic constants were reported. In general, the OTR and OUR decrease as the oxygen transfer decreases towards the limited-oxygen transfer conditions, i.e. LimOT. Damköhler number (Da) that is the maximum possible oxygen utilization rate (OD) per maximum possible oxygen-transfer rate (OTRmax) decreases with the decrease in QO/VR and/or N, that is, the oxygen-transfer resistance increases. The fructose consumption rate is the highest at MOT and the lowest at LimOT condition. In general, the cell growth rate is maximum when both the oxygen-transfer and biochemical reaction resistances are effective, i.e. at MOT. Trp production favours limited oxygen transfer conditions as B. acidocaldarius produces 0.34 kg m−3 Trp with the highest yield at LimOT condition, where the highest total amino acid concentration was obtained as 0.36 kg m−3, and overall Trp yield on fructose was 0.3 kg kg−1 at t = 17 h. Total amino acid concentration increased with the decrease in the oxygen transfer, despite the fact that the other aromatic group amino acids, i.e. l-tyrosine and l-phenylalanine, favour MOT condition. To open a new avenue to the production of Trp probably due to the increased reaction rates and the weakness of the regulatory mechanisms in the aromatic amino acid pathway, the fermentation characteristics of B. acidocaldarius were determined to start strain improvement research using metabolic engineering techniques.