Adaptive Laboratory Evolution of Bacillus subtilis 168 for Efficient Production of Surfactin Using NH₄Cl as a Nitrogen Source

Bacillus subtilis strain 168 is commonly used as a host to produce recombinant proteins and as a chassis for bio-based chemicals production. However, its preferred nitrogen source is organic nitrogen, which greatly increases production costs. In this study, adaptive laboratory evolution (ALE) was used to improve B. subtilis 168 growth using NH4Cl as the sole nitrogen source. The cell density (OD600) of a mutant strain LJ-3 was 208.7% higher than that of the original strain. We also optimized the metal ions in the medium and this resulted in a further increase in growth rate by 151.3%. Reintroduction of the sfp+ gene into strain LJ-3 led to the LJ-31 clone, which restored LJ-3's ability to synthesize surfactin. The fermentation system was optimized (C/N, aeration, pH) in a 5 L bioreactor. Dry cell weight of 7.4 g/L and surfactin concentration of 4.1 g/L were achieved using the optimized mineral salt medium after 22 h of batch fermentation with a Y-P/S value of 0.082 g/g and a Y-P/X of 0.55 g/g. HPLC analysis identified the surfactin isoforms produced by strain LJ-31 in the synthetic medium as C-13-surfactin 13.3%, C-14-surfactin 44.02%, and C-15-surfactin 32.79%. Hence, the variant LJ-3 isolated by ALE is a promising engineering chassis for efficient and cost-effective production of a variety of metabolites.