Molecular weight control of poly-γ-glutamic acid reveals novel insights into extracellular polymeric substance synthesis in Bacillus licheniformis

Background The structural diversity of extracellular polymeric substances produced by microorganisms is attracting particular attention. Poly-gamma-glutamic acid (gamma-PGA) is a widely studied extracellular polymeric substance from Bacillus species. The function of gamma-PGA varies with its molecular weight (Mw). Results Herein, different endogenous promoters in Bacillus licheniformis were selected to regulate the expression levels of pgdS, resulting in the formation of gamma-PGA with Mw values ranging from 1.61 x 10(3) to 2.03 x 10(4) kDa. The yields of gamma-PGA and exopolysaccharides (EPS) both increased in the pgdS engineered strain with the lowest Mw and viscosity, in which the EPS content was almost tenfold higher than that of the wild-type strain. Subsequently, the compositions of EPS from the pgdS engineered strain also changed. Metabolomics and RT-qPCR further revealed that improving the transportation efficiency of EPS and the regulation of carbon flow of monosaccharide synthesis could affect the EPS yield. Conclusions Here, we present a novel insight that increased pgdS expression led to the degradation of gamma-PGA Mw and changes in EPS composition, thereby stimulating EPS and gamma-PGA production. The results indicated a close relationship between gamma-PGA and EPS in B. licheniformis and provided an effective strategy for the controlled synthesis of extracellular polymeric substances.