Online measurement of the viscosity in shake flasks enables monitoring of γ-PGA production in depolymerase knockout mutants of Bacillus subtilis with the phosphate-starvation inducible promoter Ppst

Poly-gamma-glutamic acid (gamma-PGA) is a biopolymer with a wide range of applications, mainly produced using Bacillus strains. The formation and concomitant secretion of gamma-PGA increases the culture broth viscosity, while enzymatic depolymerisation and degradation of gamma-PGA decreases the culture broth viscosity. In this study, the recently published ViMOS (Viscosity Monitoring Online System) is applied for optical online measurements of broth viscosity in eight parallel shake flasks. It is shown that the ViMOS is suitable to monitor gamma-PGA production and degradation online in shake flasks. This online monitoring enables the detailed analysis of the P-pst promoter and gamma-PGA depolymerase knockout mutants in genetically modified Bacillus subtilis 168. The P-pst promoter becomes active under phosphate starvation. The different single depolymerase knockout mutants are Delta ggt, Delta pgdS, Delta cwlO and a triple knockout mutant. An increase in gamma-PGA yield in g(gamma-PGA)/g(glucose) of 190% could be achieved with the triple knockout mutant compared to the P-pst reference strain. The single cwlO knockout also increased gamma-PGA production, while the other single knockouts of ggt and pgdS showed no impact. Partial depolymerisation of gamma-PGA occurred despite the triple knockout. The online measured data are confirmed with offline measurements. The online viscosity system directly reflects gamma-PGA synthesis, gamma-PGA depolymerisation, and changes in the molecular weight. Thus, the ViMOS has great potential to rapidly gain detailed and reliable information about new strains and cultivation conditions. The broadened knowledge will facilitate the further optimization of gamma-PGA production.