In a batch culture with poly-γ-glutamic acid (γ-PGA) production growth medium without glutamic acid a mutant Bacillus subtilis PS-216 srf-cfp strain produced the most γ-PGA, (4,1 ± 0,5) g/L, among all »glutamic acid independent strains«. Rotational rheology demonstrated that the viscosity of the mutant Bacillus subtilis PS-216 srf-cfp was also the highest. At the time of the highest γ-PGA production viscosity of production medium with addition of glutamic acid doubled compared to the unsupplemented production medium. By analyzing the results of the selected strains we showed that γ-PGA concentration in the spent production medium strongly positively correlates with its viscosity measured at high shear rates. It is thus possible to predict the concentration of γ-PGA by measuring the viscosity of its production medium. We showed that despite relatively high biomass concentration (⡈ 2 x 109 cells/mL) the main factor increasing the viscosity of the production medium is γ-PGA. Cells, however, can significantly increase viscosity of the production medium at low shear rates by forming a network with entangled molecules of γ-PGA. Accordingly, oscillatory rheology, the amplitude sweep test, showed that spent medium has properties of a gel-like viscoelastic solid. When the cells were removed, the spent medium had properties of a viscoelastic liquid. By applying a frequency sweep test we demonstrated that spent medium gains properties of a viscoelastic solid, different than in the case of samples with separated γ-PGA or cells that retained properties of a viscoelastic liquid in the entire range of tested frequencies. This indicates that cells are embedded in a network of entangled molecules of γ-PGA.
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