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Poly-γ-glutamic acid (γ-PGA) is an important biopolymer with many applications due to its biodegradable and non-toxic characteristics. γ-PGA is produced industrially by fermentation of Bacillus species. The optimal pH range for producing γ-PGA by Bacillus subtilis PG-001 was firstly studied by glucose fed-batch fermentation with non-controlled pH. Result showed that both cell growth and γ-PGA synthesis were repressed when pH was lower than pH 6. Further investigation with γ-PGA fed-batch fermentation showed that pH 6.5 is more suitable for γ-PGA fermentation than pH 7. Under comparable consumption of glutamic acid and glucose, 11.8 g/L γ-PGA and 0.7 g/g yield were achieved by fermentation at pH 6.5, which was significantly higher than 10.5 g/L and 0.56 g/g yield of fermentation at pH 7. In addition, γ-PGA degradation during later phase of fermentation was repressed at pH 6.5 as 9238cP of final broth viscosity was achieved from fermentation at pH 6.5 while it was only 346 cP for fermentation at pH 7. Finally, a glucose feedback control pH-stat strategy was performed for reducing alkali consumption during γ-PGA fermentation, which further increased final γ-PGA concentration to 15.5 g/L with much higher viscosity (11458 cP); meanwhile the consumption of alkali decreased 57%. The fed-batch γ-PGA fermentation with glucose feedback control pH-stat strategy showed high feasibility for industrial scaling-up.

Endophytic Bacillus subtilis strain E1R-j, isolated from wheat roots, inhibits a wide range of microorganisms. In this study, we determined its inhibitory effect on urediniospore germination of Puccinia striiformis f. sp. tritici (Pst), the causal agent of wheat stripe rust and biocontrol effect on the rust development using bacterial cell suspension (BCS) and fermentation liquid with and without bacterial cells (FLBC and FL). Urediniospore germination was significantly inhibited by all three formulations in vitro. The maximum inhibition rate was 84.1% treated with the 10-fold dilution of original formulation of FLBC. Microscopic observation revealed that the bacterial treatments caused urediniospores and germ tubes ruptured, which resulted in protoplasm release and dysfunction of fungal structures. Urediniospore germination was also significantly inhibited by spraying E1R-j on leaf surface before inoculation. Formulations FLBC and FL were both better than formulation BCS. In a condition-controlled greenhouse, application of FLBC and FL 24 and 0 h before Pst inoculation displayed a protective mode in reducing disease severity, and the control efficacy was between 54.0% and 87.7%. For formulation BCS, only the application of 24 h before Pst inoculation significantly reduced rust severity with control efficacy 51.8%. The incidence of diseased leaves in the treatments of FLBC and FL were significantly reduced compared to the non-treated plots in the field experiments conducted in the season 2007–2008. The control efficacy was 51% and 43% for the two treatments, respectively. This study is helpful for further developing a biocontrol approach for managing stripe rust.