Effects of pH and aeration on γ-poly(glutamic acid) formation byBacillus licheniformis in controlled batch fermentor cultures

Abstract

Bacillus licheniformis ATCC 9945A was grown on Medium E in batch fermentations in which the pH was maintained at 5. 5, 6.5, 7.4, and 8.25. The effects of pH on cell growth, carbon source utilization, and y-polyglutamic acid (y-PGA) production, molecular weight, and polymer stereochemistry were determined. The y-PGA yield was highest (15 g/L, 96 h growth time) at pH 6.5. The increase in y-PGA formation at pH 6.5 corresponded with a relatively high specific production rate at high y-PGA concentration (0.09 h-', -15 g/L y-PGA). In contrast, the specific y-PGA production rates at fermentor pH values of 5.5 and 7.4 decreased significantly for y-PGA fermentor yields >-5 g/L. Interestingly, alteration of the medium pH had little to no significant effects on the product quality as measured by stereochemical composition and molecular weight. While glutamate and glycerol utilization were similar as a function of pH, citrate consumption increased at pH 6.5, indicating that the formation of y-PGA from citrate at pH 6.5 was of increased importance. The effect of aeration was evaluated by increasing the agitation speed (250 to 800 rpm) and aeration rate (0.5 t o 2.0 L/min) at pH 6.5, the pH of maximal y-PGA production. Increased aeration resulted in doubling of the cell dry weights (2 t o 4 g/ L), increasing y-PGA yields (6.3 to 23 g/L by 48 h) and increasing in the maximum y-PGA-specific production rate (0.09 to 0.1 1 h-'). Other effects of increased agitation included a rapid depletion of glutamate and citrate (by 50 h) and a decrease in product molecularweight. Despite the increase in agitation and aeration, oxygen limitation of the culture was not avoided, because the partial pressure decreased to <1.0% by 29 h.

Contribution of Glycerol on Production of Poly(γ-Glutamic Acid) in Bacillus subtilis NX-2

“…While in B. licheniformis ATCC 9945a, glycerol was utilized from the very beginning, and a total amount of 30 to 40 g/l glycerol had been consumed until the end of fermentation when 80 g/l glycerol was added in the medium [2,9]. The difference of glycerol consumption between the two strains might be due to the differences of species and medium composition.…”

Enhanced Production of Poly (γ-glutamic acid) from Bacillus licheniformis NCIM 2324 by Using Metabolic Precursors

“…The maximum PGA production reported till date by using most widely used strain B. lichenifomis ATCC 9945A was 23 g/l [23]. B. subtilis IFO 3335, which is also extensively studied for PGA production, could produce maximum of 20 g/l of PGA [24].…”

Isolation of Halotolerant Bacillus licheniformis WX-02 and Regulatory Effects of Sodium Chloride on Yield and Molecular Sizes of Poly-γ-Glutamic Acid

“…The present work provided a simple method for producing γ-PGA with different molecular sizes. The effects of NaCl on γ-PGA molecular sizes were different among Bacillus species [3,6,18]. It was still unclear how the NaCl resulted in such a variation in γ-PGA molecular sizes among different Bacillus species.…”

“…The increased broth viscosity was likely to result in O 2 limitation by decreasing the volumetric O 2 transfer, thus would lead to a decrease in γ-PGA yields [3,4]. The broth viscosity could be decreased by addition of NaCl to the medium [5,6].…”