Removing the by-products acetic acid and NH4 + from the L-tryptophan broth by vacuum thin film evaporation during L-tryptophan production

Background: During L-tryptophan production by Escherichia coli, the by-products, acetic acid and NH4 +, accumulate in the fermentation broth, resulting in inhibited cell growth and activity and decreased L-tryptophan production. To improve the L-tryptophan yield and glucose conversion rate, acetic acid and NH4 + were removed under low-temperature vacuum conditions by vacuum scraper concentrator evaporation; the fermentation broth after evaporation was pressed into another fermenter to continue fermentation. To increase the volatilisation rate of acetic acid and NH4 + and reduce damage to bacteria during evaporation, different vacuum evaporation conditions were studied. Results: The optimum operating conditions were as follows: vacuum degree, 720 mm Hg; concentration ratio, 10%; temperature, 60°C; and feeding rate, 300 mL/min. The biomass yield of the control fermentation (CF) and fermentation by vacuum evaporation (VEF) broths was 55.1 g/L and 58.3 g/L at 38 h, respectively, (an increase of 5.8%); the living biomass yield increased from 8.9 (CF) to 10.2 pF (VEF; an increase of 14.6%). L-tryptophan production increased from 50.2 g/L (CF) to 60.2 g/L (VEF) (an increase of 19.9%), and glucose conversion increased from 18.2% (CF) to 19.5% (VEF; an increase of 7.1%). The acetic acid concentrations were 2.74 g/L and 6.70 g/L, and the NH4 + concentrations were 85.3 mmol/L and 130.9 mmol/L in VEF and CF broths, respectively. Conclusions: The acetic acid and NH4 + in the fermentation broth were quickly removed using the vacuum scraper concentrator, which reduced bacterial inhibition, enhanced bacterial activity, and improved the production of L-tryptophan and glucose conversion rate.

Genetic analysis of violacein biosynthesis by Chromobacterium violaceum

Chromobacterium violaceum presents a distinctive phenotypic characteristic, the production of a deep violet pigment named violacein. Although the physiological function of this pigment is not well understood, the sequencing of the genome of this bacterium has given some insight into the mechanisms and control of violacein production. It was found that erythrose-4-phosphate (E4P), a precursor to aromatic amino acid biosynthesis, is produced by the non-oxidative portion of the hexose monophosphate pathway, since it lacks 6-phosphogluconate dehydrogenase. All genes leading from E4P plus phosphoenolpyruvate to tryptophan are present in the genome. Nevertheless, these genes are not organized in an operon, as in E. coli, indicating that other mechanisms are involved in expression. The sequencing data also indicated the presence and organization of an operon for violacein biosynthesis. Three of the four gene products of this operon presented similarity with nucleotide-dependent monooxygenases and one with a limiting enzyme polyketide synthase. As previously suggested, genes encoding proteins involved in quorum sensing control by N-hexanoyl-homoserine-lactone, an autoinducer signal molecule, are present in the bacterial genome. These data should help guide strategies to increase violacein biosynthesis, a potentially useful molecule

Microbial production of L-tryptophan: a review.

Microbial production of L-tryptophan has been reviewed with 172 references. The review includes different tryptophan producing microorganisms, their optimal cultural conditions, yields, assay and process of recovery. It also includes a discussion on the pathway of tryptophan biosynthesis and its regulation. Achievements in this regard made through genetic engineering have also been included.