Influences of Lactate on the High-density Fermentation by Bacillus licheniformis HK

To improve the biomass and spore rate of Bacillus licheniformis HK, influences of lactate on cell growth and metabolism and spore production were investigated at a 20 L bioreactor, and lactate addition was conducted by coupling with pH7.0 from 16 h to 32 h. Basing on the data differences in transcriptome and metabolome, the metabolic regulation mechanism of lactate was analyzed. The results showed that a peak biomass (30 h) of 5.43×10¹⁰ CFU/mL was obtained in lactate supplement group, which was 54.3% higher than that of basic fermentation group (control). Meanwhile, the spore number was 5.36×10¹⁰ CFU/mL, which was 60.0% higher than the control. Compared with the control, expressions of key genes in gluconeogenesis and pentose phosphate pathway (ldh, pckA, gapB, zwf, and tkt) were up-regulated by 2.8~9.3 fold in the lactate supplement group, the abundance of marker metabolites (pyruvate, fructose-6-phosphate, glyceric acid-1,3-diphosphate, 6-phosphate-gluconate, 5-phosphate-ribulose, and sedoheptulose 7-phosphate) in the pathways were increased by 1.55~12.6 fold compared with the control. Additionally, expressions of key genes in TCA cycle and oxidative phosphorylation system (citZ, icd, citB, odhB, atpAB, ndh, yumB, qcrABC, ctaODC, ythA, and sdhAB) were down-regulated by 34.2%~96.8%, while nitrate respiration metabolism gene (nasBCD) was up-regulated by 107.4~287.3 fold compared with the control. Abundances of corresponding key metabolites, such as aconitate, isocitrate and citrate, were decreased by 52.6%~62.5%, while the abundances of NAD⁺ and NADH were increased by 1.6~2.7 fold. Meanwhile, the precursors abundances of teichoic acid (glycerol-3-phosphate) and peptidoglycan (UDP-N-acetylmuramic acid, UDP-N-acetylmuramyl-L-alanine-D-glutamate) were increased by 1.5~14 fold compared with the control. Moreover, gene expressions involved in sporogenesis, such as phrAC, cotP, cotA, yeek, yheD, tasA, gerQ, yuzJ, were up-regulated by 1.4~5.7 fold, while rapAB, abrB, yisI, ynzD, and spo0E were down-regulated by 60.7%~96.7% compared with the control. The results suggested that lactate promoted gluconeogenesis, pentose phosphate pathway, nitrate respiration, and cell wall synthesis, thus promoting cell growth and spore formation efficiency. The results provided theoretical supports for high-density fermentation production of Bacillus licheniformis.