Abstract
Lactate production is enhanced by adding calcium carbonate or sodium hydroxide during fermentation. However, Bacillus coagulans 2-6 can produce more than 180 g/L L-lactic acid when calcium lactate is accumulated, but less than 120 g/L L-lactic acid when sodium lactate is formed. The molecular mechanisms by which B. coagulans responds to calcium lactate and sodium lactate remain unclear. In this study, comparative transcriptomic methods based on high-throughput RNA sequencing were applied to study gene expression changes in B. coagulans 2-6 cultured in non-stress, sodium lactate stress and calcium lactate stress conditions. Gene expression profiling identified 712 and 1213 significantly regulated genes in response to calcium lactate stress and sodium lactate stress, respectively. Gene ontology assignments of the differentially expressed genes were performed. KEGG pathway enrichment analysis revealed that ‘ATP-binding cassette transporters’ were significantly affected by calcium lactate stress, and ‘amino sugar and nucleotide sugar metabolism’ was significantly affected by sodium lactate stress. It was also found that lactate fermentation was less affected by calcium lactate stress than by sodium lactate stress. Sodium lactate stress had negative effect on the expression of ‘glycolysis/gluconeogenesis’ genes but positive effect on the expression of ‘citrate cycle (TCA cycle)’ genes. However, calcium lactate stress had positive influence on the expression of ‘glycolysis/gluconeogenesis’ genes and had minor influence on ‘citrate cycle (TCA cycle)’ genes. Thus, our findings offer new insights into the responses of B. coagulans to different lactate stresses. Notably, our RNA-seq dataset constitute a robust database for investigating the functions of genes induced by lactate stress in the future and identify potential targets for genetic engineering to further improve L-lactic acid production by B. coagulans.
Highlights
Lactic acid has wide applications in the industries of food, medicine, brewing, leather, textiles, environmental protection and agriculture [1,2,3]
Large volumes of data were generated with Illumina HiSeq 2000 sequencing of the three B. coagulans cDNA samples, namely, GY, CA, and NA
After removing low quality reads and trimming off adapter sequences, 13,747,126 (GY), 13,464,898 (CA) and 14,974,752 (NA) high-quality, clean PE sequencing reads with a total of 1,334,784,442 (GY), 1,306,406,094 (CA) and 1,456,148,628 (NA) nucleotides were obtained for the three samples
Summary
Lactic acid has wide applications in the industries of food, medicine, brewing, leather, textiles, environmental protection and agriculture [1,2,3]. Biodegradability and excellent mechanical properties of polylactic acid, it is considered the most promising novel packaging material of this century. It can replace non-degradable petrochemical materials such as polyethylene, polypropylene and polystyrene. Calcium lactate or another lactate can accumulate in the fermentation broth depending on the type of neutraliser selected. Previous studies indicated that the maximum yield of lactic acid can exceed 200 g/L when calcium lactate accumulates in the fermentation broth [6, 7]; the lactic acid yield is generally less than 125 g/L when sodium lactate is formed [5, 8]. The influences of lactate on lactic acid production may be different depending on the type of lactic acid formed
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