## 3 Results and Discussions
### Expression Change of Genes Related to Phosphotransferase System (PTS)  
Phosphotransferase system (PTS) is widely used in prokaryotes and archaea, through which prokaryotes transport carbohydrate or other substances to adapt to the environment and save resources. Specific transporter components in PTS system specifically deliver substances into the cells. Compared with the cells from solid-state fermentation, the cells from liquid-state fermentation exhibited an extensively reduced the expression of the genes related to sugar transport, especially those genes in the PTS system (Table S1). In liquid-state fermented cells, the expressions of 9 genes related to mannose transport (including 4 \(manX\) genes, 2 \(manY\) genes, 2 \(manZ\) genes and 1 \(manA\) gene) were downregulated by 2.01-39.63 times. These 9 genes were responsible for the transport of D-mannose from extracellular to intracellular to form mannose 6-phosphate, and the obtained mannose 6-phosphate was further catalyzed by mannose-6-phosphate isomerase encoded by gene \(manA\) to form fructose-6-phosphate. The expressions of 10 genes (3 _celA_ genes, 4 _celB_ genes, and 3 _celC_ genes) related to cellobiose transport were down-regulated by 2.02-9.02 times. These 10 genes were responsible for the transport of cellobiose from extracellular to intracellular to form cellobiose-phosphate. The resultant cellobiose-phosphate was further catalyzed by \(\beta\)-glucoside-6-phosphate isomerase to form \(\alpha\)-D-glucose-6-phosphate. The expressions of three _bgIA_ genes encoding \(\beta\)-glucoside-6-phosphate isomerase were down-regulated by 3.01-38.85 times. Five genes (2 \(malX\) genes, 1 _galU_ gene, 1 _fpB_ gene, and 1 _pgi_ gene) associated with the maltose/glucose transport, which were responsible for converting extracellular D-glucose and maltose to intracellular D-glucose 6-phosphate and maltose 6-phosphate,  
Figure 5: Schematic diagram of metabolism differences and acid stress tolerance mechanisms of \(L\). _paracasei_ Zhang obtained from LSF. The serial number (I, II, III, IV) indicates the acid stress tolerance mechanism of cells from LSF. (**I**) improving fatty acid metabolic pathway and changing the composition of cell membrane fatty acid; (**II**) maintaining the stability of intracellular microenvironment, maintaining and increasing enzyme activity; (**III**) changing glutamate metabolism, producing ammonia and GABA to alleviate acid stress; (**IV**) upregulating the expression of several genes related to acid stress response. Glu-6-P, glucose-6-phosphate; Fru-6-P, fructose-6-phosphate; FBP, fructose-1,6-diphosphate; DHAP, dihydroxyacetone-phosphate; GAP, glyceraldehyde-3-phosphate; 1,3-PG, 1,3-diphosphoglycerate; 3-PG, 3-phosphoglycerate; 2-PG, 2-phosphoglycerate; PEP, phosphoenolpyruvate; GR, Glutathione reductase. Red color indicates up-regulation; black color represents no change; green color indicates down-regulation at mRNA level. The blue lines show transportation, neutralization, and other coping processes. were down-regulated by 2.37 to 9.48 times. Meanwhile, the expression of gene _gluA_ was down-regulated by 12.80 times, and this gene was responsible for further catalyzing the formation of D-glucose 6-phosphate and D-glucose from maltose 6-phosphate. Six genes (2 _srlA_ genes, 2 _srlB_ genes, and 2 _srlE_ genes) associated with the glucitol/sorbitol transport, which transformed extracellular D-sorbitol into intracellular sorbitol-6-phosphate, were down-regulated by 4.10-4.84 times. In addition, 2 _srlD_ genes were down-regulated by 2.40 and 4.01 times, respectively, and these 2 genes further catalyzed sorbitol-6-phosphate into fructose-6-phosphate. Four genes (2 _fruA_ genes and 2 _fruB_ genes) associated with the fructose transport, which converted extracellular D-fructose into intracellular fructose-1-phosphate, were down-regulated by 2.15-3.0 times. The resultant fructose-1-phosphate was further catalyzed into fructose-1,6-bisphosphatase (FBP) by the fructose-1-phosphate kinase encoded by intracellular gene _fruK_.