## 3 Results and Discussions
### Differences in Cell Morphology and Cell Membrane Fatty Acid Composition  
In order to explore morphological differences between the cells from two fermentations and cell morphological changes after acid stress treatment, we used transmission electron microscopy to observe the cell morphology. Before acid stress, the membrane structure of the cells from the two fermentation methods was intact, and the membrane wall was clear (Figure 2A,B). As shown in Figure 2C, after acid stress treatment, the membrane structure of cells obtained from SSF was damaged, and cell wall was blurred. By comparison, after acid stress treatment, the cell wall of the bacteria obtained from LSF was clear, and no cell membrane was damaged, which was not different from that before acid treatment (Figure 2D). Figure 1: Differences in acid tolerance of \(L\). _paracasei_ Zhang cultured from solid-state fermentation and liquid-state fermentation. (**a**) Survival rate of solid-state and liquid-state fermented cells under different pH values; (**b**) Plate diagram of solid-state and liquid-state fermented cells under different pH values. Three biological and three technical replicates were measured. ** statistically significant differences (\(p<0.01\)). The cell membrane is a barrier between bacteria and the external environment, and it plays an important role in resisting the stimulation of the external environment and maintaining the stability of intracellular environment [44]. Since a large number of metabolic enzymes and signal receptors are distributed on the membrane, the cell membrane plays a very important role in the physiological metabolism of cells and signal energy conduction. Therefore, cell structure, cell membrane integrity, and cell membrane lipid composition are critical to the tolerance of the cells to environmental stimuli [45; 46]. In this study, we found that the cell membrane obtained from liquid-state fermentation was more tolerant to acid stress, we speculated that it may be due to the fact that LSF was more beneficial to cell structure integrity maintenance. Membrane fatty acid can maintain cellular viability under different conditions, and the relationship between membrane fatty acid composition and environmental stimuli is complex [47; 48; 49]. In LAB, temperature has been reported to induce changes in fatty acids such as the proportion of unsaturated fatty acids, the degree of cyclization, and the proportion of long-chain fatty acids containing 20 to 24 carbons [50; 51]. Changing the cell membrane composition of fatty acids can effectively regulate the fluidity of the cell membrane and a response of cells to environmental stress [52; 53]. In order to explore the differences in cell membrane fatty acid composition between the two types of cells obtained respectively by the two fermentation methods (SSF and LSF) under acid stress, we measured the membrane fatty acid of two types of cells. The results showed that (Table 1) the proportion of unsaturated fatty acids (palmitoleic acid C16:1, octapenoic acid C18:1u 8C, oleic acid C18:1u 9C, linoleic acid C18:2, linoleic acid C18:3, and docoenoic acid C22:1) in cell membrane of the bacteria obtained by solid state fermentation was significantly lower than that of saturated fatty acids (myristic acid C14:0, pentadecanoic acid C15:0, palmitic acid C16:0, heptadecanoic acid C17:0, stearic acid C18:0, and arachidonic acid C20:0) with the ratio of saturated to unsaturated fatty acids (S/U) of 1.92, whereas  
Figure 2: Transmission electron microscopy observation of cell morphological changes of \(L\). _pancasei_ Zhang under different stresses. (**A**). Cell morphology from solid-state fermentation before acid stress. (**B**) Cell morphology from liquid-state fermentation before acid stress. (**C**) Cell morphology from solid-state fermentation under pH 2 acid stress. (**D**) Cell morphology from liquid-state fermentation under pH 2 acid stress.