## 3 Results and Discussions
### Differences in Cell Morphology and Cell Membrane Fatty Acid Composition
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**FA or Parameter**} & \multicolumn{4}{c}{**Fatty Acid Distribution (\(\upmu\)g, M/M \%)**} \\ \cline{2-5}  & **G** & **GP** & **Y** & **YP** \\ \hline C\({}_{14.0}\) & 116.56 \(\pm\) 12.69 & 309.84 \(\pm\) 27.05 & 194.78 \(\pm\) 10.13 & 595.60 \(\pm\) 87.63 \\ C\({}_{15.0}\) & 36.93 \(\pm\) 4.35 & 99.61 \(\pm\) 8.87 & 29.30 \(\pm\) 1.80 & 83.66 \(\pm\) 7.74 \\ C\({}_{16.0}\) & 3589.90 \(\pm\) 351.80 & 10,490.40 \(\pm\) 773.05 & 5036.58 \(\pm\) 220.69 & 22,115.55 \(\pm\) 2961.51 \\ C\({}_{16.1}\) & 40.77 \(\pm\) 5.92 & 66.61 \(\pm\) 7.39 & 163.85 \(\pm\) 9.60 & 230.47 \(\pm\) 33.66 \\ C\({}_{17.0}\) & 24.45 \(\pm\) 2.78 & 43.44 \(\pm\) 3.61 & 29.22 \(\pm\) 1.90 & 87.81 \(\pm\) 8.62 \\ C\({}_{18.0}\) & 1341.62 \(\pm\) 137.26 & 4190.22 \(\pm\) 308.72 & 2025.53 \(\pm\) 89.15 & 13065.27 \(\pm\) 1781.62 \\ C\({}_{18.10}\)\(\upmu\)g & 26.95 \(\pm\) 4.79 & 26.84 \(\pm\) 1.30 & 17.50 \(\pm\) 1.00 & 34.00 \(\pm\) 13.05 \\ C\({}_{18.1}\)\(\upmu\)g & 682.13 \(\pm\) 65.14 & 1746.20 \(\pm\) 137.60 & 1320.58 \(\pm\) 40.83 & 1351.64 \(\pm\) 162.82 \\ C\({}_{18.2}\) & 1530.59 \(\pm\) 163.38 & 2972.23 \(\pm\) 221.16 & 5241.26 \(\pm\) 214.60 & 4685.70 \(\pm\) 543.25 \\ C\({}_{18.3}\) & 229.96 \(\pm\) 23.94 & 366.59 \(\pm\) 30.10 & 835.87 \(\pm\) 36.46 & 686.66 \(\pm\) 89.92 \\ C\({}_{20.0}\) & 140.07 \(\pm\) 15.32 & 62.10 \(\pm\) 4.66 & 35.95 \(\pm\) 1.80 & 186.65 \(\pm\) 16.90 \\ C\({}_{22.1}\) & 219.28 \(\pm\) 25.63 & 588.33 \(\pm\) 49.55 & 173.98 \(\pm\) 6.75 & 2229.79 \(\pm\) 311.61 \\ Saturated/unsaturated ratio & 1.92 & 2.64 & 0.95 & 3.92 \\ \hline \hline \end{tabular} Notes: G, solid state-fermented cells; Y, liquid state-fermented cells. GP, solid state-fermented cells treated with pH 2 for 10 min; YP, liquid state-fermented cells treated with pH 2 for 10 min. Results are based on three technical replicates.  
\end{table} 
Table 1: Fatty acid composition of cell membrane under environmental stress. Differences in Intracellular Microenvironment and Activity of Metabolic Enzymes after Acid Stress Treatment  
In order to reveal physiological differences of bacteria obtained by two fermentation methods under acid stress, we measured intracellular pH, intracellular ATP content, H\({}^{+}\)-ATPase and glutathione reductase activities of the bacteria. The results showed (Figure 4a) that before acid stress treatment, the intracellular pH values of the bacteria from the two fermentations were basically the same (6.43 for solid state and 6.38 for liquid state). After acid stress treatment, intracellular pH values of two types of bacteria were decreased. The intracellular pH of the liquid-state fermented bacteria decreased from 6.38 to 5.98, but basically maintained around 6.0. However, that of the solid-state fermented bacteria decreased significantly from 6.43 to 4.25, indicating that significant damage of the intracellular microenvironment by acid stress, which would seriously affect the activity of important intracellular metabolic enzymes and many important metabolic processes. The higher ability of liquid state-fermented bacteria to maintain intracellular pH under acid stress might be one of the reasons for their better tolerance to acid stress. Since the energy charge of the cell is an important indicator evaluating intracellular microenvironment, we measured the content of ATP in the cell. As shown in Figure 4b, before acid stress treatment, the content of ATP in cells from liquid-state fermentation (51.28 nmol/mg protein) was higher than that from solid-state fermentation (39.51 nmol/mg protein). After acid stress treatment, ATP content in solid-state fermented bacteria decreased to 30.08 nmol/mg protein, while ATP content in liquid-state fermented bacteria significantly increased to 79.15 nmol/mg protein. ATP and other energy substances can help cells to cope with the stimulation of adverse environment.