## 3 Results and Discussion
### Analysis of Microbial Community during Fermentation of BBP-Meju with Different Salt Concentrations
#### 3.6.2 Analysis of Microbial Community Structure  
Differential analysis of OTUs for BBP-meju samples with different salt concentrations was conducted, and the results are shown in Figure 7. It could be seen that the unique bacterial OTU numbers in L7, M7, H7, L35, M35, and H35 samples were 932, 896, 839, 1171, 968, and 853, respectively, while the shared bacterial OTU number was 50 (Figure 7a). Relative to fungi, the number of unique fungal OTUs in each BBP-meju sample was notably lower  
\begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{4}{c}{**Bacteria**} & \multicolumn{4}{c}{**Fungal**} \\ \cline{2-9}  & **Effective** & **High-Quality** & **Proportions** & **Effective** & **High-Quality** & **Proportions** \\  & **Sequences** & **Sequences** & **(\%)** & **Sequences** & **Sequences** & **(\%)** \\ \hline L7a & 58,786 & 51,006 & 86.77 & 79,408 & 77,311 & 97.36 \\ \hline L7b & 55,060 & 48,152 & 87.45 & 79,349 & 76,322 & 96.19 \\ \hline L7  
than in bacteria. However, the shared fungal OTU number was 94 (Figure 7b), significantly higher than the bacterial community. This result suggested a higher similarity in the fungal community during the fermentation with different salt concentrations. Nevertheless, the overall abundance of the bacterial community was significantly higher than the fungal community, with more pronounced differentiation across different salt concentrations. According to the histograms of the distribution of bacterial communities at genus level during BBP-meju fermentation, the top 10 relatively abundant bacterial genera were _Bacillus_, _Enterobacter_, _unclassified_Cyanobacterial_, _Staphylococcus_, _unclassified_Bacteria_, _Pantoea_, _unclassified_Enterobacteriaceae_, _Brevundimonas_, and _Weissella_ (Figure 7c). Among them, _Bacillus_ was the dominant bacterial community in all BBP-meju samples (Figure 7c). On day 7, _Bacillus_ exhibited the highest relative abundance in low-salt meju, ranging from 42.76% to 47.34%, whereas its highest abundance was reached in medium-salt meju on day 35 (43.66-51.51%) (Figure 7c). _Bacillus_ was reported to be a functional core microbial community during BBP fermentation. _Bacillus_ secreted abundant proteolytic enzymes, leading to the accumulation of flavor compounds, such as amino acids, crucial for the distinctive flavor formation of BBP [49]. Thus, the high abundance of _Bacillus_ on day 7 may explain the rapid increase in AAN levels in early-stage fermentation. The decrease in _Bacillus_ abundance on day 35 corresponded to the decline in AAN levels in middle and late fermentation. Meanwhile, the low-salt conditions could alleviate the osmotic stress of salt concentration on microorganisms and their secreted proteases, which caused the elevation of amino acid levels in low-salt meju. Aside from _Bacillus_, _Staphylo  
Figure 7: Structural analysis of bacterial and fungal communities in BBP-meju samples: (**a**,**b**) comparison of OTUs of bacteria (**a**) and fungi (**b**); (**c**,**d**) relative abundance of bacteria (**c**) and fungi (**d**) at the genus level. _coccus_ is also a core dominant microbiota during BBP fermentation, which is one of the key factors influencing the flavor and quality of the BBP [55]. The relative abundance of _Staphylococcus_ in BBP-meju increased significantly as fermentation progressed. On day 7, the relative abundance of _Staphylococcus_ in medium-salt meju ranged from 14.32% to 36.92%, the highest among the three salt concentrations. However, by day 35, the relative abundance of _Staphylococcus_ in low-salt meju reached 41.93-50.21%, significantly higher than that in medium- and high-salt meju (\(p<0.05\)) (Figure 7c). It is known that the growth of _Staphylococcus_ can cause the accumulation of large amounts of acids during BBP fermentation [56].