## 4 Discussion  
Bacterial EPSs have received a great deal of attention in recent years due to wide applications in various industries, such as pharmacy, cosmetics, and food. Among them, only dextran extracted from bacteria is used commercially in the food and pharmaceutical \begin{table}
\begin{tabular}{l c c c} \hline \hline
**EPS-Producing Bacteria** & **Monosaccharides** & **Antioxidant Activity** & **References** \\ \hline _B. mojavensis_ MT012152 & Fructose & ND & [42] \\ _B. amyloliquefaciens_ C-1 & Glucose, mannose, galactose, arabinose & Superoxide, hydroxyl radicals & [25] \\ _B. tequilensis_ PS2 & Glucose, xylose,ribose, rhamnose, & DPPH, ABTS & [43] \\ _B. altitudini_ MSH2014 & Glucose & DPPH & [44] \\ _B. tequilensis_ GM & Fructose & ND & [41] \\ _B. velezensis_ VTX20 & Fructose, glucose & DPPH, hydroxyl radicals & This study \\ \hline \hline \end{tabular} Abbreviation: ND, Not determined; ABTS: 2,2°-azino-bis(3-ethylbenzothazoline-6-sulfonic acid); DPPH, 1,1-diphenyl-2-picrylhydrazyl.  
\end{table} 
Table 1: Monosaccharide composition of EPS produced by Bacillus strains and their antioxidant activities. Figure 6: Antioxidant activities of EPS produced by _B. velezensis_ VTX20. Scavenging activity on DPPH free radicals (**A**) and hydroxyl radicals (**B**). industries [46]. Commercial applications of EPS are limited due to the production costs, including fermentation productivity and purification process [47]. Until now, there have been few reports on the production and biological function of EPS from _B. velezensis_. In this study, we demonstrated for the first time that _B. velezensis_ isolated from traditional fermented soybean paste is a producer of levan at both genotypic and phenotypic levels, which showed great antioxidant activity, offering the potential for use as medical compounds and functional additives. The EPS biosynthesis capabilities rely on the bacterial strain and its medium composition. A low yield (112.1 mg/L) of EPS was observed in _B. tequilensis_ PS21 when grown in the medium supplemented with 80 g/L lactose [43]. Conversely, _B. mojavensis_ MT012152 isolated from soil produced a high levan amount (22 g/L) using a 5% sucrose concentration [42]. Under an optimized condition that contains 120 g/L molasses and 6 g/L yeast extract, and 30 \({}^{\circ}\)C for cultivation temperature, 7.6 g/L EPS consisting of glucose, mannose, and galactose was produced by _B. velezensis_ KY471306 derived from soil [23]. Comparing to our study, it is 9.9-fold lower than levan harvested from _B. velezensis_ VTX20 using 20% sucrose as the main carbon source. To the best of our knowledge, it was the highest _Bacillus_ EPS recorded to date. However, when using a supplement of glucose (200 g/L), _B. velezensis_ VTX20 only produced 12.5 g/L EPS. These results indicated that the type and concentration of carbon source significantly influence EPS production in _Bacillus_ strains. Optimization of levan production by _B. velezensis_ VTX20 using response surface methodology will be an interesting subject for further studies, which will help to reduce production costs. Using complete genome sequencing of _B. velezensis_ VTX20, the genome of _B. velezensis_ VTX20 contains genes involved in EPS and levan-type EPS biosynthesis. Surprisingly, they are in the opposite strand in comparison to other compared _B. velezensis_ (Figure 5). The _slrR-epsAO_ operon is around 16.4 kb that includes the genes encoding for the regulation, determination of chain length, synthesis, polymerization, and exportation of the repeating sugar units concerned. Excessive glucose might lead to inactivation of the SlrR repressor activity, dissociation of SlrR from its promoter, and derepression of _epsA-O_ genes. Deletion of transcriptional factor SlrR could be an effective strategy for overproduction of EPS encoded by _epsA-O_ operon in the medium supplemented with glucose. By contrast, under excessive sucrose, the _ls_ encoding for levansucrase hydrolyzes sucrose to yield levan and release glucose as a by-product [48].