*3.3. Antibacterial Activity of NEAU-SSA 1<sup>T</sup> against Ralstonia solanacearum*

Strain NEAU-SSA 1<sup>T</sup> exhibited antibacterial activity against *Ralstonia solanacearum* with inhibitory zone diameters of 23 mm (Figure 4a). However, no inhibitory effect on the growth of *Micrococcus luteus* (Figure 4b) was observed. Comparison of the antibacterial activity of the extract of the supernatant with that of the cell pellet suggested that the antibacterial substances of strain NEAU-SSA 1<sup>T</sup> were in both the supernatant and cell pellet since the extracts all showed inhibition of the growth of *Ralstonia solanacearum* with the inhibitory zone diameters of 31.5 and 26.4 mm, respectively (Figure 5a,b). The antibacterial substances in the supernatant were stable after they were placed in a water bath at 40 and 60 ◦C for 30 min, while they did not show antibacterial activity after 80 and 100 ◦C bath (Figure 6a), which indicated that they were sensitive to temperature. In contrast, the antibacterial substances in the cell pellet were insensitive to temperature (Figure 6b), which demonstrated that the antibacterial substances in the supernatant and cell pellet were different. The antiSMASH analysis led to the identification of 49 gene clusters, including 24 gene clusters that showed very low similarity to the known gene clusters of mediomycin A, cremimycin, primycin, ibomycin, naphthomycin, lasalocid, informatipeptin, polyoxypeptin, kutznerides, anisomycin, paulomycin, himastatin, desotamide, nystatin, tiacumicin B, oxazolomycin, and 4-Z-annimycin. Therefore, the relationships between the corresponding secondary metabolites produced by NEAU-SSA 1<sup>T</sup> and the antibacterial activity are still ambiguous. *Streptomyces* are well known as important biological resources for their biologically active secondary metabolites, which play important roles in protecting plants against pathogens [50]. Strain NEAU-SSA 1T, which shows a stronger antibacterial activity against *Ralstonia solanacearum*, is a novel species of the

genus *Streptomyces*, and possesses 24 lower similarity gene clusters. Therefore, it is interesting and significant to isolate and identify the secondary metabolites of the strain in further studies.

**Figure 4.** The antibacterial activity of strain NEAU-SSA 1T against *Ralstonia solanacearum* (**a**) and *Micrococcus luteus* (**b**).

**Figure 5.** The antibacterial activity of the extract of the supernatant (**a**) and cell pellet of strain NEAU-SSA 1T (**b**) against *Ralstonia solanacearum*.

**Figure 6.** The effect of temperature on the antibacterial activity of the extract of the supernatant (**a**) and cell pellet of strain NEAU-SSA 1T (**b**) against *Ralstonia solanacearum*.
