**A** *Streptomyces* **sp. NEAU-HV9: Isolation, Identification, and Potential as a Biocontrol Agent against** *Ralstonia solanacearum* **of Tomato Plants**

**Ling Ling 1, Xiaoyang Han 1, Xiao Li 1, Xue Zhang 1, Han Wang 1, Lida Zhang 1, Peng Cao 1, Yutong Wu 1, Xiangjing Wang 1, Junwei Zhao 1,\* and Wensheng Xiang 1,2,\***


Received: 25 October 2019; Accepted: 12 February 2020; Published: 1 March 2020

**Abstract:** *Ralstonia solanacearum* is an important soil-borne bacterial plant pathogen. In this study, an actinomycete strain named NEAU-HV9 that showed strong antibacterial activity against *Ralstonia solanacearum* was isolated from soil using an in vitro screening technique. Based on physiological and morphological characteristics and 98.90% of 16S rRNA gene sequence similarity with *Streptomyces panaciradicis* 1MR-8T, the strain was identified as a member of the genus *Streptomyces*. Tomato seedling and pot culture experiments showed that after pre-inoculation with the strain NEAU-HV9, the disease occurrence of tomato seedlings was effectively prevented for *R. solanacearum*. Then, a bioactivity-guided approach was employed to isolate and determine the chemical identity of bioactive constituents with antibacterial activity from strain NEAU-HV9. The structure of the antibacterial metabolite was determined as actinomycin D on the basis of extensive spectroscopic analysis. To our knowledge, this is the first report that actinomycin D has strong antibacterial activity against *R. solanacearum* with a MIC (minimum inhibitory concentration) of 0.6 mg L−<sup>1</sup> (0.48 μmol L<sup>−</sup>1). The in vivo antibacterial activity experiment showed that actinomycin D possessed significant preventive efficacy against *R. solanacearum* in tomato seedlings. Thus, strain NEAU-HV9 could be used as BCA (biological control agent) against *R. solanacearum*, and actinomycin D might be a promising candidate for a new antibacterial agent against *R. solanacearum*.

**Keywords:** antibacterial activity; *Ralstonia solanacearum*; *Streptomyces* sp. NEAU-HV9; actinomycin D
