**1. Introduction**

Infectious diseases caused by infectious microorganisms continue to threaten human health. Moreover, the development of drug resistance by *Candida albicans*, *Staphylococcus aureus*, *Escherichia coli*, and *Mycobacterium tuberculosi* is becoming more and more serious in hospitals and the community [1–4]. There is an urgen<sup>t</sup> need to develop new drugs to fight against these pathogens.

*Streptomyces* belongs to actinomycetes which are highly diverse Gram-positive bacteria with high guanine and cytosine content in their DNA. Actinomycetes are well known as an important resource for screening new antibiotics [5], representing 45% of the bioactive secondary metabolites originating from microorganisms [6]. Moreover, *Streptomyces* are the key source of many of the world's antibiotics in clinics [7,8]. With the detailed investigation on marine microorganisms, marine-derived actinomycetes have proven to be an inexhaustible source for bioactive secondary metabolites [9]. A number of new bioactive compounds were characterized from marine-derived *Streptomyces*, such as isoquinolinequinones [10], terpenoid derivatives [11,12], angucycline derivatives [13–15], glycosylated aromatic polyketides [16], bicyclic peptides [17], depsipeptides [18], benzodiazepines [19], and piericidin derivatives [20].

In the course of our screening of antibacterial secondary metabolites from marinederived actinomycetes [21–24], the EtOAc extract of *Streptomyces* sp. BTBU20218885, isolated from a mud sample collected from the coastal area of Xiamen, Fujian Province, China,

**Citation:** Song, F.; Hu, J.; Zhang, X.; Xu, W.; Yang, J.; Li, S.; Xu, X. Unique Cyclized Thiolopyrrolones from the Marine-Derived *Streptomyces* sp. BTBU20218885. *Mar. Drugs* **2022**, *20*, 214. https://doi.org/10.3390/ md20030214

Academic Editor: Asunción Barbero

Received: 15 February 2022 Accepted: 16 March 2022 Published: 18 March 2022

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showed antibacterial activity against Bacille Calmette–Guérin (BCG), the live attenuated vaccine form of *Mycobacterium bovis*, with minimum inhibitory concentration (MIC) of 20 μg/mL. A chemical investigation of this *Streptomyces* strain resulted in the isolation of two new cyclized thiolopyrrolone derivatives, namely, thiolopyrrolone A (**1**) and 2,2- dioxido thiolutin (**2**), together with the known compound, thiolutin (**3**) (Figure 1). Details of fermentation, isolation, structural elucidation, and antibacterial activities are reported here.

**Figure 1.** Chemical structures of **1**–**3**.
