Rare Actinobacteria, known as non-Streptomyces, hold great potential to produce new bioactive compounds for drug development. A strain designated DSW09, which belongs those rare Actinobacteria, was isolated from surface seawater of the East China Sea. The cells were aerobic, Gram-positive, non-motile, non-spore-forming and rod-shaped (0.4 µm wide and 1.5-4.0 µm long). The closest relative was Euzebya tangerina F10 (96.46 % of 16S rRNA gene similarity). Cell growth occurred at 15-45 °C (optimum, 25-30 °C), at pH 6.0-9.0 (pH 6.0-7.0) and at NaCl concentrations of 0.5-5.0 % (w/v; 1.0-4.0 %). The major cellular fatty acids were summed feature 3 (comprising C16 : 1ω7c and/or C15 : 0iso 2OH), C17 : 1ω8c and C16 : 0. The predominant polar lipid was diphosphatidylglycerol. The predominant menaquinone was MK-9(H4). The cell-wall peptidoglycan was A1 γ-type, containing meso-DPA. The major cell-wall sugars were rhamnose and ribose. The genome size was 5 509 297 bp with a 71.29 mol% G+C content for strain DSW09, while 4 781 440 bp with a 68.87 mol% G+C content for E. tangerina F10. The average nucleotide identity and digital DNA-DNA hybridization values between strain DSW09 and E. tangerina F10 were 73.44 % and 16.43 %, respectively. Based on phylogenetic, phenotypic, chemotaxonomic evidence and genomic analyses, strain DSW09 is a novel species of genus Euzebya, for which the name Euzebya rosea sp. nov. is proposed. The type strain is DSW09 (=DMS 104446=MCCC 1K03290).

Biofilms formed by methicillin-resistant Staphylococcus aureus (MRSA) are one of the contributing factors to recurrent nosocomial infection in humans. There is currently no specific treatment targeting on biofilms in clinical trials approved by FDA, and antibiotics remain the primary therapeutic strategy. In this study, two anthraquinone compounds isolated from a rare actinobacterial strain Kitasatospora albolonga R62, 3,8-dihydroxy-l-methylanthraquinon-2-carboxylic acid (1) and 3,6,8-trihydroxy-1-methylanthraquinone-2-carboxylic acid (2), together with their 10 commercial analogs 3–12 were evaluated for antibacterial and antibiofilm activities against MRSA, which led to the discovery of two potential antibiofilm anthraquinone compounds anthraquinone-2-carboxlic acid (6) and rhein (12). The structure-activity relationship analysis of these anthraquinones indicated that the hydroxyl group at the C-2 position of the anthraquinone skeleton played an important role in inhibiting biofilm formation at high concentrations, while the carboxyl group at the same C-2 position had a great influence on the antibacterial activity and biofilm eradication activity. The results of crystal violet and methyl thiazolyl tetrazolium staining assays, as well as scanning electron microscope and confocal scanning laser microscopy imaging of compounds 6 and 12 treatment groups showed that both compounds could disrupt preformed MRSA biofilms possibly by killing or dispersing biofilm cells. RNA-Seq was subsequently used for the preliminary elucidation of the mechanism of biofilm eradication, and the results showed upregulation of phosphate transport-related genes in the overlapping differentially expressed genes of both compound treatment groups. Herein, we propose that anthraquinone compounds 6 and 12 could be considered promising candidates for the development of antibiofilm agents.

An aerobic, Gram-stain-negative, rod-shaped and non-motile strain (XY-359T) was isolated from the mouth of a marine invertebrate Onchidium species from the South China Sea. It grew at pH 6.0–8.5 (optimum, pH 7.5), at 15−37 °C (optimum, 30 °C) and in the presence of 0.5–4.5 % (w/v) NaCl (optimum, 2.5 %). It could not hydrolyse Tweens 20, 40, 60 or 80 and no flexirubin-type pigments were produced. The major polar lipids were phosphatidylethanolamine, one unidentified aminolipid, six unidentified phospholipids and two unidentified polar lipids. The major fatty acids were iso-C17:0 3-OH, iso-C15:1 G and iso-C15:0 3-OH. The respiratory quinone was MK-6. Strain XY-359T showed the greatest degree of 16S rRNA sequence similarity to Flagellimonas algicola AsT0115T (96.54 %), followed by Muricauda flava DSM 22638T (96.27 %). Phylogenetic analysis based on 16S rRNA gene sequences and 31 core genes indicated that strain XY-359T belongs to the genus Muricauda . The genome size of strain XY-359T was 4 207 872 bp, with 39.1 mol% of DNA G+C content. The average nucleotide identity and digital DNA–DNA hybridization values between strain XY-359T and F. algicola AsT0115T were 74.58 % and 18.5 %, respectively, and those between strain XY-359T and M. flava DSM 22638T were 74.2 % and 18.3 %. The combined phenotypic, chemotaxonomic and phylogenetic data suggest that strain XY-359T represents a novel species of the genus Muricauda , for which the name Muricauda onchidii sp. nov. is proposed. The type strain is XY-359T (=MCCC 1K03658T =KCTC 72218T). Moreover, based on the proposal of nesting Spongiibacterium and Flagellimonas within Muricauda by García (Validation List No. 193) and the analyses of phylogenetic trees and average amino acid identities in this study, the transfers of F. algicola , F. pacifica and F. maritima to the genus Muricauda as Muricauda algicola comb. nov., Muricauda parva nom. nov. and M. aurantiaca nom. nov., respectively, are proposed, with an emended description of the genus Muricauda .