Reclassification of Streptomyces diastaticus subsp. ardesiacus, Streptomyces gougerotii and Streptomyces rutgersensis

Streptomyces salinarius sp. nov., an actinomycete isolated from solar saltern soil

An actinobacterium, designated strain SS06011^T, was isolated from solar saltern soil collected from Samut Sakhon province, Thailand. The taxonomic position of this strain was established using the polyphasic taxonomic approach. The strain produced grey aerial spore mass on International Streptomyces Project 2 seawater agar that differentiated into spiral spore chains with rugose-surfaced spores. Strain SS06011^T was found to have ll-diaminopimelic acid in the cell peptidoglycan. Whole-cell hydrolysates contained galactose, glucose and ribose. MK-9(H₆) and MK-9(H₄) were major menaquinones. The major cellular fatty acids comprised iso-C_16 : 0, anteiso-C_15 : 0 and anteiso-C_17 : 0. Diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylinositol were detected in cells. These characteristics were coincident with the typical morphological and chemotaxonomic properties of the genus Streptomyces. The taxonomic affiliation at the genus level of this strain could also be confirmed by its 16S rRNA gene sequence data. Strain SS06011^T showed the highest 16S rRNA gene sequence similarity to Streptomyces ardesiacus NRRL B-1773^T (99.1 %), Streptomyces coelicoflavus NBRC 15399^T (99.1 %) and Streptomyces hyderabadensis OU-40^T (99.1 %). Digital DNA-DNA hybridization (dDDH), average nucleotide identity-blast (ANIb) and average amino acid identity (AAI) values between strain SS06011^T and its closely related type strains, S. ardesiacus NBRC 15402^T, S. coelicoflavus NBRC 15399^T and S. hyderabadensis JCM 17657^T, were in the range of 45.4-48.4 % (for dDDH), 90.8-91.9 % (for ANIb) and 90.8-91.7 % (for AAI), respectively, which are lower than the cut-off criteria for species delineation. The DNA G+C content of genomic DNA was 71.9 mol%. With the differences in physiological, biochemical and genotypic data, strain SS06011^T could be discriminated from its closest neighbours. Thus, strain SS06011^T should be recognized as representing a novel species of the genus Streptomyces, for which the name Streptomyces salinarius sp. nov. is proposed. The type strain is SS06011^T (=TBRC 9951^T=NBRC 113998^T).

Recent Progress of Reclassification of the Genus Streptomyces

The genus Streptomyces is a representative group of actinomycetes and one of the largest taxa in bacteria, including approximately 700 species with validly published names. Since the classification was mainly based on phenotypic characteristics in old days, many members needed to be reclassified according to recent molecular-based taxonomies. Recent developments of molecular-based analysis methods and availability of whole genome sequences of type strains enables researchers to reclassify these phylogenetically complex members on a large scale. This review introduces reclassifications of the genus Streptomyces reported in the past decade. Appropriately 34 Streptomyces species were transferred to the other genera, such as Kitasatospora, Streptacidiphilus, Actinoalloteichus and recently proposed new genera. As a result of reclassifications of 14 subspecies, the genus Streptomyces includes only four subspecies at present in practice. A total of 63 species were reclassified as later heterotypic synonyms of previously recognized species in 24 published reports. As strong relationships between species and the secondary metabolite-biosynthetic gene clusters become clarified, appropriate classifications of this genus will not only contribute to systematics, but also provide significant information when searching for useful bioactive substances.

Characterization of the surugamide biosynthetic gene cluster of TUA-NKU25, a Streptomyces diastaticus strain isolated from Kusaya, and its effects on salt-dependent growth

Kusaya, a traditional Japanese fermented fish product, is known for its high preservability, as it contains natural antibiotics derived from microorganisms, and therefore molds and yeasts do not colonize it easily. In this study, the Streptomyces diastaticus strain TUA-NKU25 was isolated from Kusaya, and its growth as well as the production of antibiotics were investigated. Strain TUA-NKU25 showed advantageous growth characteristics in the presence, but not in the absence, of sodium chloride (NaCl). Antimicrobial assay, high-performance liquid chromatography, and electrospray ionization-mass spectrometry analysis showed that this strain produced surugamide A and uncharacterized antimicrobial compound(s) during growth in the presence of NaCl, suggesting that the biosynthesis of these compounds was upregulated by NaCl. Draft genomic analysis revealed that strain TUA-NKU25 possesses a surugamide biosynthetic gene cluster (sur BGC), although it is incomplete, lacking surB/surC. Phylogenetic analysis of strain TUA-NKU25 and surugamide-producing Streptomyces showed that sur BGC formed a clade distinct from other known groups.

Taxonomic Positions and Secondary Metabolite-Biosynthetic Gene Clusters of Akazaoxime- and Levantilide-Producers

Micromonospora sp. AKA109 is a producer of akazaoxime and A-76356, whereas Micromonospora sp. AKA38 is that of levantilide C. We aimed to clarify their taxonomic positions and identify biosynthetic gene clusters (BGCs) of these compounds. In 16S rRNA gene and DNA gyrase subunit B gene (gyrB) sequence analyses, strains AKA109 and AKA38 were the most closely related to Micromonospora humidisoli MMS20-R2-29^T and Micromonospora schwarzwaldensis HKI0641^T, respectively. Although Micromonospora sp. AKA109 was identified as M. humidisoli by the gyrB sequence similarity and DNA-DNA relatedness based on whole genome sequences, Micromonospora sp. AKA38 was classified to a new genomospecies. M. humidisoli AKA109 harbored six type-I polyketide synthase (PKS), one type-II PKS, one type-III PKS, three non-ribosomal peptide synthetase (NRPS) and three hybrid PKS/NRPS gene clusters, among which the BGC of akazaoxime and A-76356 was identified. These gene clusters are conserved in M. humidisoli MMS20-R2-29^T. Micromonospora sp. AKA38 harbored two type-I PKS, one of which was responsible for levantilide C, one type-II PKS, one type-III PKS, two NRPS and five hybrid PKS/NRPS gene clusters. We predicted products derived from these gene clusters through bioinformatic analyses. Consequently, these two strains are revealed to be promising sources for diverse non-ribosomal peptide and polyketide compounds.

Genome mining of Streptomyces sp. BRB081 reveals the production of the antitumor pyrrolobenzodiazepine sibiromycin

Employing a genome mining approach, this work aimed to further explore the secondary metabolism associated genes of Streptomyces sp. BRB081, a marine isolate. The genomic DNA of BRB081 was sequenced and assembled in a synteny-based pipeline for biosynthetic gene clusters (BGCs) annotation. A total of 27 BGCs were annotated, including a sibiromycin complete cluster, a bioactive compound with potent antitumor activity. The production of sibiromycin, a pyrrolobenzodiazepine, was confirmed by the analysis of obtained BRB081 extract by HPLC-MS/MS, which showed the presence of the sibiromycin ions themselves, as well as its imine and methoxylated forms. To verify the presence of this cluster in other genomes available in public databases, a genome neighborhood network (GNN) was constructed with the non-ribosomal peptide synthetase (NRPS) gene from Streptomyces sp. BRB081. Although the literature does not report the occurrence of the sibiromycin BGC in any other microorganism than Streptosporangium sibiricum, we have located this BGC in 10 other genomes besides the BRB081 isolate, all of them belonging to the Actinomycetia class. These findings strengthen the importance of uninterrupted research for new producer strains of secondary metabolites with uncommon biological activities. These results reinforced the accuracy and robustness of genomics in the screening of natural products. Furthermore, the unprecedented nature of this discovery confirms the unknown metabolic potential of the Actinobacteria phylum and the importance of continuing screening studies in this taxon.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03305-0.

Reclassification of four subspecies in the genus Streptomyces to Streptomyces rubradiris sp. nov., Streptomyces asoensis sp. nov., Streptomyces fructofermentans sp. nov. and Streptomyces ossamyceticus sp. nov

The genus Streptomyces includes, at the time of writing, eight subspecies with validly published names. Streptomyces thermoviolaceus subsp. apingens NBRC 15459^T and Streptomyces lavendulae subsp. grasserius NBRC 13045^T show 16S rRNA gene sequence similarities of >99.7% to their parent species Streptomyces thermoviolaceus subsp. thermoviolaceus NBRC 13905^T and Streptomyces lavendulae subsp. lavendulae NRRL B-2774^T, respectively. In contrast, the type strains of the remaining six subspecies, Streptomyces achromogenes subsp. rubradiris, Streptomyces albosporeus subsp. labilomyceticus , Streptomyces cacaoi subsp. asoensis , Streptomyces chrysomallus subsp. fumigatus , Streptomyces cinereoruber subsp. fructofermentans and Streptomyces hygroscopicus subsp. ossamyceticus, do not show >99.0% 16S rRNA gene sequence similarity to that of each parent species. Although S. chrysomallus subsp. fumigatus and S. hygroscopicus subsp. ossamyceticus were respectively reclassified to 'Kitasatospora fumigata' and 'Streptomyces ossamyceticus', these names have not been validly published yet. In this study, we investigated the taxonomic positions of S. achromogenes subsp. rubradiris, S. cacaoi subsp. asoensis, S. cinereoruber subsp. fructofermentans, S. hygroscopicus subsp. ossamyceticus and S. thermoviolaceus subsp. apingens given that their whole genome sequences are available. Except for S. thermoviolaceus subsp. apingens, these subspecies were discriminated from the parent and closely related species based on phylogenetic, genomic and phenotypic differences. Thus, we reclassify S. achromogenes subsp. rubradiris, S. cacaoi subsp. asoensis , S. cinereoruber subsp. fructofermentans and S. hygroscopicus subsp. ossamyceticus as Streptomyces rubradiris sp. nov., Streptomyces asoensis sp. nov., Streptomyces fructofermentans sp. nov. and Streptomyces ossamyceticus sp. nov., respectively. Multilocus sequence and 16S rRNA gene sequence analyses suggested that S. albosporeus subsp. labilomyceticus and S. lavendulae subsp. grasserius may also be reclassified as independent species.

Differences at Species Level and in Repertoires of Secondary Metabolite Biosynthetic Gene Clusters among Streptomyces coelicolor A3(2) and Type Strains of S. coelicolor and Its Taxonomic Neighbors

Streptomyces coelicolor A3(2) is used worldwide for genetic studies, and its complete genome sequence was published in 2002. However, as the whole genome of the type strain of S. coelicolor has not been analyzed, the relationship between S. coelicolor A3(2) and the type strain is not yet well known. To clarify differences in their biosynthetic potential, as well as their taxonomic positions, we sequenced whole genomes of S. coelicolor NBRC 12854T and type strains of its closely related species—such as Streptomyces daghestanicus, Streptomyces hydrogenans, and Streptomyces violascens—via PacBio. Biosynthetic gene clusters for polyketides and non-ribosomal peptides were surveyed by antiSMASH, followed by bioinformatic analyses. Type strains of Streptomyces albidoflavus, S. coelicolor, S. daghestanicus, S. hydrogenans, and S. violascens shared the same 16S rDNA sequence, but S. coelicolor A3(2) did not. S. coelicolor A3(2) and S. coelicolor NBRC 12854T can be classified as Streptomycesanthocyanicus and S. albidoflavus, respectively. In contrast, S. daghestanicus, S. hydrogenans, and S. violascens are independent species, despite their identical 16S rDNA sequences. S. coelicolor A3(2), S. coelicolor NBRC 12854T, S. daghestanicus NBRC 12762T, S. hydrogenans NBRC 13475T, and S. violascens NBRC 12920T each harbor specific polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) gene clusters in their genomes, whereas PKS and NRPS gene clusters are well conserved between S. coelicolor A3(2) and S. anthocyanicus JCM 5058T, and between S. coelicolor NBRC 12854T and S. albidoflavus DSM 40455T, belonging to the same species. These results support our hypothesis that the repertoires of PKS and NRPS gene clusters are different between different species.

Reclassification of Streptomyces cinnamonensis as a later heterotypic synonym of Streptomyces virginiae

We studied the taxonomic relationship between Streptomyces cinnamonensis and Streptomyces virginiae. These type strains shared the same 16S rRNA gene sequence. Phylogenomic analysis supported them being closely related. Digital DNA-DNA relatedness and average nucleotide identity using whole genome sequences indicated that the two species represent the same genomospecies. They shared similar phenotypic characteristics and harboured the same set of secondary metabolite-biosynthetic gene clusters for polyketides and nonribosomal peptides in the genomes. Therefore, according to Rule 24b of the Bacteriological Code, S. cinnamonensis Okami 1952, 572^AL (Approved Lists 1980) should be reclassified as a later heterotypic synonym of S. virginiae Grundy et al. 1952, 399^AL (Approved Lists 1980) emend. Nouioui et al. 2018. Although 16S rRNA gene sequences were identical among type strains of Streptomyces xanthophaeus, Streptomyces spororaveus and Streptomyces nojiriensis and between those of Streptomyces vinaceus and Streptomyces cirratus, respectively, digital DNA-DNA relatedness indicated that these species are not synonymous.

Genome-based classification of the Streptomyces violaceusniger clade and description of Streptomyces sabulosicollis sp. nov. from an Indonesian sand dune

A polyphasic study was designed to determine the taxonomic provenance of a strain, isolate PRKS01-29T, recovered from an Indonesian sand dune and provisionally assigned to the Streptomyces violaceusniger clade. Genomic, genotypic and phenotypic data confirmed this classification. The isolate formed an extensively branched substrate mycelium which carried aerial hyphae that differentiated into spiral chains of rugose ornamented spores, contained LL-as the wall diaminopimelic acid, MK-9 (H6, H8) as predominant isoprenologues, phosphatidylethanolamine as the diagnostic phospholipid and major proportions of saturated, iso- and anteiso- fatty acids. Whole-genome sequences generated for the isolate and Streptomyces albiflaviniger DSM 41598T and Streptomyces javensis DSM 41764T were compared with phylogenetically closely related strains, the isolate formed a branch within the S. violaceusniger clade in the resultant phylogenomic tree. Whole-genome sequences data showed that isolate PRKS01-29T was most closely related to the S. albiflaviniger strain but was distinguished from the latter and from other members of the clade using combinations of phenotypic properties and average nucleotide identity and digital DNA:DNA hybridization scores. Consequently, it is proposed that isolate PRKS01-29T (= CCMM B1303T = ICEBB-02T = NCIMB 15210T) should be classified in the genus Streptomyces as Streptomyces sabulosicollis sp. nov. It is also clear that streptomycetes which produce spiral chains of rugose ornamented spores form a well-defined monophyletic clade in the Streptomyces phylogenomic tree., the taxonomic status of which requires further study. The genome of the type strain of S. sabulosicollis contains biosynthetic gene clusters predicted to produce new natural products.

Reclassification of Streptomyces costaricanus and Streptomyces phaeogriseichromatogenes as later heterotypic synonyms of Streptomyces murinus

This study aimed to clarify the taxonomic relationships among Streptomyces costaricanus, Streptomyces graminearus, Streptomyces murinus and Streptomyces phaeogriseichromatogenes. These strains share the same 16S rRNA gene sequence. Multilocus sequence analysis revealed that S. costaricanus, S. murinus and S. phaeogriseichromatogenes belong to the same species, but S. graminearus does not. Digital DNA-DNA relatedness and average nucleotide identity among S. costaricanus, S. murinus and S. phaeogriseichromatogenes were 70.9-74.6% and 96.5-97.0 %, respectively. In addition to the previously reported phenotypic data, the presence of a similar set of secondary metabolite-biosynthetic gene clusters for polyketides and nonribosomal peptides supported the similarity among the three species. Therefore, S. costaricanus and S. phaeogriseichromatogenes should be reclassified as later heterotypic synonyms of S. murinus.

Reclassification of 15 Streptomyces species as synonyms of Streptomyces albogriseolus, Streptomyces althioticus, Streptomyces anthocyanicus, Streptomyces calvus, Streptomyces griseoincarnatus, Streptomyces mutabilis, Streptomyces pilosus or Streptomyces rochei

Taxonomic relationships in eight sets of Streptomyces species, (1a) Streptomyces enissocaesilis, Streptomyces plicatus, Streptomyces rochei and Streptomyces vinaceusdrappus, (1b) Streptomyces geysiriensis, (2) Streptomyces luteus and Streptomyces mutabilis, (3) Streptomyces flavoviridis and Streptomyces pilosus, (4) Streptomyces asterosporus and Streptomyces calvus, (5) Streptomyces erythrogriseus, Streptomyces griseoincarnatus, Streptomyces labedae and Streptomyces variabilis, (6a) Streptomyces griseorubens, (6b) Streptomyces matensis, (6c) Streptomyces althioticus, (7) Streptomyces albogriseolus and Streptomyces viridodiastaticus, (8a) Streptomyces humiferus and Streptomyces violaceolatus, (8b) Streptomyces anthocyanicus, Streptomyces coelescens and Streptomyces violaceoruber, were investigated. Type strains within each subset of 1a to 8b shared completely identical 16S rRNA gene sequences. In MLSA, subsets 1a and 1b, 6a to 6c, and 8a and 8b formed an independent clade, respectively, but the evolutionary distances between S. violaceoruber and the other members in set 8 and between S. griseorubens and those in set 6 were 0.022-0.023 and 0.0064-0.0076, respectively. Members in each of the other sets, except for S. labedae, formed an independent clade. In each clade, evolutionary distances between/among the members were <0.007 except for that between S. griseorubens and S. matensis in set 6, suggesting the same species. Digital DNA-DNA relatedness using whole genome sequences and phenotypic similarities supported the synonymies of sets 1 to 3, set 4 except for S. labedae, sets 5 to 7, and set 8 except for S. violaceoruber, respectively. Therefore, S. enissocaesilis, S. geysiriensis, S. plicatus and S. vinaceusdrappus were considered as later heterotypic synonyms of S. rochei; S. luteus as that of S. mutabilis; S. flavoviridis as that of S. pilosus; S. asterosporus as that of S. calvus; S. erythrogriseus and S. variabilis as those of S. griseoincarnatus; S. griseorubens and S. matensis as that of S. althioticus; S. viridodiastaticus as that of S. albogriseolus; S. coelescens, S. humiferus and S. violaceolatus as those of S. anthocyanicus.