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Currently, we are at the threshold of the 'post-antibiotic era' due to the global emergence of antimicrobial resistance (AMR), and hence there is a dire need to discover new antibiotics. Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a diverse class of natural products (NPs), some of which are under clinical trials for their antimicrobial potential. Thiopeptides are structurally one of the most complex classes of RiPPs due to numerous post-translational modifications (PTMs), with [4+2] cycloaddition being the core PTM and are active against several gram-positive pathogens. Genome mining coupled with experimental work can harness the unexplored 'cryptic' gene clusters while minimizing the rate of the rediscovery of known metabolites and expand the molecular diversity of NPs with medicinal potential. Employing the genome mining approach using a series of freely available bioinformatics tools, we have identified eight novel putative thiopeptide encoding biosynthetic gene clusters (BGCs) from different bacterial genomes, most of which belong to the class Actinobacteria. Our results provide confidence in the newly identified BGCs, to proceed with wet-bench experiments and discover novel thiopeptide(s).

The cell wall of Clavibacter michiganensis subsp. michiganensis VKM Ас-1403Т (family Microbacteriaceae, class Actinobacteria) contains two polysaccharides. The first one is neutral (1 → 6) linked galactofuranan in which every second galactofuranose residue in the main chain substituted at position 3 by side trisaccharide, β-D-GlcpNAc-(1 → 3)-α-L-Rhap-(1 → 2)-α-D-Fucp-(1 →. The second polymer is pyruvylated galactomannan with the repeating unit, →3)-α-D-Galp-(1 → 3)-α-D-[4,6-S-Pyr]-Manp-(1 → 3)-α-D-Manp-(1 →. The cell wall glycopolymer structures were established by chemical and NMR spectroscopic methods. The obtained results provide new data on the cell wall composition of plant pathogenic species of the genus Clavibacter and can promote understanding the molecular mechanisms involved in colonization and infection of plants.

In the class Actinobacteria, the renowned genus Streptomyces comprised of a group of uniquely complex bacteria that capable of synthesizing a great variety of bioactive metabolites. Streptomycetes are noted to possess several special qualities such as multicellular life cycle and large linearized chromosomes. The significant contribution of Streptomyces in microbial drug discovery as witnessed through the discovery of many important antibiotic drugs has undeniably encourage the exploration of these bacteria from different environments, especially the mangrove environments. This review emphasizes on the genus Streptomyces and reports on the diversity of actinobacterial population from mangroves at different regions of the world as well as discovery of mangrove-derived novel Streptomyces species. Overall, the research on diversity of Actinobacteria in the mangrove environments remains limited. A total of 19 novel Streptomyces spp. isolated from mangroves between the year 2009 - early 2019, notably from China, India, Malaysia, and Thailand. Hence, it will be worthwhile to encourage the study of these bacteria from mangroves of different locations.

To investigate differences in culturable freshwater bacterioplankton between summer and winter, we examined specific bacterioplankton in the low temperature surface water of two shallow, hypereutrophic, temperate lakes in Japan over two winters, and compared our results with the previously published data. We used the size-exclusion assay method (SEAM), a cultivation-based approach that simply and effectively isolates typical freshwater bacterioplankton. The specific clusters detected in the winter samples were the Rhodoferax sp. BAL47 cluster (LimA and LimC), a predominant and ubiquitous freshwater lineage, and the LiUU-5-340 cluster. To confirm tolerance to low water temperature of winter-specific groups, we also compared growth rates at 5°C among several pure strains of typical freshwater bacterioplankton clusters belonging to the Rhodoferax sp. BAL47, Polynucleobacter necessarius, GKS98, LiUU-5-340, and IRD18C08 of the class Betaproteobacteria, and the Luna-1 and Luna-2 of the class Actinobacteria. Specific detectability of freshwater bacterioplankton clusters/subclusters by SEAM in the winter sample substantially correlated with the low temperature-specific growth characteristics of each isolate. Response to water temperature is a key control factor in freshwater bacterioplankton assemblage composition. These results provide important insights into the specific response to water temperature of several ubiquitous culturable freshwater bacterioplankton clusters inhabiting a temperate climate zone.

In order to obtain much more un-known actinomycetes for discovering new drug lead, one hundred soil samples were collected from five national natural protection areas of tropical rain forests, Mengla, Menglun, Mandian, Xiaomengyang and Guanping, in Xishuangbanna, Yunnan, China. 1652 purified cultures of actinobacteria were isolated from these samples by using 5 media. The 16S rRNA gene sequences of 388 selected strains were analyzed, and the phylogenetic analysis was carried out. 35 genera which belong to 8 orders and 14 families of the Class actinobacteria were identified. It is showed from research results that actinomycete diversity in tropical rain forest of Xishuangbanna is the highest comparing with all areas studied in our laboratories before. Selective isolation methods for un-known actinomycetes from soil samples, including medium and inhibitors are discussed in this paper.

The name of the class Actinobacteria is illegitimate according to Rules 15, 22 and 27(3) because it was proposed without the designation of a nomenclatural type. I therefore propose to designate the order Actinomycetales Buchanan 1917, 162 (Approved Lists 1980) as its nomenclatural type, based on Rule 22 of the International Code of Nomenclature of Prokaryotes.

The higher ranks of the class Actinobacteria were proposed and described in 1997. At each rank, the taxa were delineated from each other solely on the basis of 16S rRNA gene sequence phylogenetic clustering and taxon-specific 16S rRNA signature nucleotides. In the past 10 years, many novel members have been assigned to this class while, at the same time, some members have been reclassified. The new 16S rRNA gene sequence information and the changes in phylogenetic positions of some taxa influence decisions about which 16S rRNA nucleotides to define as taxon-specific. As a consequence, the phylogenetic relationships of Actinobacteria at higher levels may need to be reconstructed. Here, we present new 16S rRNA signature nucleotide patterns of taxa above the family level and indicate the affiliation of genera to families. These sets replace the signatures published in 1997. In addition, Actinopolysporineae subord. nov. and Actinopolysporaceae fam. nov. are proposed to accommodate the genus Actinopolyspora, Kineosporiineae subord. nov. and Kineosporiaceae fam. nov. are proposed to accommodate the genera Kineococcus, Kineosporia and Quadrisphaera, Beutenbergiaceae fam. nov. is proposed to accommodate the genera Beutenbergia, Georgenia and Salana and Cryptosporangiaceae fam. nov. is proposed to accommodate the genus Cryptosporangium. The families Nocardiaceae and Gordoniaceae are proposed to be combined in an emended family Nocardiaceae. Emended descriptions are also proposed for most of the other higher taxa.

Different groups of as yet uncultured, phylogenetically deeply rooting organisms of the class Actinobacteria have been shown previously by analysis of 16S rDNA clone libraries to be present in different soil and marine environments of worldwide distribution. In order to monitor specifically the occurrence of two of the actinobacterial groups among bacterial DNA extracted from different environments, a nested polymerase chain reaction (PCR) method, aiming at the sensitive detection of 16S rDNA, was used. Analysis of a clone library generated from one of these 16S rDNA-derived, group-specific PCR products of DNA from a forest soil broadened the spectrum of putative taxa within the two phylogenetic groups. Application of this PCR system demonstrated the presence of these organisms in natural environments and in a sample of dried and moistened peat. Fluctuations in the detectability of the target groups in different samples from the same site strongly suggest that the taxon diversity of a 16S rDNA clone library must be regarded as a temporal genomic snapshot taken from this environmental community.

Globally the change towards the establishment of a bio-based economy has resulted in an increased need for bio-based applications. This, in turn, has served as a driving force for the discovery and application of novel biosurfactants. The class Actinobacteria represents a vast group of microorganisms with the ability to produce a diverse range of secondary metabolites, including surfactants. Understanding the extensive nature of the biosurfactants produced by actinobacterial strains can assist in finding novel biosurfactants with new potential applications. This review therefore presents a comprehensive overview of the knowledge available on actinobacterial surfactants, the chemical structures that have been completely or partly elucidated, as well as the identity of the biosurfactant-producing strains. Producer strains of not yet elucidated compounds are discussed, as well as the original habitats of all the producer strains, which seems to indicate that biosurfactant production is environmentally driven. Methodology applied in the isolation, purification and structural elucidation of the different types of surface active compounds, as well as surfactant activity tests, are also discussed. Overall, actinobacterial surfactants can be summarized to include the dominantly occurring trehalose-comprising surfactants, other non-trehalose containing glycolipids, lipopeptides and the more rare actinobacterial surfactants. The lack of structural information on a large proportion of actinobacterial surfactants should be considered as a driving force to further explore the abundance and diversity of these compounds. This would allow for a better understanding of actinobacterial surface active compounds and their potential for biotechnological application.