Two Streptomyces species producing antibiotic, antitumor, and anti-inflammatory compounds are widespread among intertidal macroalgae and deep-sea coral reef invertebrates from the central Cantabrian Sea

Unveiling the culturable and non-culturable actinobacterial diversity in two macroalgae species from the northern Portuguese coast

Actinomycetota, associated with macroalgae, remains one of the least explored marine niches. The secondary metabolism of Actinomycetota, the primary microbial source of compounds relevant to biotechnology, continues to drive research into the distribution, dynamics, and metabolome of these microorganisms. In this study, we employed a combination of traditional cultivation and metagenomic analysis to investigate the diversity of Actinomycetota in two native macroalgae species from the Portuguese coast. We obtained and taxonomically identified a collection of 380 strains, which were distributed across 12 orders, 15 families, and 25 genera affiliated with the Actinomycetia class, with Streptomyces making up approximately 60% of the composition. Metagenomic results revealed the presence of Actinomycetota in both Chondrus crispus and Codium tomentosum datasets, with relative abundances of 11% and 2%, respectively. This approach identified 12 orders, 16 families, and 17 genera affiliated with Actinomycetota, with minimal overlap with the cultivation results. Acidimicrobiales emerged as the dominant actinobacterial order in both macroalgae, although no strain affiliated with this taxonomic group was successfully isolated. Our findings suggest that macroalgae represent a hotspot for Actinomycetota. The synergistic use of both culture-dependent and independent approaches proved beneficial, enabling the identification and recovery of not only abundant but also rare taxonomic members.

Culturable Streptomyces spp. from high-altitude, oligotrophic North Western Himalaya: a comprehensive study on the diversity, bioactivity and insights into the proteome of potential species

The increasing global concern of antimicrobial resistance and shortage of new antimicrobials necessitates exploring untapped terrestrial environments for new bioactive microbiome diversity. The low-temperature and oligotrophic North Western Himalaya (NWH) region has a vast diversity of Streptomyces with potential antimicrobial properties that remain largely unexplored. This study evaluates the diversity of culturable Streptomyces from high-altitude NWH and their potential as a source of new antimicrobials through genus-specific isolation and identification. The results demonstrate a distinct phylogenetic clustering of Streptomyces from different sampling regions of NWH, site-specific variation in culturable β-diversity and species commonness with varying intersite bioactivity among different sites. Further, the study optimized the media selection for large-scale culture cultivation in antibiotic production processes and demonstrated the antimicrobial efficacy of Streptomyces against a range of pathogens through in vitro bioassays using minimum inhibitory concentration determination and antibiofilm activity. Untargeted label-free proteomic profiling also revealed variable expression of stress-response proteins and antibiotic regulators as a competitive survival strategy for selective antagonistic Streptomyces. The findings highlight the potential of NWH in augmenting antimicrobial discovery and combating antimicrobial resistance through the isolation and study of novel bioactive Streptomyces.

Natural Products and Biological Activity from Actinomycetes Associated with Marine Algae

Marine natural products have been recognized as the most promising source of bioactive substances for drug discovery research. This review illustrates the diversity of culturable actinobacteria associated with marine algae, their bioactivity and metabolites, and approaches to their isolation and determination of their biological properties. Furthermore, actinobacteria associated with marine algae are presented as a new subject for an extensive investigation to find novel and active natural products, which make them a potentially rich and innovative source for new drug development deserving more attention and exploration.

Antibiotic Activity Altered by Competitive Interactions Between Two Coral Reef-Associated Bacteria

Microbes produce natural products that mediate interactions with each other and with their environments, representing a potential source of antibiotics for human use. The biosynthesis of some antibiotics whose constitutive production otherwise remains low has been shown to be induced by competing microbes. Competition among macroorganism hosts may further influence the metabolic outputs of members of their microbiomes, especially near host surfaces where hosts and microbial symbionts come into close contact. At multiple field sites in Fiji, we collected matched samples of corals and algae that were freestanding or in physical contact with each other, cultivated bacteria from their surfaces, and explored growth-inhibitory activities of these bacteria against marine and human pathogens. In the course of the investigation, an interaction was discovered between two coral-associated actinomycetes in which an Agrococcus sp. interfered with the antibiotic output of a Streptomyces sp. Several diketopiperazines identified from the antibiotic-producing bacterium could not, on their own, account for the antibiotic activity indicating that other, as yet unidentified molecule(s) or molecular blends, possibly including diketopiperazines, are likely involved. This observation highlights the complex molecular dynamics at play among microbiome constituents. The mechanisms through which microbial interactions impact the biological activities of specialized metabolites deserve further attention considering the ecological and commercial importance of bacterial natural products.

Streptomyces isolated from bird feathers as a potential source for novel antibiotics

Streptomyces, the main source of antibiotics essential for human health, are widely distributed in nature among terrestrial, oceanic and atmospheric environments. New trends in antibiotic discovery are focused in the search for novel bioactive strains in unexplored habitats. We provide here evidence of the presence of diverse Streptomyces populations in wild bird feathers, such as the seagull, Larus michahellis, collected at Northern Spain; the sparrow, Passer domesticus, and the hoopoe, Upupa epops, both collected in Southern Spain. Taxonomic identification of fourteen bioactive strains, by sequencing their 16S rRNA gene and phylogenetic analyses, revealed that all of them are homologous to a total of 10 different Streptomyces. Strains from seagull samples are homologous to other antibiotic producers previously isolated from atmospheric, marine and terrestrial environments in the Cantabrian Sea region, Northern Spain. Isolates form Southern feather samples, from a house sparrow and a Eurasian hoopoe, are homologues to Streptomyces strains previously isolated mainly from soils along the Mediterranean region. The most relevant feature is that they are producers of diverse antibiotics with activity against Gram-positive, Gram-negative bacteria and fungi. We report here the successful activation of silent antibiotic biosynthetic pathways in response to changes in environmental conditions, such as incubation temperature and salinity of the culture medium, in agreement with the OSMAC approach, One Strain Many Compounds. The finding of bioactive Streptomyces in bird's plumage might be of relevance, not only in the ecology of Streptomyces-birds associations, but also in medicine and biotechnology since they can be regarded as a potential source for novel antibiotics.

Bioactivity of bacteria associated with Red Sea nudibranchs and whole genome sequence of Nocardiopsis dassonvillei RACA-4

Microorganisms associated with marine invertebrates consider an important source of bioactive products. This study aimed to screen for antimicrobial and anticancer activity of crude extracts of bacteria associated with Red sea nudibranchs and molecular identification of the bioactive isolates using 16Sr RNA sequencing, in addition to whole-genome sequencing of one of the most bioactive bacteria. This study showed that bacteria associated with Red sea nudibranchs are highly bioactive and 16Sr RNA sequencing results showed that two isolates belonged to Firmicutes, and two isolates belonged to Proteobacteria, and Actinobacteria. The whole genome sequence data of the isolated Nocardiopsis RACA4 isolate has an estimated genome length of 6,721,839 bp and the taxonomy showed it belongs to the bacteria Nocardiopsis dassonvillei. The De novo assembly of RACA-4 paired reads using Unicycler v0.4.8 initially yielded 97 contigs with an N50 value of 214,568 bp and L50 value of 10, The resulting assembly was further mapped to the reference genome Nocardiopsis dassonvillei strain NCTC10488 using RagTag software v.2.1.0 and a final genome assembly resulted in 39 contigs and N50 value of 6,726,007 and L50 of 1. Genome mining using anti-smash showed around 9.1% of the genome occupied with genes related to secondary metabolites biosynthesis. A wide variety of secondary metabolites belonging to Polyketides, Terpenes, and nonribosomal peptides were predicted with high degree of similarity to known compounds. Non-characterized clusters were also found which suggest new natural compounds discovered by further studies.

Actinobacteria from Arctic and Atlantic deep-sea sediments-Biodiversity and bioactive potential

The deep-sea covers over 70% of the Earth's surface and harbors predominantly uncharacterized bacterial communities. Actinobacteria are the major prokaryotic source of bioactive natural products that find their way into drug discovery programs, and the deep-sea is a promising source of biotechnologically relevant actinobacteria. Previous studies on actinobacteria in deep-sea sediments were either regionally restricted or did not combine a community characterization with the analysis of their bioactive potential. Here we characterized the actinobacterial communities of upper layers of deep-sea sediments from the Arctic and the Atlantic (Azores and Madeira) ocean basins, employing 16S rRNA metabarcoding, and studied the biosynthetic potential of cultivable actinobacteria retrieved from those samples. Metabarcoding analysis showed that the actinobacterial composition varied between the sampled regions, with higher abundance in the Arctic samples but higher diversity in the Atlantic ones. Twenty actinobacterial genera were detected using metabarcoding, as a culture-independent method, while culture-dependent methods only allowed the identification of nine genera. Isolation of actinobacteria resulted on the retrieval of 44 isolates, mainly associated with Brachybacterium, Microbacterium, and Brevibacterium genera. Some of these isolates were only identified on a specific sampled region. Chemical extracts of the actinobacterial isolates were subsequently screened for their antimicrobial, anticancer and anti-inflammatory activities. Extracts from two Streptomyces strains demonstrated activity against Candida albicans. Additionally, eight extracts (obtained from Brachybacterium, Brevibacterium, Microbacterium, Rhodococcus, and Streptomyces isolates) showed significant activity against at least one of the tested cancer cell lines (HepG2 and T-47D). Furthermore, 15 actinobacterial extracts showed anti-inflammatory potential in the RAW 264.4 cell model assay, with no concomitant cytotoxic response. Dereplication and molecular networking analysis of the bioactive actinobacterial extracts showed the presence of some metabolites associated with known natural products, but one of the analyzed clusters did not show any match with the natural products described as responsible for these bioactivities. Overall, we were able to recover taxonomically diverse actinobacteria with different bioactivities from the studied deep-sea samples. The conjugation of culture-dependent and -independent methods allows a better understanding of the actinobacterial diversity of deep-sea environments, which is important for the optimization of approaches to obtain novel chemically-rich isolates.

Natural products, including a new caboxamycin, from Streptomyces and other Actinobacteria isolated in Spain from storm clouds transported by Northern winds of Arctic origin

Actinobacteria, mostly Streptomyces species, are the main source of natural products essential in medicine. While the majority of producer microorganisms of secondary metabolite are reported from terrestrial or marine environments, there are limited reports of their isolation from atmospheric precipitations. Clouds are considered as atmospheric oases for microorganisms and there is a recent paradigm shift whereby atmospheric-derived Actinobacteria emerge as an alternative source for drug discovery. In this context, we studied a total of 18 bioactive Actinobacteria strains, isolated by sampling nine precipitation events with prevailing Northern winds in the Cantabrian Sea coast, Northern Spain. Backward trajectories meteorological analyses indicate that air masses were originated mostly in the Arctic Ocean, and their trajectory to downwind areas involved the Atlantic Ocean and also terrestrial sources from continental Europe, and in some events from Canada, Greenland, Mauritania and Canary Islands. Taxonomic identification of the isolates, by 16S rRNA gene sequencing and phylogenetic analyses, revealed that they are members of three Actinobacteria genera. Fifteen of the isolates are Streptomyces species, thus increasing the number of bioactive species of this genus in the atmosphere to a 6.8% of the total currently validated species. In addition, two of the strains belong to the genus Micromonospora and one to genus Nocardiopsis. These findings reinforce a previous atmospheric dispersal model, extended herein to the genus Micromonospora. Production of bioactive secondary metabolites was screened in ethyl acetate extracts of the strains by LC-UV-MS and a total of 94 secondary metabolites were detected after LC/MS dereplication. Comparative analyses with natural products databases allowed the identification of 69 structurally diverse natural products with contrasted biological activities, mostly as antibiotics and antitumor agents, but also anti-inflammatory, antiviral, antiparasitic, immunosuppressant and neuroprotective among others. The molecular formulae of the 25 remaining compounds were determined by HRMS. None of these molecules had been previously reported in natural product databases indicating potentially novel metabolites. As a proof of concept, a new metabolite caboxamycin B (1) was isolated from the culture broth of Streptomyces sp. A-177 and its structure was determined by various spectrometric methods. To the best of our knowledge, this is the first novel natural product obtained from an atmospheric Streptomyces, thus pointing out precipitations as an innovative source for discovering new pharmaceutical natural products.

Marine Actinomycetes Associated with Stony Corals: A Potential Hotspot for Specialized Metabolites

Microbial secondary metabolites are an important source of antibiotics currently available for combating drug-resistant pathogens. These important secondary metabolites are produced by various microorganisms, including Actinobacteria. Actinobacteria have a colossal genome with a wide array of genes that code for several bioactive metabolites and enzymes. Numerous studies have reported the isolation and screening of millions of strains of actinomycetes from various habitats for specialized metabolites worldwide. Looking at the extent of the importance of actinomycetes in various fields, corals are highlighted as a potential hotspot for untapped secondary metabolites and new bioactive metabolites. Unfortunately, knowledge about the diversity, distribution and biochemistry of marine actinomycetes compared to hard corals is limited. In this review, we aim to summarize the recent knowledge on the isolation, diversity, distribution and discovery of natural compounds from marine actinomycetes associated with hard corals. A total of 11 new species of actinomycetes, representing nine different families of actinomycetes, were recovered from hard corals during the period from 2007 to 2022. In addition, this study examined a total of 13 new compounds produced by five genera of actinomycetes reported from 2017 to 2022 with antibacterial, antifungal and cytotoxic activities. Coral-derived actinomycetes have different mechanisms of action against their competitors.

Bioactive Natural Products in Actinobacteria Isolated in Rainwater From Storm Clouds Transported by Western Winds in Spain

Actinobacteria are the main producers of bioactive natural products essential for human health. Although their diversity in the atmosphere remains largely unexplored, using a multidisciplinary approach, we studied here 27 antibiotic producing Actinobacteria strains, isolated from 13 different precipitation events at three locations in Northern and Southern Spain. Rain samples were collected throughout 2013-2016, from events with prevailing Western winds. NOAA HYSPLIT meteorological analyses were used to estimate the sources and trajectories of the air-mass that caused the rainfall events. Five-day backward air masses trajectories of the diverse events reveals a main oceanic source from the North Atlantic Ocean, and in some events long range transport from the Pacific and the Arctic Oceans; terrestrial sources from continental North America and Western Europe were also estimated. Different strains were isolated depending on the precipitation event and the latitude of the sampling site. Taxonomic identification by 16S rRNA sequencing and phylogenetic analysis revealed these strains to belong to two Actinobacteria genera. Most of the isolates belong to the genus Streptomyces, thus increasing the number of species of this genus isolated from the atmosphere. Furthermore, five strains belonging to the rare Actinobacterial genus Nocardiopsis were isolated in some events. These results reinforce our previous Streptomyces atmospheric dispersion model, which we extend herein to the genus Nocardiopsis. Production of bioactive secondary metabolites was analyzed by LC-UV-MS. Comparative analyses of Streptomyces and Nocardiopsis metabolites with natural product databases led to the identification of multiple, chemically diverse, compounds. Among bioactive natural products identified 55% are antibiotics, both antibacterial and antifungal, and 23% have antitumor or cytotoxic properties; also compounds with antiparasitic, anti-inflammatory, immunosuppressive, antiviral, insecticidal, neuroprotective, anti-arthritic activities were found. Our findings suggest that over time, through samples collected from different precipitation events, and space, in different sampling places, we can have access to a great diversity of Actinobacteria producing an extraordinary reservoir of bioactive natural products, from remote and very distant origins, thus highlighting the atmosphere as a contrasted source for the discovery of novel compounds of relevance in medicine and biotechnology.

Natural Products from Actinomycetes Associated with Marine Organisms

The actinomycetes have proven to be a rich source of bioactive secondary metabolites and play a critical role in the development of pharmaceutical researches. With interactions of host organisms and having special ecological status, the actinomycetes associated with marine animals, marine plants, macroalgae, cyanobacteria, and lichens have more potential to produce active metabolites acting as chemical defenses to protect the host from predators as well as microbial infection. This review focuses on 536 secondary metabolites (SMs) from actinomycetes associated with these marine organisms covering the literature to mid-2021, which will highlight the taxonomic diversity of actinomycetes and the structural classes, biological activities of SMs. Among all the actinomycetes listed, members of Streptomyces (68%), Micromonospora (6%), and Nocardiopsis (3%) are dominant producers of secondary metabolites. Additionally, alkaloids (37%), polyketides (33%), and peptides (15%) comprise the largest proportion of natural products with mostly antimicrobial activity and cytotoxicity. Furthermore, the data analysis and clinical information of SMs have been summarized in this article, suggesting that some of these actinomycetes with multiple host organisms deserve more attention to their special ecological status and genetic factors.

Mining Natural Products with Anticancer Biological Activity through a Systems Biology Approach

Natural products, like turmeric, are considered powerful antioxidants which exhibit tumor-inhibiting activity and chemoradioprotective properties. Nowadays, there is a great demand for developing novel, affordable, efficacious, and effective anticancer drugs from natural resources. In the present study, we have employed a stringent in silico methodology to mine and finally propose a number of natural products, retrieved from the biomedical literature. Our main target was the systematic search of anticancer products as anticancer agents compatible to the human organism for future use. In this case and due to the great plethora of such products, we have followed stringent bioinformatics methodologies. Our results taken together suggest that natural products of a great diverse may exert cytotoxic effects in a maximum of the studied cancer cell lines. These natural compounds and active ingredients could possibly be combined to exert potential chemopreventive effects. Furthermore, in order to substantiate our findings and their application potency at a systems biology level, we have developed a representative, user-friendly, publicly accessible biodatabase, NaturaProDB, containing the retrieved natural resources, their active ingredients/fractional mixtures, the types of cancers that they affect, and the corresponding experimentally verified target genes.

Whole-genome sequencing of two Streptomyces strains isolated from the sand dunes of Sahara

BACKGROUND

Sahara is one of the largest deserts in the world. The harsh climatic conditions, especially high temperature and aridity lead to unique adaptation of organisms, which could be a potential source of new metabolites. In this respect, two Saharan soils from El Oued Souf and Beni Abbes in Algeria were collected. The bacterial isolates were selected by screening for antibacterial, antifungal, and enzymatic activities. The whole genomes of the two native Saharan strains were sequenced to study desert Streptomyces microbiology and ecology from a genomic perspective.

RESULTS

Strains Babs14 (from Beni Abbes, Algeria) and Osf17 (from El Oued Souf, Algeria) were initially identified by 16S rRNA sequencing as belonging to the Streptomyces genus. The whole genome sequencing of the two strains was performed using Pacific Biosciences Sequel II technology (PacBio), which showed that Babs14 and Osf17 have a linear chromosome of 8.00 Mb and 7.97 Mb, respectively. The number of identified protein coding genes was 6910 in Babs14 and 6894 in Osf17. No plasmids were found in Babs14, whereas three plasmids were detected in Osf17. Although the strains have different phenotypes and are from different regions, they showed very high similarities at the DNA level. The two strains are more similar to each other than either is to the closest database strain. The search for potential secondary metabolites was performed using antiSMASH and predicted 29 biosynthetic gene clusters (BGCs). Several BGCs and proteins were related to the biosynthesis of factors needed in response to environmental stress in temperature, UV light and osmolarity.

CONCLUSION

The genome sequencing of Saharan Streptomyces strains revealed factors that are related to their adaptation to an extreme environment and stress conditions. The genome information provides tools to study ecological adaptation in a desert environment and to explore the bioactive compounds of these microorganisms. The two whole genome sequences are among the first to be sequenced for the Streptomyces genus of Algerian Sahara. The present research was undertaken as a first step to more profoundly explore the desert microbiome.

Steptomyces fagopyri sp. nov., a novel actinomycete isolated from rhizospheric soil of Fagopyrum dibotrys

A novel actinomycete, designated strain QMT-28^T, was isolated from rhizosphere soil of Fagopyrum dibotrys collected from Shuangfeng, Hunan Province, PR China. Strain QMT-28^T grew well on International Streptomyces Project series media and formed well-developed, branched substrate hyphae and aerial mycelium that differentiated into loose spiral spore chains consisting of cylindrical spores with smooth surfaces. The diagnostic diamino acid was ll-diaminopimelic acid and the whole-cell sugars were galactose and glucose. The predominant fatty acids were C_18 : 1 cis9, summed feature 6 (C_18 : 2 cis 9,12/C_18 : 0 a) and C_16 : 0. The polar lipids included diphosphatidylglycerol, hydroxy phospatidylethanolamine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylinositol mannosides, phospholipids of unknown structure containing glucosamine and several unidentified phospholipids. The major menaquinones were MK-9, MK-9(H₂), MK-9(H₄), MK-9(H₆) and MK-9(H₈). The genome size of strain QMT-28^T was about 8.7 Mbp with a G+C content of 71.2 mol%. Phylogenetic analysis showed that the novel strain was closely related to Streptomyces olivochromogenes DSM 40451^T (99.5 % similarity), Streptomyces mirabilis NBRC 13450^T (98.9 %), Streptomyces kanamyceticus NBRC 13414^T (98.9 %), Streptomyces kaempferi I37^T (98.9 %) and Streptomyces arcticus ZLN234^T (98.8 %). However, the average nucleotide identity values, the digital DNA-DNA hybridization values and the multilocus sequence analysis evolutionary distances between this strain and closely related strains showed that it belonged to a distinct species. In addition, these results were also supported by differences in the phenotypic characteristics between QMT-28^T and five closely related type strains. Consequently, strain QMT-28^T should represent a novel species of the genus Streptomyces, with the suggested name Streptomyces fagopyri sp. nov. The type strain is QMT-28^T (=CICC 24808^T=JCM 33796^T).

Ecological and biotechnological importance of secondary metabolites produced by coral-associated bacteria

Symbiotic relationships between corals and their associated micro-organisms are essential to maintain host homeostasis. Coral-associated bacteria (CAB) can have different beneficial roles in the coral metaorganism, such as metabolizing essential nutrients for the coral host and protecting the coral from pathogens. Many CAB exert these functions via secondary metabolites, which include antibacterial, antifouling, antitumour, antiparasitic and antiviral compounds. This review describes how analysis of CAB has led to the discovery of secondary metabolites with potential biotechnological applications. The most commonly found types of secondary metabolites, antimicrobial and antibiofilm compounds, are emphasized and described. Recently developed methods that can be applied to enhance the culturing of CAB from shallow-water reefs and the less-studied deep-sea coral reefs are also discussed. Last, we suggest how the combined use of meta-omics and innovative growth-diffusion techniques can vastly improve the discovery of novel compounds in coral environments.

Genome-based analysis for the bioactive potential of Streptomyces yeochonensis CN732, an acidophilic filamentous soil actinobacterium

BACKGROUND

Acidophilic members of the genus Streptomyces can be a good source for novel secondary metabolites and degradative enzymes of biopolymers. In this study, a genome-based approach on Streptomyces yeochonensis CN732, a representative neutrotolerant acidophilic streptomycete, was employed to examine the biosynthetic as well as enzymatic potential, and also presence of any genetic tools for adaptation in acidic environment.

RESULTS

A high quality draft genome (7.8 Mb) of S. yeochonensis CN732 was obtained with a G + C content of 73.53% and 6549 protein coding genes. The in silico analysis predicted presence of multiple biosynthetic gene clusters (BGCs), which showed similarity with those for antimicrobial, anticancer or antiparasitic compounds. However, the low levels of similarity with known BGCs for most cases suggested novelty of the metabolites from those predicted gene clusters. The production of various novel metabolites was also confirmed from the combined high performance liquid chromatography-mass spectrometry analysis. Through comparative genome analysis with related Streptomyces species, genes specific to strain CN732 and also those specific to neutrotolerant acidophilic species could be identified, which showed that genes for metabolism in diverse environment were enriched among acidophilic species. In addition, the presence of strain specific genes for carbohydrate active enzymes (CAZyme) along with many other singletons indicated uniqueness of the genetic makeup of strain CN732. The presence of cysteine transpeptidases (sortases) among the BGCs was also observed from this study, which implies their putative roles in the biosynthesis of secondary metabolites.

CONCLUSIONS

This study highlights the bioactive potential of strain CN732, an acidophilic streptomycete with regard to secondary metabolite production and biodegradation potential using genomics based approach. The comparative genome analysis revealed genes specific to CN732 and also those among acidophilic species, which could give some insights into the adaptation of microbial life in acidic environment.

A new uridine derivative and a new indole derivative from the coral-associated actinomycete Pseudonocardia sp. SCSIO 11457

A new uridine derivative 11457 A (1), and a new indole derivative 11457B (2), together with a known compound 1H-indole-2-carbaldehyde (3), were characterized from the fermentation broth of the actinomycete Pseudonocardia sp. SCSIO 11457, an isolate associated with the scleractinian coral Galaxea fascicularis. Upon detailed spectroscopic analysis, 11457 A (1) was identified as a uridine analog, and 11457B (2) was elucidated as an indole derivative 2-hydroxy-1-(1H-indol-2-yl)pentane-1,4-dione. Biological evaluation indicated that none of compounds 1-3 showed antibacterial activities against pathogenic bacteria and cytotoxic activities against human cancer cell lines.

Actinobacteria Isolated From Laminaria ochroleuca: A Source of New Bioactive Compounds

Nature is the major reservoir of biologically active molecules. The urgent need of finding novel molecules for pharmaceutical application is prompting the research of underexplored environments, such as marine ecosystems. Here, we investigated cultivable actinobacteria associated with the macroalgae Laminaria ochroleuca and assessed their potential to produce compounds with antimicrobial or anticancer activities. A specimen of L. ochroleuca was collected in a rocky shore in northern Portugal, and fragments of tissues from different parts of the macroalgae (holdfast, stipe, and blades) were surface sterilized and plated in three culture media selective for actinobacteria. A total of 90 actinobacterial strains were isolated, most of which affiliated with the genus Streptomyces. Isolates associated with the genera Isoptericola, Rhodococcus, Nonomuraeae, Nocardiopsis, Microbispora, and Microbacterium were also obtained. Organic extracts from the isolates were tested for their antimicrobial activity using the agar-based disk diffusion method, followed by determination of minimum inhibitory concentration (MIC) values. Forty-five isolates inhibited the growth of Candida albicans and/or Staphylococcus aureus, with MIC values ranging from <0.5 to 1000 μg mL⁻¹. The actinobacterial isolates were also tested for their anticancer potential on two human cancer cell lines. Twenty-eight extracts affected the viability of at least one human cancer cell line (breast carcinoma T-47D and neuroblastoma SH-SY5Y) and non-carcinogenic endothelial cell line (hCMEC/D3). Seven extracts affected the viability of cancer cells only. This study revealed that L. ochroleuca is a rich source of actinobacteria with promising antimicrobial and anticancer activities and suggests that macroalgae may be a valuable source of actinobacteria and, consequently, of new molecules with biotechnological importance.

Desertomycin G, a New Antibiotic with Activity against Mycobacterium tuberculosis and Human Breast Tumor Cell Lines Produced by Streptomyces althioticus MSM3, Isolated from the Cantabrian Sea Intertidal Macroalgae Ulva sp

The isolation and structural elucidation of a structurally new desertomycin, designated as desertomycin G (1), with strong antibiotic activity against several clinically relevant antibiotic resistant pathogens are described herein. This new natural product was obtained from cultures of the marine actinomycete Streptomyces althioticus MSM3, isolated from samples of the intertidal seaweed Ulva sp. collected in the Cantabrian Sea (Northeast Atlantic Ocean). Particularly interesting is its strong antibiotic activity against Mycobacterium tuberculosis clinical isolates, resistant to antibiotics in clinical use. To the best of our knowledge, this is the first report on a member of the desertomycin family displaying such activity. Additionally, desertomycin G shows strong antibiotic activities against other relevant Gram-positive clinical pathogens such as Corynebacterium urealyticum, Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, Enterococcus faecium, Enterococcus faecalis, and Clostridium perfringens. Desertomycin G also displays moderate antibiotic activity against relevant Gram-negative clinical pathogens such as Bacteroides fragilis, Haemophilus influenzae and Neisseria meningitidis. In addition, the compound affects viability of tumor cell lines, such as human breast adenocarcinoma (MCF-7) and colon carcinoma (DLD-1), but not normal mammary fibroblasts.

The homeostasis-maintaining metabolites from bacterial stress response to bacteriophage infection suppress tumor metastasis

The antiviral metabolites from bacterial stress response to bacteriophage infection can maintain homeostasis of host cells, while metabolism disorder is a remarkable characteristic of tumorigenesis. In the aspect of metabolic homeostasis, therefore, the antiviral homeostasis-maintaining metabolites of bacteria may possess anti-tumor activity. However, this issue has not been addressed. Here we show that the homeostasis-challenged maintaining metabolites from deep-sea bacteriophage-challenged thermophile can suppress tumor metastasis. The results indicated that the metabolic profiles of the bacteriophage GVE2-infected and virus-free thermophile Geobacillus sp. E263 from a deep-sea hydrothermal vent were remarkably different. Thirteen metabolites were significantly elevated and two metabolites were downregulated in thermophile stress response to GVE2 infection. As an example, the upregulated L-norleucine was characterized. The data showed that L-norleucine had antiviral activity in thermophile. Furthermore, the in vitro and in vivo assays revealed that L-norleucine, as well as its derivative, significantly suppressed metastasis of gastric and breast cancer cells. L-norleucine interacted with hnRNPA2/B1 protein to inhibit the expressions of Twist1 and Snail, two inhibitors of E-cadherin, and promote the E-cadherin expression, leading to the inhibition of tumor metastasis. Therefore, our study presented that antiviral homeostasis-maintaining metabolites of microbes might be a promising source for anti-tumor drugs.

Diversity and antimicrobial potential in sea anemone and holothurian microbiomes

Marine invertebrates, as holobionts, contain symbiotic bacteria that coevolve and develop antimicrobial substances. These symbiotic bacteria are an underexplored source of new bioactive molecules to face the emerging antibiotic resistance in pathogens. Here, we explored the antimicrobial activity of bacteria retrieved from the microbiota of two sea anemones (Anemonia sulcata, Actinia equina) and two holothurians (Holothuria tubulosa, Holothuria forskali). We tested the antimicrobial activity of the isolated bacteria against pathogens with interest for human health, agriculture and aquaculture. We isolated 27 strains with antibacterial activity and 12 of these isolates also showed antifungal activity. We taxonomically identified these strains being Bacillus and Vibrio species the most representative producers of antimicrobial substances. Microbiome species composition of the two sea anemones was similar between them but differed substantially of seawater bacteria. In contrast, microbiome species composition of the two holothurian species was different between them and in comparison with the bacteria in holothurian feces and seawater. In all the holobiont microbiomes Bacteroidetes was the predominant phylum. For each microbiome, we determined diversity and the rank-abundance dominance using five fitted models (null, pre-emption, log-Normal, Zipf and Zipf-Mandelbrot). The models with less evenness (i.e. Zipf and Zipf-Mandelblot) showed the best fits in all the microbiomes. Finally, we tracked (using the V4 hypervariable region of 16S rRNA gene) the relative abundance of these 27 isolates with antibacterial activity in the total pool of sequences obtained for the microbiome of each holobiont. Coincidences, although with extremely low frequencies, were detected only in the microbiome of H. forskali. This fact suggests that these isolated bacteria belong to the long tail of rare symbiotic bacteria. Therefore, more and more sophisticated culture techniques are necessary to explore this apparently vast pool of rare symbiontic bacteria and to determine their biotechnological potentiality.

Atmospheric Precipitations, Hailstone and Rainwater, as a Novel Source of Streptomyces Producing Bioactive Natural Products

A cultivation-dependent approach revealed that highly diverse populations of Streptomyces were present in atmospheric precipitations from a hailstorm event sampled in February 2016 in the Cantabrian Sea coast, North of Spain. A total of 29 bioactive Streptomyces strains isolated from small samples of hailstone and rainwater, collected from this hailstorm event, were studied here. Taxonomic identification by 16S rRNA sequencing revealed more than 20 different Streptomyces species, with their closest homologs displaying mainly oceanic but also terrestrial origins. Backward trajectory analysis revealed that the air-mass sources of the hailstorm event, with North Western winds, were originated in the Arctic Ocean (West Greenland and North Iceland) and Canada (Labrador), depending on the altitude. After traveling across the North Atlantic Ocean during 4 days the air mass reached Europe and precipitated as hailstone and rain water at the sampling place in Spain. The finding of Streptomyces species able to survive and disperse through the atmosphere increases our knowledge of the biogeography of genus Streptomyces on Earth, and reinforces our previous dispersion model, suggesting a generalized feature for the genus which could have been essential in his evolution. This unique atmospheric-derived Streptomyces collection was screened for production of bioactive secondary metabolites. Analyses of isolates ethyl acetate extracts by LC-UV-MS and further database comparison revealed an extraordinary diversity of bioactive natural products. One hundred molecules were identified, mostly displaying contrasted antibiotic and antitumor/cytotoxic activities, but also antiparasitic, antiviral, anti-inflammatory, neuroprotector, and insecticide properties. More interestingly, 38 molecules not identified in natural products databases might represent new natural products. Our results revealed for the first time an extraordinary diversity of Streptomyces species in the atmosphere able to produce an extraordinary repertoire of bioactive molecules, thus providing a very promising source for the discovery of novel pharmaceutical natural products.

Chemical analysis of the Alphaproteobacterium strain MOLA1416 associated with the marine lichen Lichina pygmaea

Alphaproteobacterium strain MOLA1416, related to Mycoplana ramosa DSM 7292 and Chelativorans intermedius CC-MHSW-5 (93.6% 16S rRNA sequence identity) was isolated from the marine lichen, Lichina pygmaea and its chemical composition was characterized by a metabolomic network analysis using LC-MS/MS data. Twenty-five putative different compounds were revealed using a dereplication workflow based on MS/MS signatures available through GNPS (https://gnps.ucsd.edu/). In total, ten chemical families were highlighted including isocoumarins, macrolactones, erythrinan alkaloids, prodiginines, isoflavones, cyclohexane-diones, sterols, diketopiperazines, amino-acids and most likely glucocorticoids. Among those compounds, two known metabolites (13 and 26) were isolated and structurally identified and metabolite 26 showed a high cytotoxic activity against B16 melanoma cell lines with an IC₅₀ 0.6 ± 0.07 μg/mL.

Microbial diversity of saline environments: searching for cytotoxic activities

In order to select halophilic microorganisms as a source of compounds with cytotoxic activities, a total of 135 bacterial strains were isolated from water and sediment samples collected from the Zipaquirá salt mine in the Colombian Andes. We determined the cytotoxic effects of 100 crude extracts from 54 selected organisms on the adherent murine mammary cell carcinoma 4T1 and human mammary adenocarcinoma MCF-7 cell lines. These extracts were obtained from strains of Isoptericola, Ornithinimicrobium, Janibacter, Nesterenkonia, Alkalibacterium, Bacillus, Halomonas, Chromohalobacter, Shewanella, Salipiger, Martellela, Oceanibaculum, Caenispirillum and Labrenzia. The extracts of 23 strains showed an IC₅₀ of less than 100 μg mL⁻¹. They were subsequently analyzed by LC/MS allowing dereplication of 20 compounds. The cytotoxic effect was related to a complex mixture of diketopiperazines present in many of the extracts analyzed. The greatest cytotoxic activity against both of the evaluated cell lines was obtained from the chloroform extract of Labrenzia aggregata USBA 371 which had an IC₅₀ < 6 μg mL⁻¹. Other extracts with high levels of cytotoxic activity were obtained from Bacillus sp. (IC₅₀ < 50 μg mL⁻¹) which contained several compounds such as macrolactin L and A, 7-O-succinoylmacrolactin F and iturin. Shewanella chilikensis USBA 344 also showed high levels of cytotoxic activity against both cell lines in the crude extract: an IC₅₀ < 15 μg mL⁻¹ against the 4T1 cell line and an IC₅₀ < 68 μg mL⁻¹ against the MCF-7 cell line. Nesterenkonia sandarakina CG 35, which has an IC₅₀ of 118 µg mL⁻¹ against 4T1, is a producer of diketopiperazines and 1-acetyl-β-carboline. Also, Ornithinimicrobium kibberense CG 24, which has IC₅₀ < 50 μg mL⁻¹, was a producer of diketopiperazines and lagunamycin. Our study demonstrates that these saline environments are habitats of halophilic and halotolerant bacteria that have previously unreported cytotoxic activity.

The Orphan Response Regulator Aor1 Is a New Relevant Piece in the Complex Puzzle of Streptomyces coelicolor Antibiotic Regulatory Network

Streptomyces coelicolor, the best-known biological antibiotic producer, encodes 29 predicted orphan response regulators (RR) with a putative role in the response to environmental stimuli. However, their implication in relation to secondary metabolite production is mostly unexplored. Here, we show how the deletion of the orphan RR Aor1 (SCO2281) provoked a drastic decrease in the production of the three main antibiotics produced by S. coelicolor and a delay in morphological differentiation. With the aim to better understand the transcriptional events underpinning these phenotypes, and the global role of Aor1 in Streptomyces, a transcriptional fingerprint of the Δaor1 mutant was compared to a wild-type strain. RNA-Seq analysis revealed that the deletion of this orphan regulator affects a strikingly high number of genes, such as the genes involved in secondary metabolism, which matches the antibiotic production profiles observed. Of particular note, the sigma factor SigB and all of the genes comprising its regulon were up regulated in the mutant. Our results show that this event links osmotic stress to secondary metabolite production in S. coelicolor and indicates that the RR encoded by aor1 could be a key regulator in both of these processes.

Isolation and characterization of a novel tyrosinase produced by Sahara soil actinobacteria and immobilization on nylon nanofiber membranes

In the present study different actinomycete strains were collected and isolated from Algerian Sahara soil with the aim to select novel enzymes with promising features for biotechnological applications. The Ms1 strain was selected, amongst the others, for its capability to produce melanin in different solid media. Ms1 chromosomal DNA was sequenced and the strain assigned to Streptomyces cyaneofuscatus sp. A tyrosinase (MW∼30kD) encoding sequence was identified and the corresponding enzyme was isolated and biochemically characterized. The tyrosinase showed the highest activity and stability at neutral and alkaline pH and it was able to oxidize l-DOPA at T=55°C and pH 7. The enzyme showed variable stability in presence of various water-miscible organic solvents, while it was inactivated by reducing agents. The tyrosinase activity was unaffected by NaCl and enhanced by different cations. Furthermore, the enzyme showed a higher specificity for diphenols than monophenols showing a higher diphenolase than monophenolase activity. Finally, tyrosinase was stabilized by immobilization on nylon nanofiber membranes with a payload of 82% when 1% glutaraldeyde was used. Taken all together, these results show that the enzyme displays interesting properties for biotechnological purposes.

Streptomyces sediminis sp. nov. isolated from crater lake sediment

A novel actinobacterial strain, MKSP12^T, was isolated from coastal sediment of a crater lake in central Anatolia, Turkey. The taxonomic position of the strain was clarified using a polyphasic approach. Phylogenetic analyses based on 16S rRNA gene sequences showed that strain MKSP12^T is closely related to Streptomyces specialis GW 41-1564^T with 97.1% sequence similarity. The strain produces aerial hyphae that differentiate into spiral chains of smooth surfaced spores and grows over a temperature range of 20-37 °C, at pH 7-11 and in the presence of 3% (w/v) sodium chloride. The cell wall amino acid is LL-diaminopimelic acid and the whole cell sugars are glucose and ribose. The polar lipids profile consists of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol, an unidentified aminophospholipid, two unidentified phospholipids, an unidentified glycophospholipid and eight unidentified glycolipids; iso-C_16:0, iso-C_16:1 G, anteiso-C_17:0 and anteiso-C_17:1 ω9c were identified as the predominant cellular fatty acids (> 10%). Based on morphological and chemotaxonomic characteristics, and phylogenetic analyses, the strain is considered to represent a novel species in the genus Streptomyces, for which the name Streptomyces sediminis sp. nov. is proposed with the type strain MKSP12^T (= DSM 100692^T = KCTC 39613^T).

Paulomycin G, a New Natural Product with Cytotoxic Activity against Tumor Cell Lines Produced by Deep-Sea Sediment Derived Micromonospora matsumotoense M-412 from the Avilés Canyon in the Cantabrian Sea

The present article describes a structurally novel natural product of the paulomycin family, designated as paulomycin G (1), obtained from the marine strain Micromonospora matsumotoense M-412, isolated from Cantabrian Sea sediments collected at 2000 m depth during an oceanographic expedition to the submarine Avilés Canyon. Paulomycin G is structurally unique since-to our knowledge-it is the first member of the paulomycin family of antibiotics lacking the paulomycose moiety. It is also the smallest bioactive paulomycin reported. Its structure was determined using HRMS and 1D and 2D NMR spectroscopy. This novel natural product displays strong cytotoxic activities against different human tumour cell lines, such as pancreatic adenocarcinoma (MiaPaca_2), breast adenocarcinoma (MCF-7), and hepatocellular carcinoma (HepG2). The compound did not show any significant bioactivity when tested against a panel of bacterial and fungal pathogens.

Symbiosis-inspired approaches to antibiotic discovery

Covering: 2010 up to 2017Life on Earth is characterized by a remarkable abundance of symbiotic and highly refined relationships among life forms. Defined as any kind of close, long-term association between two organisms, symbioses can be mutualistic, commensalistic or parasitic. Historically speaking, selective pressures have shaped symbioses in which one organism (typically a bacterium or fungus) generates bioactive small molecules that impact the host (and possibly other symbionts); the symbiosis is driven fundamentally by the genetic machineries available to the small molecule producer. The human microbiome is now integral to the most recent chapter in animal-microbe symbiosis studies and plant-microbe symbioses have significantly advanced our understanding of natural products biosynthesis; this also is the case for studies of fungal-microbe symbioses. However, much less is known about microbe-microbe systems involving interspecies interactions. Microbe-derived small molecules (i.e. antibiotics and quorum sensing molecules, etc.) have been shown to regulate transcription in microbes within the same environmental niche, suggesting interspecies interactions whereas, intraspecies interactions, such as those that exploit autoinducing small molecules, also modulate gene expression based on environmental cues. We, and others, contend that symbioses provide almost unlimited opportunities for the discovery of new bioactive compounds whose activities and applications have been evolutionarily optimized. Particularly intriguing is the possibility that environmental effectors can guide laboratory expression of secondary metabolites from "orphan", or silent, biosynthetic gene clusters (BGCs). Notably, many of the studies summarized here result from advances in "omics" technologies and highlight how symbioses have given rise to new anti-bacterial and antifungal natural products now being discovered.

Lobophorin K, a New Natural Product with Cytotoxic Activity Produced by Streptomyces sp. M-207 Associated with the Deep-Sea Coral Lophelia pertusa

The present article describes the isolation of a new natural product of the lobophorin family, designated as lobophorin K (1), from cultures of the marine actinobacteria Streptomyces sp. M-207, previously isolated from the cold-water coral Lophelia pertusa collected at 1800 m depth during an expedition to the submarine Avilés Canyon. Its structure was determined using a combination of spectroscopic techniques, mainly ESI-TOF MS and 1D and 2D NMR. This new natural product displayed cytotoxic activity against two human tumor cell lines, such as pancreatic carcinoma (MiaPaca-2) and breast adenocarcinoma (MCF-7). Lobophorin K also displayed moderate and selective antibiotic activity against pathogenic Gram-positive bacteria such as Staphylococcus aureus.

Branimycins B and C, Antibiotics Produced by the Abyssal Actinobacterium Pseudonocardia carboxydivorans M-227

Two new antibiotics, branimycins B (2) and C (3), were produced by fermentation of the abyssal actinobacterium Pseudonocardia carboxydivorans M-227, isolated from deep seawater of the Avilés submarine Canyon. Their structures were elucidated by HRMS and NMR analyses. These compounds exhibit antibacterial activities against a panel of Gram-positive bacteria, including Corynebacterium urealyticum, Clostridium perfringens, and Micrococcus luteus, and against the Gram-negative bacterium Neisseria meningitidis. Additionally, branimycin B displayed moderate antibacterial activity against other Gram-negative bacteria such as Bacteroides fragilis, Haemophilus influenzae, and Escherichia coli, and branimycin C against the Gram-positive Enterococcus faecalis and methicillin-sensitive and methicillin-resistant Staphylococcus aureus.

Pharmacological Potential of Phylogenetically Diverse Actinobacteria Isolated from Deep-Sea Coral Ecosystems of the Submarine Avilés Canyon in the Cantabrian Sea

Marine Actinobacteria are emerging as an unexplored source for natural product discovery. Eighty-seven deep-sea coral reef invertebrates were collected during an oceanographic expedition at the submarine Avilés Canyon (Asturias, Spain) in a range of 1500 to 4700 m depth. From these, 18 cultivable bioactive Actinobacteria were isolated, mainly from corals, phylum Cnidaria, and some specimens of phyla Echinodermata, Porifera, Annelida, Arthropoda, Mollusca and Sipuncula. As determined by 16S rRNA sequencing and phylogenetic analyses, all isolates belong to the phylum Actinobacteria, mainly to the Streptomyces genus and also to Micromonospora, Pseudonocardia and Myceligenerans. Production of bioactive compounds of pharmacological interest was investigated by high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) techniques and subsequent database comparison. Results reveal that deep-sea isolated Actinobacteria display a wide repertoire of secondary metabolite production with a high chemical diversity. Most identified products (both diffusible and volatiles) are known by their contrasted antibiotic or antitumor activities. Bioassays with ethyl acetate extracts from isolates displayed strong antibiotic activities against a panel of important resistant clinical pathogens, including Gram-positive and Gram-negative bacteria, as well as fungi, all of them isolated at two main hospitals (HUCA and Cabueñes) from the same geographical region. The identity of the active extracts components of these producing Actinobacteria is currently being investigated, given its potential for the discovery of pharmaceuticals and other products of biotechnological interest.

Production of Potent Antimicrobial Compounds from Streptomyces cyaneofuscatus Associated with Fresh Water Sediment

The genus Streptomyces under phylum actinobacteria has been recognized as a prolific source for the production of bioactive secondary metabolites. An actinobacterial strain designated as DST103 isolated from a wetland fresh water sediment of Tamdil Lake, Mizoram, Northeast, India was identified as Streptomyces cyaneofuscatus (KY287599) using 16SrRNA gene sequencing which shares 99.87% sequence similarity with Streptomyces cyaneofuscatus NRRL B-2570^T. The strain showed broad spectrum antimicrobial activities against Gram negative bacteria (Escherichia coli MTCC 739 and Pseudomonas aeruginosa MTCC 2453), Gram positive bacteria (Micrococcus luteus NCIM 2170 and Staphylococcus aureus MTCC 96) and yeast pathogen Candida albicans MTCC 3017). The methanolic extract of the strain DST103 exhibited highest antimicrobial activity against E. coli (IC₅₀ = 2.10 μg/mL) and minimum activity against S. aureus (IC₅₀ = 43.63 μg/mL). Five antibiotics [trimethoprim (18 μg/g), fluconazole (6 μg/g), ketoconazole (18 μg/g), nalidixic acid (135 μg/g), and rifampicin (56 μg/g)] were detected and quantified using ultra-performance liquid chromatography (UPLC-ESI-MS/MS). Further, biosynthetic potential genes [polyketide synthases type II, non-ribosomal peptide synthetases, and aminodeoxyisochorismate synthase (phzE)] were also detected in strain DST103 which may possibly be responsible for the production of antimicrobial compounds. Additionally, gas chromatography-mass spectrometry analysis showed the presence of four volatile compounds which might be responsible for their diverse biological activity. The present study revealed the presence of bioactive compounds in strain DST103, which may be a promising resource for the discovery of novel bioactive metabolites against wide range of pathogens.

Molecules to Ecosystems: Actinomycete Natural Products In situ

Actinomycetes, filamentous actinobacteria found in numerous ecosystems around the globe, produce a wide range of clinically useful natural products (NP). In natural environments, actinomycetes live in dynamic communities where environmental cues and ecological interactions likely influence NP biosynthesis. Our current understating of these cues, and the ecological roles of NP, is in its infancy. We postulate that understanding the ecological context in which actinomycete metabolites are made is fundamental to advancing the discovery of novel NP. In this review we explore the ecological relevance of actinomycetes and their secondary metabolites from varying ecosystems, and suggest that investigating the ecology of actinomycete interactions warrants particular attention with respect to metabolite discovery. Furthermore, we focus on the chemical ecology and in situ analysis of actinomycete NP and consider the implications for NP biosynthesis at ecosystem scales.

Lichens as natural sources of biotechnologically relevant bacteria

The search for microorganisms from novel sources and in particular microbial symbioses represents a promising approach in biotechnology. In this context, lichens have increasingly become a subject of research in microbial biotechnology, particularly after the recognition that a diverse community of bacteria other than cyanobacteria is an additional partner to the traditionally recognized algae-fungus mutualism. Here, we review recent studies using culture-dependent as well as culture-independent approaches showing that lichens can harbor diverse bacterial families known for the production of compounds of biotechnological interest and that several microorganisms isolated from lichens, in particular Actinobacteria and Cyanobacteria, can produce a number of bioactive compounds, many of them with biotechnological potential.

Littoral lichens as a novel source of potentially bioactive Actinobacteria

Cultivable Actinobacteria are the largest source of microbially derived bioactive molecules. The high demand for novel antibiotics highlights the need for exploring novel sources of these bacteria. Microbial symbioses with sessile macro-organisms, known to contain bioactive compounds likely of bacterial origin, represent an interesting and underexplored source of Actinobacteria. We studied the diversity and potential for bioactive-metabolite production of Actinobacteria associated with two marine lichens (Lichina confinis and L. pygmaea; from intertidal and subtidal zones) and one littoral lichen (Roccella fuciformis; from supratidal zone) from the Brittany coast (France), as well as the terrestrial lichen Collema auriforme (from a riparian zone, Austria). A total of 247 bacterial strains were isolated using two selective media. Isolates were identified and clustered into 101 OTUs (98% identity) including 51 actinobacterial OTUs. The actinobacterial families observed were: Brevibacteriaceae, Cellulomonadaceae, Gordoniaceae, Micrococcaceae, Mycobacteriaceae, Nocardioidaceae, Promicromonosporaceae, Pseudonocardiaceae, Sanguibacteraceae and Streptomycetaceae. Interestingly, the diversity was most influenced by the selective media rather than lichen species or the level of lichen thallus association. The potential for bioactive-metabolite biosynthesis of the isolates was confirmed by screening genes coding for polyketide synthases types I and II. These results show that littoral lichens are a source of diverse potentially bioactive Actinobacteria.