Salinispora arenicola gen. nov., sp. nov. and Salinispora tropica sp. nov., obligate marine actinomycetes belonging to the family Micromonosporaceae

A guide to successful bioprospecting: informed by actinobacterial systematics

New structurally diverse natural products are discovered when novel screening procedures are introduced or when high quality biological materials from new sources are examined in existing screens, hence it is important to foster these two aspects of novelty in drug discovery programmes. Amongst prokaryotes, actinomycetes, notably streptomycetes, remain a rich source of new natural products though it has become increasingly difficult to find such metabolites from common actinomycetes as screening 'old friends' leads to the costly rediscovery of known compounds. The bioprospecting strategy which is the subject of this review is based upon the premise that new secondary metabolites can be found by screening relatively small numbers of dereplicated, novel actinomycetes isolated from marine sediments. The success of the strategy is exemplified by the discovery of a range of novel bioactive compounds, notably atrop-abyssomicin C and proximicins A, B and C from Verrucosispora strains isolated from sediment samples taken from the Sea of Japan and the Raune Fjord, respectively, and the dermacozines derived from Dermacoccus strains isolated from the Challenger Deep of the Mariana Trench in the Pacific Ocean. The importance of current advances in prokaryotic systematics in work of this nature is stressed and a plea made that resources be sought to train, support and employ the next generation of actinobacterial systematists.

Jishengella endophytica gen. nov., sp. nov., a new member of the family Micromonosporaceae

A novel endophytic actinomycete, designated strain 202201(T), was isolated from an Acanthus illicifolius root collected from the mangrove reserve zone in Hainan Province, China. Phylogenetic analysis based on 16S rRNA gene sequences suggested that strain 202201(T) fell within the family Micromonosporaceae. The strain formed an extensively branched substrate mycelium, which carried uneven warty-surfaced spores. Cell walls of strain 202201(T) contained meso-diaminopimelic acid and xylose, mannose, arabinose, ribose and glucose were detected as whole-cell sugars. The acyl type of the cell-wall polysaccharides was glycolyl. The major menaquinones were MK-9(H(4)), MK-9(H(6)), MK-9(H(8)) and MK-10(H(4)). The polar lipids were phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylinositol mannoside and phosphatidylserine. The major cellular fatty acids were 10-methyl-C(17 : 0), iso-C(15 : 0), iso-C(16 : 0) and C(17 : 1)ω8c. The DNA G+C content was 72.3 mol%. On the basis of the morphological and chemotaxonomic characteristics, phylogenetic analysis and characteristic patterns of 16S rRNA gene signature nucleotides, strain 202201(T) ( = CGMCC 4.5597(T ) = DSM 45430(T)) represents a novel species of a new genus within the family Micromonosporaceae, for which the name Jishengella endophytica gen. nov., sp. nov. is proposed.

Growth, cell division and sporulation in mycobacteria

Bacteria have the ability to adapt to different growth conditions and to survive in various environments. They have also the capacity to enter into dormant states and some bacteria form spores when exposed to stresses such as starvation and oxygen deprivation. Sporulation has been demonstrated in a number of different bacteria but Mycobacterium spp. have been considered to be non-sporulating bacteria. We recently provided evidence that Mycobacterium marinum and likely also Mycobacterium bovis bacillus Calmette–Guérin can form spores. Mycobacterial spores were detected in old cultures and our findings suggest that sporulation might be an adaptation of lifestyle for mycobacteria under stress. Here we will discuss our current understanding of growth, cell division, and sporulation in mycobacteria.

Culture-dependent and culture-independent diversity of Actinobacteria associated with the marine sponge Hymeniacidon perleve from the South China Sea

In this report, the diversity of Actinobacteria associated with the marine sponge Hymeniacidon perleve collected from a remote island of the South China Sea was investigated employing classical cultivation and characterization, 16S rDNA library construction, 16S rDNA-restriction fragment length polymorphism (rDNA-RFLP) and phylogenetic analysis. A total of 184 strains were isolated using seven different media and 24 isolates were selected according to their morphological characteristics for phylogenetic analysis on the basis of their 16S rRNA gene sequences. Results showed that the 24 isolates were assigned to six genera including Salinispora, Gordonia, Mycobacterium, Nocardia, Rhodococcus and Streptomyces. This is the first report that Salinispora is present in a marine sponge from the South China Sea. Subsequently, 26 rDNA clones were selected from 191 clones in an Actinobacteria-specific 16S rDNA library of the H. perleve sample, using the RFLP technique for sequencing and phylogenetic analysis. In total, 26 phylotypes were clustered in eight known genera of Actinobacteria including Mycobacterium, Amycolatopsis, Arthrobacter, Brevibacterium, Microlunatus, Nocardioides, Pseudonocardia and Streptomyces. This study contributes to our understanding of actinobacterial diversity in the marine sponge H. perleve from the South China Sea.

Generating a generation of proteasome inhibitors: from microbial fermentation to total synthesis of salinosporamide a (marizomib) and other salinosporamides

The salinosporamides are potent proteasome inhibitors among which the parent marine-derived natural product salinosporamide A (marizomib; NPI-0052; 1) is currently in clinical trials for the treatment of various cancers. Methods to generate this class of compounds include fermentation and natural products chemistry, precursor-directed biosynthesis, mutasynthesis, semi-synthesis, and total synthesis. The end products range from biochemical tools for probing mechanism of action to clinical trials materials; in turn, the considerable efforts to produce the target molecules have expanded the technologies used to generate them. Here, the full complement of methods is reviewed, reflecting remarkable contributions from scientists of various disciplines over a period of 7 years since the first publication of the structure of 1.

Development of novel drugs from marine surface associated microorganisms

While the oceans cover more than 70% of the Earth's surface, marine derived microbial natural products have been largely unexplored. The marine environment is a habitat for many unique microorganisms, which produce biologically active compounds ("bioactives") to adapt to particular environmental conditions. For example, marine surface associated microorganisms have proven to be a rich source for novel bioactives because of the necessity to evolve allelochemicals capable of protecting the producer from the fierce competition that exists between microorganisms on the surfaces of marine eukaryotes. Chemically driven interactions are also important for the establishment of cross-relationships between microbes and their eukaryotic hosts, in which organisms producing antimicrobial compounds ("antimicrobials"), may protect the host surface against over colonisation in return for a nutrient rich environment. As is the case for bioactive discovery in general, progress in the detection and characterization of marine microbial bioactives has been limited by a number of obstacles, such as unsuitable culture conditions, laborious purification processes, and a lack of de-replication. However many of these limitations are now being overcome due to improved microbial cultivation techniques, microbial (meta-) genomic analysis and novel sensitive analytical tools for structural elucidation. Here we discuss how these technical advances, together with a better understanding of microbial and chemical ecology, will inevitably translate into an increase in the discovery and development of novel drugs from marine microbial sources in the future.

Isolation, phylogenetic analysis and anti-infective activity screening of marine sponge-associated actinomycetes

Terrestrial actinomycetes are noteworthy producers of a multitude of antibiotics, however the marine representatives are much less studied in this regard. In this study, 90 actinomycetes were isolated from 11 different species of marine sponges that had been collected from offshore Ras Mohamed (Egypt) and from Rovinj (Croatia). Phylogenetic characterization of the isolates based on 16S rRNA gene sequencing supported their assignment to 18 different actinomycete genera representing seven different suborders. Fourteen putatively novel species were identified based on sequence similarity values below 98.2% to other strains in the NCBI database. A putative new genus related to Rubrobacter was isolated on M1 agar that had been amended with sponge extract, thus highlighting the need for innovative cultivation protocols. Testing for anti-infective activities was performed against clinically relevant, Gram-positive (Enterococcus faecalis, Staphylococcus aureus) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria, fungi (Candida albicans) and human parasites (Leishmania major, Trypanosoma brucei). Bioactivities against these pathogens were documented for 10 actinomycete isolates. These results show a high diversity of actinomycetes associated with marine sponges as well as highlight their potential to produce anti-infective agents.

A preliminary investigation on the growth requirement for monovalent cations, divalent cations and medium ionic strength of marine actinomycete Salinispora

In this paper, we report that three species of Salinispora, S. arenicola, S. tropica, and S. pacifica, require magnesium and calcium, for growth, with S. pacifica having the most stringent growth requirement for these ions. Interaction between these ions in supporting the growth of Salinispora was observed. We also demonstrated that the absolute requirement of sodium to support the growth of Salinispora has not been established as all three species of Salinispora can use either potassium or lithium to replace sodium to support maximum growth. While lithium can replace sodium to support maximum growth of Salinispora, it is more toxic to S. arenicola than S. tropica and S. pacifica, inhibiting the growth of S. arenicola at 189 mM but without effect on the growth of S. tropica and S. pacifica. Using both sodium chloride-based and lithium chloride-based media, we showed that Salinispora has a growth requirement for divalent ions, magnesium and calcium as well as growth requirement for ionic strength (8.29 to 15.2 mS/cm). S. arenicola has a lower growth requirement for ionic strength than S. tropica and S. pacifica.

Saliniquinones A-F, New Members of the Highly Cytotoxic Anthraquinone-γ-Pyrones from the Marine Actinomycete Salinispora arenicola

Six new anthraquinone-γ-pyrones, saliniquinones A-F (1-6), which are related to metabolites of the pluramycin/altromycin class, were isolated from a fermentation broth of the marine actinomycete Salinispora arenicola (strain CNS-325). Their structures were determined by analysis of one- and two-dimensional NMR spectroscopic and high-resolution mass spectrometric data. The relative and absolute configurations of compounds 1-6 were determined by analysis of NOESY NMR spectroscopic data and by comparison of circular dichroism and optical rotation data with model compounds found in the literature. Saliniquinone A (1) exhibited potent inhibition of the human colon adenocarcinoma cell line (HCT-116) with an IC₅₀ of 9.9 × 10^-9 M. In the context of the biosynthetic diversity of S. arenicola, compounds 1-6 represent secondary metabolites that appear to be strain specific and thus occur outside of the core group of compounds commonly observed from this species.

Linking species concepts to natural product discovery in the post-genomic era

A widely accepted species concept for bacteria has yet to be established. As a result, species designations are inconsistently applied and tied to what can be considered arbitrary metrics. Increasing access to DNA sequence data and clear evidence that bacterial genomes are dynamic entities that include large numbers of horizontally acquired genes have added a new level of insight to the ongoing species concept debate. Despite uncertainties over how to apply species concepts to bacteria, there is clear evidence that sequence-based approaches can be used to resolve cohesive groups that maintain the properties of species. This cohesion is clearly evidenced in the genus Salinispora, where three species have been discerned despite very close relationships based on 16S rRNA sequence analysis. The major phenotypic differences among the three species are associated with secondary metabolite production, which occurs in species-specific patterns. These patterns are maintained on a global basis and provide evidence that secondary metabolites have important ecological functions. These patterns also suggest that an effective strategy for natural product discovery is to target the cultivation of new Salinispora taxa. Alternatively, bioinformatic analyses of biosynthetic genes provide opportunities to predict secondary metabolite novelty and reduce the redundant isolation of well-known metabolites. Although much remains to be learned about the evolutionary relationships among bacteria and how fundamental units of diversity can be resolved, genus and species descriptions remain the most effective method of scientific communication.

Genomic basis for natural product biosynthetic diversity in the actinomycetes

The phylum Actinobacteria hosts diverse high G + C, Gram-positive bacteria that have evolved a complex chemical language of natural product chemistry to help navigate their fascinatingly varied lifestyles. To date, 71 Actinobacteria genomes have been completed and annotated, with the vast majority representing the Actinomycetales, which are the source of numerous antibiotics and other drugs from genera such as Streptomyces, Saccharopolyspora and Salinispora . These genomic analyses have illuminated the secondary metabolic proficiency of these microbes – underappreciated for years based on conventional isolation programs – and have helped set the foundation for a new natural product discovery paradigm based on genome mining. Trends in the secondary metabolomes of natural product-rich actinomycetes are highlighted in this review article, which contains 199 references.

Isolation, diversity, and antimicrobial activity of rare actinobacteria from medicinal plants of tropical rain forests in Xishuangbanna, China

Endophytic actinobacteria are relatively unexplored as potential sources of novel species and novel natural products for medical and commercial exploitation. Xishuangbanna is recognized throughout the world for its diverse flora, especially the rain forest plants, many of which have indigenous pharmaceutical histories. However, little is known about the endophytic actinobacteria of this tropical area. In this work, we studied the diversity of actinobacteria isolated from medicinal plants collected from tropical rain forests in Xishuangbanna. By the use of different selective isolation media and methods, a total of 2,174 actinobacteria were isolated. Forty-six isolates were selected on the basis of their morphologies on different media and were further characterized by 16S rRNA gene sequencing. The results showed an unexpected level of diversity, with 32 different genera. To our knowledge, this is the first report describing the isolation of Saccharopolyspora, Dietzia, Blastococcus, Dactylosporangium, Promicromonospora, Oerskovia, Actinocorallia, and Jiangella species from endophytic environments. At least 19 isolates are considered novel taxa by our current research. In addition, all 46 isolates were tested for antimicrobial activity and were screened for the presence of genes encoding polyketide synthetases and nonribosomal peptide synthetases. The results confirm that the medicinal plants of Xishuangbanna represent an extremely rich reservoir for the isolation of a significant diversity of actinobacteria, including novel species, that are potential sources for the discovery of biologically active compounds.

Actinaurispora siamensis gen. nov., sp. nov., a new member of the family Micromonosporaceae

Two actinomycete strains, CM2-8(T) and CM2-12, were isolated from temperate peat swamp forest soil in Chiang Mai Province, Thailand. Their taxonomic positions were determined using a polyphasic approach. Chemotaxonomic characteristics of these strains coincided with those of the family Micromonosporaceae, i.e. cell wall chemotype II, N-glycolyl type of muramic acid, and type II phospholipids. Phylogenetic analysis based on 16S rRNA gene sequence data also indicated that these strains fell within the family Micromonosporaceae and formed a distinct taxon in the Micromonosporaceae phylogenetic tree. On the basis of phylogenetic analysis, characteristic patterns of 16S rRNA gene signature nucleotides and chemotaxonomic data, it is proposed that the novel isolates belong to a new genus, Actinaurispora gen. nov. The type species of the genus is proposed as Actinaurispora siamensis sp. nov., with strain CM2-8(T) (=JCM 15677(T)=BCC 34762(T)) as the type strain.

Plantactinospora mayteni gen. nov., sp. nov., a member of the family Micromonosporaceae

A novel Gram-positive, aerobic, spore-forming, endophytic actinomycete, designated strain YIM 61359(T), was isolated from the roots of Maytenus austroyunnanensis plants collected from tropical rainforest in Xishuangbanna, Yunnan Province, south-west China. The strain formed single or cluster spores with smooth surfaces from substrate mycelia. The strain contained meso-diaminopimelic acid in the cell wall and arabinose, xylose, galactose and glucose in whole-cell hydrolysates. The acyl type of the cell-wall polysaccharides was glycolyl. MK-10(H(6)), MK-10(H(8)) and MK-10(H(4)) were the predominant menaquinones. The polar lipids were phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylinositol and several unknown phospholipids. The major fatty acids were iso-C(15 : 0), anteiso-C(15 : 0), C(17 : 0), anteiso-C(17 : 0) and iso-C(16 : 0). The DNA G+C content of strain YIM 61359(T) was 69.7 mol%. These chemotaxonomic data indicated that the strain belongs to the family Micromonosporaceae. Phylogenetic analysis based on 16S rRNA gene sequences also suggested that strain YIM 61359(T) fell within the family Micromonosporaceae. On the basis of morphological and chemotaxonomic data, phylogenetic analysis and characteristic patterns of 16S rRNA gene signature nucleotides, strain YIM 61359(T) is considered to represent a novel species of a new genus within the family Micromonosporaceae, for which the name Plantactinospora mayteni gen. nov., sp. nov. is proposed. The type strain of Plantactinospora mayteni is YIM 61359(T) (=CCTCC AA 208022(T)=DSM 45238(T)).

Proteasome regulators: activators and inhibitors

This mini review covers the drug discovery aspect of both proteasome activators and inhibitors. The proteasome is involved in many essential cellular functions, such as regulation of cell cycle, cell differentiation, signal transduction pathways, antigen processing for appropriate immune responses, stress signaling, inflammatory responses, and apoptosis. Due to the importance of the proteasome in cellular functions, inhibition or activation of the proteasome could become a useful therapeutic strategy for a variety of diseases. Many proteasome inhibitors have been identified and can be classified into two groups according to their source: chemically synthesized small molecules and compounds derived from natural products. A successful example of development of a proteasome inhibitor as a clinically useful drug is the peptide boronate, PS341 (Bortezomib), was approved for the treatment of multiple myeloma. In contrast to proteasome inhibitors, small molecules that can activate or enhance proteasome activity are rare and are not well studied. The fact that over-expression of the cellular proteasome activator PA28 exhibited beneficial effects on the Huntington's disease neuronal model cells raised the prospect that small molecule proteasome activators could become useful therapeutics. The beneficial effect of oleuropein, a small molecule proteasome activator, on senescence of human fibroblasts also suggested that proteasome activators might have the potential to be developed into anti-aging agents.

Study of the diversity of culturable actinomycetes in the North Pacific and Caribbean coasts of Costa Rica

In this study, 137 actinomycetes were isolated from subtidal marine sediments in the North Pacific and Caribbean coasts of Costa Rica. Bioinformatics analysis of the 16S rRNA gene sequences assigned the isolates to 15 families and 21 genera. Streptomyces was the dominant genus while the remaining 20 genera were poorly represented. Nearly 70% of the phylotypes presented a coastal-restricted distribution whereas the other 30% were common inhabitants of both shores. The coastal tropical waters of Costa Rica showed a high diversity of actinomycetes, both in terms of the number of species and phylogenetic composition, although significant differences were observed between and within shores. The observed pattern of species distribution might be the result of several factors including the characteristics of the ecosystems, presence of endemic species and the influence of terrestrial runoff.

Genomic islands link secondary metabolism to functional adaptation in marine Actinobacteria

Genomic islands have been shown to harbor functional traits that differentiate ecologically distinct populations of environmental bacteria. A comparative analysis of the complete genome sequences of the marine Actinobacteria Salinispora tropica and S. arenicola reveals that 75% of the species-specific genes are located in 21 genomic islands. These islands are enriched in genes associated with secondary metabolite biosynthesis providing evidence that secondary metabolism is linked to functional adaptation. Secondary metabolism accounts for 8.8% and 10.9% of the genes in the S. tropica and S. arenicola genomes, respectively, and represents the major functional category of annotated genes that differentiates the two species. Genomic islands harbor all 25 of the species-specific biosynthetic pathways, the majority of which occur in S. arenicola and may contribute to the cosmopolitan distribution of this species. Genome evolution is dominated by gene duplication and acquisition, which in the case of secondary metabolism provide immediate opportunities for the production of new bioactive products. Evidence that secondary metabolic pathways are exchanged horizontally, coupled with prior evidence for fixation among globally distributed populations, supports a functional role and suggests that the acquisition of natural product biosynthetic gene clusters represents a previously unrecognized force driving bacterial diversification. Species-specific differences observed in CRISPR (clustered regularly interspaced short palindromic repeat) sequences suggest that S. arenicola may possess a higher level of phage immunity, while a highly duplicated family of polymorphic membrane proteins provides evidence of a new mechanism of marine adaptation in Gram-positive bacteria.

An update of the structure and 16S rRNA gene sequence-based definition of higher ranks of the class Actinobacteria, with the proposal of two new suborders and four new families and emended descriptions of the existing higher taxa

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.

Exploration and engineering of biosynthetic pathways in the marine actinomycete Salinispora tropica

In recent years, members of the marine actinomycete genus Salinispora have proven to be a precious source of structurally diverse secondary metabolites, including the potent anticancer agent salinosporamide A and the enediyne-derived sporolides. The tremendous potential of these marine-dwelling microbes for natural products biosynthesis, however, was not fully realized until sequencing of the Salinispora tropica genome revealed the presence of numerous orphan biosynthetic loci besides a plethora of rare metabolic pathways. This contribution summarizes the biochemical exploration of this prolific organism, highlighting studies in which genome-based information was exploited for the discovery of new enzymatic processes and the engineering of unnatural natural products. Inactivation of key genes within the salinosporamide pathway has expanded its inherent metabolic plasticity and enabled access to various salinosporamide derivatives by mutasynthesis. New insights into the biosynthesis of the sporolides allowed us to increase production titers of these structurally complex molecules, thereby providing the means to search for the DNA cleaving presporolide enediyne.

Advances in the understanding and use of the genomic base of microbial secondary metabolite biosynthesis for the discovery of new natural products

Over the past decade major changes have occurred in the access to genome sequences that encode the enzymes responsible for the biosynthesis of secondary metabolites, knowledge of how those sequences translate into the final structure of the metabolite, and the ability to alter the sequence to obtain predicted products via both homologous and heterologous expression. Novel genera have been discovered leading to new chemotypes, but more surprisingly several instances have been uncovered where the apparently general rules of modular translation have not applied. Several new biosynthetic pathways have been unearthed, and our general knowledge grows rapidly. This review aims to highlight some of the more striking discoveries and advances of the decade.

Discovery and development of the anticancer agent salinosporamide A (NPI-0052)

The discovery of the anticancer agent salinosporamide A (NPI-0052) resulted from the exploration of new marine environments and a commitment to the potential of the ocean to yield new natural products for drug discovery and development. Driving the success of this process was the linkage of academic research together with the ability and commitment of industry to undertake drug development and provide the resources and expertise to advance the entry of salinosporamide A (NPI-0052) into human clinical trials. This paper offers a chronicle of the important events that facilitated the rapid clinical development of this exciting molecule.

Actinomycete bacteria isolated from the sediments at coastal and offshore area of Nagasaki Prefecture, Japan: diversity and biological activity

About 800 strains of actinomycetes were isolated from marine environments around Nagasaki Prefecture, Japan. The isolates were compared with taxa and biological activities of their secondary metabolites. It is suggested that a variety of actinomycetes are isolated from different marine environments.

Drug development from marine natural products

Drug discovery from marine natural products has enjoyed a renaissance in the past few years. Ziconotide (Prialt; Elan Pharmaceuticals), a peptide originally discovered in a tropical cone snail, was the first marine-derived compound to be approved in the United States in December 2004 for the treatment of pain. Then, in October 2007, trabectedin (Yondelis; PharmaMar) became the first marine anticancer drug to be approved in the European Union. Here, we review the history of drug discovery from marine natural products, and by describing selected examples, we examine the factors that contribute to new discoveries and the difficulties associated with translating marine-derived compounds into clinical trials. Providing an outlook into the future, we also examine the advances that may further expand the promise of drugs from the sea.

Marineosins A and B, cytotoxic spiroaminals from a marine-derived actinomycete

Two novel spiroaminals, marineosins A and B (1, 2), containing two pyrrole functionalities, were isolated from cultures of a marine sediment-derived actinomycete related to the genus Streptomyces. The marineosins, which appear to be derived from unknown modifications of prodigiosin-like pigment pathways, showed significant inhibition of human colon carcinoma (HCT-116) in an in vitro assay (IC50 = 0.5 microM for marineosin A) and selective activities in diverse cancer cell types.

Panning for chemical gold: marine bacteria as a source of new therapeutics

Marine bacteria are emerging as an exciting resource for the discovery of new classes of therapeutics. The promising anticancer clinical candidates salinosporamide A and bryostatin only hint at the incredible wealth of drug leads hidden just beneath the ocean surface. For example, if properly developed, marine bacteria could provide the drugs needed to sustain us for the next 100 years in our battle against drug-resistant infectious diseases. This review will focus on several recently discovered compounds, primarily from cyanobacteria and actinobacteria, that illustrate the tremendous potential of marine bacteria as a source of new therapeutics within the areas of oncology and infectious diseases.

An N-acyl homolog of mycothiol is produced in marine actinomycetes

Marine actinomycetes have generated much recent interest as a potentially valuable source of novel antibiotics. Like terrestrial actinomycetes the marine actinomycetes are shown here to produce mycothiol as their protective thiol. However, a novel thiol, U25, was produced by MAR2 strain CNQ703 upon progression into stationary phase when secondary metabolite production occurred and became the dominant thiol. MSH and U25 were maintained in a reduced state during early stationary phase, but become significantly oxidized after 10 days in culture. Isolation and structural analysis of the monobromobimane derivative identified U25 as a homolog of mycothiol in which the acetyl group attached to the nitrogen of cysteine is replaced by a propionyl residue. This N-propionyl-desacetyl-mycothiol was present in 13 of the 17 strains of marine actinomycetes examined, including five strains of Salinispora and representatives of the MAR2, MAR3, MAR4 and MAR6 groups. Mycothiol and its precursor, the pseudodisaccharide 1-O-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol, were found in all strains. High levels of mycothiol S-conjugate amidase activity, a key enzyme in mycothiol-dependent detoxification, were found in most strains. The results demonstrate that major thiol/disulfide changes accompany secondary metabolite production and suggest that mycothiol-dependent detoxification is important at this developmental stage.

Pseudosporangium ferrugineum gen. nov., sp. nov., a new member of the family Micromonosporaceae

An actinomycete strain 3-44-a(19)(T) was isolated from sandy soil collected in Bangladesh. The strain formed irregular pseudosporangia directly from aggregated spore chains above the rudimentary aerial mycelium. The pseudosporangia developed singly. Each pseudosporangium contained many small, non-motile, spherical, smooth-surfaced spores in chains. Strain 3-44-a(19)(T) contained meso- and 3-hydroxydiaminopimelic acid in the cell wall and MK-9(H(6)) as the major menaquinone and arabinose, galactose, glucose, mannose, ribose and xylose were present in the whole-cell hydrolysate. The diagnostic phospholipid was phosphatidylethanolamine and iso-C(15 : 0) (24.6 %), C(18 : 1)omega9c (15.5 %), C(16 : 0) (10.6 %), C(18 : 0) (9.4 %), iso-C(16 : 0) (8.6 %) and anteiso-C(15 : 0) (6.0 %) were detected as the major cellular fatty acids. The acyl type of the peptidoglycan was glycolyl and mycolic acids were not detected. The G+C content of the DNA was 73.6 mol%. The chemotaxonomic data indicated that the strain belonged to the family Micromonosporaceae. Phylogenetic analysis based on 16S rRNA gene sequence data also suggested that strain 3-44-a(19)(T) fell within the family Micromonosporaceae. On the basis of phylogenetic analysis and characteristic patterns of 16S rRNA gene signature nucleotides as well as morphological and chemotaxonomic data, this strain should be classified as a member of a new genus and species, Pseudosporangium ferrugineum gen. nov., sp. nov., in the family Micromonosporaceae. The type strain of Pseudosporangium ferrugineum is 3-44-a(19)(T) (=JCM 14710(T) =MTCC9007(T)).

Growth of Salinispora tropica strains CNB440, CNB476, and NPS21184 in nonsaline, low-sodium media

We recently described the development of a potassium-chloride-based salt formulation containing low sodium concentration (5.0 mM) to support the growth of Salinispora tropica strain NPS21184 and its production of salinosporamide A (NPI-0052). In order to determine whether the above low-sodium salt formulation can also support the growth of other S. tropica strains, we examined the growth of the type strain CNB440 and the parent strain CNB476, from which strain NPS21184 was derived as a single colony isolate. We demonstrated that good growth rate and yield of S. tropica strains CNB440 and CNB476, similar to S. tropica strain NPS21184 reported earlier, were detected in both agar and liquid media containing the potassium-chloride-based salt formulation with sodium concentration of 5.0 mM. Furthermore, we also detected good growth rate and yield of all three S. tropica strains on potassium-sulfate-based salt formulation agar medium containing both low-sodium (5.7 mM) and low-chloride (14 mM) content. This finding confirms the observation that the species of S. tropica does not have a seawater growth requirement but requirement for a specific combination of salts to provide a balance of salts and maintain a high enough ionic strength for growth.

An assessment of actinobacterial diversity in the marine environment

The 16S rRNA gene sequence diversity within the Phylum Actinobacteria was assessed from four sources: PCR-generated V6 sequence tags derived from seawater samples, metagenomic data from the Global Ocean Sampling (GOS) expedition, marine-derived sequences maintained in the Ribosomal Database Project (RDP), and select cultured strains for which sequence data is not yet available in the RDP. This meta-analysis revealed remarkable levels of phylogenetic diversity and confirms the existence of major, deeply rooted, and as of yet uncharacterized lineages within the phylum. A dramatic incongruence among cultured strains and those detected using culture-independent techniques was also revealed. Redundancy among the actinobacteria detected using culture-independent techniques suggests that greater sequence coverage or improved DNA extraction efficiencies may be required to detect the rare phylotypes that can be readily cultured from marine samples. Conversely, new strategies need to be developed for the cultivation of frequently observed but yet to be cultured marine actinobacteria.

Characterisation of micromonosporae from aquatic environments using molecular taxonomic methods

Large numbers of strains assigned to the genus Micromonospora on the basis of typical colonial and pigmentation features were isolated from diverse aquatic sediments using a standard selective isolation procedure. Two hundred and six isolates and eight representatives of the genus Micromonospora were assigned to 24 multimembered groups based on a numerical analysis of banding patterns generated using BOX and ERIC primers. Representatives of multimembered groups encompassing isolated micromonosporae were the subject of 16S rRNA gene sequencing analyses. Good congruence was found between the molecular fingerprinting and 16S rRNA sequence data indicating that the groups based upon the former are taxonomically meaningful. Nearly all of the isolates that were chosen for the 16S rRNA gene sequencing analyses showed that the ecosystems studied are a rich source of novel micromonosporae. These findings have implications for high throughput screening for novel micromonosporae as BOX and ERIC fingerprinting, which is rapid and reproducible, can be applied as a robust dereplication procedure to indicate which environmental isolates have been cultured previously.

Rare actinomycete bacteria from the shallow water sediments of the Trondheim fjord, Norway: isolation, diversity and biological activity

Actinomycete bacteria produce a wide variety of secondary metabolites with diverse biological activities, some of which have been developed for human medicine. Rare actinomycetes are promising sources in search for new drugs, and their potential for producing biologically active molecules is poorly studied. In this work, we have investigated the diversity of actinomycetes in the shallow water sediments of the Trondheim fjord (Norway). Due to the use of different selective isolation methods, an unexpected variety of actinomycete genera was isolated. Although the predominant genera were clearly Streptomyces and Micromonospora, representatives of Actinocorallia, Actinomadura, Knoellia, Glycomyces, Nocardia, Nocardiopsis, Nonomuraea, Pseudonocardia, Rhodococcus and Streptosporangium genera were isolated as well. To our knowledge, this is the first report describing isolation of Knoellia and Glycomyces species from the marine environment. 35 selected actinomycete isolates were characterized by 16S rDNA sequencing, and were shown to represent strains from 11 different genera. In addition, these isolates were tested for antimicrobial activity and the presence of polyketide synthase and non-ribosomal peptide synthetase genes. This study confirms the significant biodiversity of actinobacteria in the Norwegian marine habitats, and their potential for producing biologically active compounds.

Salinipyrones and pacificanones, mixed-precursor polyketides from the marine actinomycete Salinispora pacifica

Chemical examination of a phylogenetically unique strain of the obligate marine actinomycete Salinispora pacifica led to the discovery of four new polyketides, salinipyrones A and B ( 1, 2) and pacificanones A and B ( 3, 4). These compounds appear to be derived from a mixed-precursor polyketide biosynthesis involving acetate, propionate, and butyrate building blocks. Spectral analysis, employing NMR, IR, UV, and CD methods and chemical derivatization, was used to assign the structures and absolute configurations of these new metabolites. Salinipyrones A and B displayed exactly opposite CD spectra, indicating their pseudoenantiomeric relationship. This relationship was shown to be a consequence of the geometric isomerization of one double bond. The phenomenon of polyketide module skipping is proposed to explain the unusual biosynthesis of the salinipyrones and the pacificanones.

Discovery and characterization of a marine bacterial SAM-dependent chlorinase

Halogen atom incorporation into a scaffold of bioactive compounds often amplifies biological activity, as is the case for the anticancer agent salinosporamide A (1), a chlorinated natural product from the marine bacterium Salinispora tropica. Significant effort in understanding enzymatic chlorination shows that oxidative routes predominate to form reactive electrophilic or radical chlorine species. Here we report the genetic, biochemical and structural characterization of the chlorinase SalL, which halogenates S-adenosyl-L-methionine (2) with chloride to generate 5'-chloro-5'-deoxyadenosine (3) and L-methionine (4) in a rarely observed nucleophilic substitution strategy analogous to that of Streptomyces cattleya fluorinase. Further metabolic tailoring produces a halogenated polyketide synthase substrate specific for salinosporamide A biosynthesis. SalL also accepts bromide and iodide as substrates, but not fluoride. High-resolution crystal structures of SalL and active site mutants complexed with substrates and products support the S(N)2 nucleophilic substitution mechanism and further illuminate halide specificity in this newly discovered halogenase family.

Genome sequencing reveals complex secondary metabolome in the marine actinomycete Salinispora tropica

Recent fermentation studies have identified actinomycetes of the marine-dwelling genus Salinispora as prolific natural product producers. To further evaluate their biosynthetic potential, we sequenced the 5,183,331-bp S. tropica CNB-440 circular genome and analyzed all identifiable secondary natural product gene clusters. Our analysis shows that S. tropica dedicates a large percentage of its genome ( approximately 9.9%) to natural product assembly, which is greater than previous Streptomyces genome sequences as well as other natural product-producing actinomycetes. The S. tropica genome features polyketide synthase systems of every known formally classified family, nonribosomal peptide synthetases, and several hybrid clusters. Although a few clusters appear to encode molecules previously identified in Streptomyces species, the majority of the 17 biosynthetic loci are novel. Specific chemical information about putative and observed natural product molecules is presented and discussed. In addition, our bioinformatic analysis not only was critical for the structure elucidation of the polyene macrolactam salinilactam A, but its structural analysis aided the genome assembly of the highly repetitive slm loci. This study firmly establishes the genus Salinispora as a rich source of drug-like molecules and importantly reveals the powerful interplay between genomic analysis and traditional natural product isolation studies.

Luedemannella gen. nov., a new member of the family Micromonosporaceae and description of Luedemannella helvata sp. nov. and Luedemannella flava sp. nov

Three actinomycete strains were isolated from soil samples collected in Bangladesh. The cultures formed spherical sporangia on short sporangiophores directly above the surface of the substrate mycelium. The sporangia developed singly or in clusters and each sporangium contained several nonmotile spherical to oval spores with a smooth surface. The strains 3-9(24)(T), 3-21(27) and 7-40(26)(T) contained meso-diaminopimelic acid in the cell walls, predominant menaquinone MK-9(H(6)) and MK-9(H(4)) and glucose, xylose, galactose, mannose, rhamnose, ribose and arabinose in the whole-cell hydrolysates. Diagnostic phospholipid is phosphatidylethanolamine and branched anteiso-C(17 : 0) (30.0-38.0%), anteiso-C(15 : 0) (12.5-14.0%), iso-C(16 : 0) (10.0-15.0%) and iso-C(15 : 0) (10.0-12.0%) were detected as the major cellular fatty acids. The acyl type of the peptidoglycan was glycolyl and mycolic acids were not detected. The G+C content of the DNA was 71 mol%. The chemotaxonomic data indicate that these strains belong to the family Micromonosporaceae. Phylogenetic analysis based on 16S rRNA gene sequence data suggested that the strains 3-9(24)(T), 3-21(27) and 7-40(26)(T) fall within the family Micromonosporaceae. On the basis of phylogenetic analysis and characteristic patterns of signature nucleotides as well as morphological and chemotaxonomic data, Luedemannella gen. nov. is proposed for our 3 isolates. DNA-DNA hybridization experiment and phenotypic characterization indicated that the new genus was constituted of 2 species, as Luedemannella helvata sp. nov. for the strain 3-9(24)(T) (=JCM 13249(T)=MTCC 8091(T)) and Luedemannella flava for the strain 7-40(26)(T) (=JCM 13250(T)=MTCC 8095(T)) in the family Micromonosporaceae.