Antibacterial Nucleoside-Analog Inhibitor of Bacterial RNA Polymerase.

Drug-resistant bacterial pathogens pose an urgent public-health crisis. Here, we report the discovery, from microbial-extract screening, of a nucleoside-analog inhibitor that inhibits bacterial RNA polymerase (RNAP) and exhibits antibacterial activity against drug-resistant bacterial pathogens: pseudouridimycin (PUM). PUM is a natural product comprising a formamidinylated, N-hydroxylated Gly-Gln dipeptide conjugated to 6'-amino-pseudouridine. PUM potently and selectively inhibits bacterial RNAP in vitro, inhibits bacterial growth in culture, and clears infection in a mouse model of Streptococcus pyogenes peritonitis. PUM inhibits RNAP through a binding site on RNAP (the NTP addition site) and mechanism (competition with UTP for occupancy of the NTP addition site) that differ from those of the RNAP inhibitor and current antibacterial drug rifampin (Rif). PUM exhibits additive antibacterial activity when co-administered with Rif, exhibits no cross-resistance with Rif, and exhibits a spontaneous resistance rate an order-of-magnitude lower than that of Rif. PUM is a highly promising lead for antibacterial therapy.

Reticulibacter mediterranei gen. nov., sp. nov., within the new family Reticulibacteraceae fam. nov., and Ktedonospora formicarum gen. nov., sp. nov., Ktedonobacter robiniae sp. nov., Dictyobacter formicarum sp. nov. and Dictyobacter arantiisoli sp. nov., belonging to the class Ktedonobacteria.

The aerobic, Gram-positive, mesophilic Ktedonobacteria strains, Uno17T, SOSP1-1T, 1-9T, 1-30T and 150040T, formed mycelia of irregularly branched filaments, produced spores or sporangia, and numerous secondary metabolite biosynthetic gene clusters. The five strains grew at 15-40 °C (optimally at 30 °C) and pH 4.0-8.0 (optimally at pH 6.0-7.0), and had 7.21-12.67 Mb genomes with 49.7-53.7 mol% G+C content. They shared MK9(H2) as the major menaquinone and C16 : 1-2OH and iso-C17 : 0 as the major cellular fatty acids. Phylogenetic and phylogenomic analyses showed that Uno17T and SOSP1-9T were most closely related to members of the genus Dictyobacter, with 94.43-96.21 % 16S rRNA gene similarities and 72.16-81.56% genomic average nucleotide identity. The strain most closely related to SOSP1-1T and SOSP1-30T was Ktedonobacter racemifer SOSP1-21T, with 91.33 and 98.84 % 16S rRNA similarities, and 75.13 and 92.35% average nucleotide identities, respectively. Strain 150040T formed a distinct clade within the order Ktedonobacterales, showing <90.47 % 16S rRNA gene similarity to known species in this order. Based on these results, we propose: strain 150040T as Reticulibacter mediterranei gen. nov., sp. nov. (type strain 150 040T=CGMCC 1.17052T=BCRC 81202T) within the family Reticulibacteraceae fam. nov. in the order Ktedonobacterales; strain SOSP1-1T as Ktedonospora formicarum gen. nov., sp. nov. (type strain SOSP1-1T=CGMCC 1.17205T=BCRC 81203T) and strain SOSP1-30T as Ktedonobacter robiniae sp. nov. (type strain SOSP1-30T=CGMCC 1.17733T=BCRC 81205T) within the family Ktedonobacteraceae; strain Uno17T as Dictyobacter arantiisoli sp. nov. (type strain Uno17T=NBRC 113155T=BCRC 81116T); and strain SOSP1-9T as Dictyobacter formicarum sp. nov. (type strain SOSP1-9T=CGMCC 1.17206T=BCRC 81204T) within the family Dictyobacteraceae.

Effective approaches to discover new microbial metabolites in a large strain library.

Natural products have provided many molecules to treat and prevent illnesses in humans, animals and plants. While only a small fraction of the existing microbial diversity has been explored for bioactive metabolites, tens of thousands of molecules have been reported in the literature over the past 80 years. Thus, the main challenge in microbial metabolite screening is to avoid the re-discovery of known metabolites in a cost-effective manner. In this perspective, we report and discuss different approaches used in our laboratory over the past few years, ranging from bioactivity-based screening to looking for metabolic rarity in different datasets to deeply investigating a single Streptomyces strain. Our results show that it is possible to find novel chemistry through a limited screening effort, provided that appropriate selection criteria are in place.

Antibacterial Aromatic Polyketides Incorporating the Unusual Amino Acid Enduracididine.

The increasing incidence of infections caused by drug-resistant pathogens requires new efforts for the discovery of novel antibiotics. By screening microbial extracts in an assay aimed at identifying compounds interfering with cell wall biosynthesis, based on differential activity against a Staphylococcus aureus strain and its isogenic l-form, the potent enduracyclinones (1, 2), containing the uncommon amino acid enduracididine linked to a six-ring aromatic skeleton, were discovered from different Nonomuraea strains. The structures of 1 and 2 were established through a combination of derivatizations, oxidative cleavages, and NMR analyses of natural and 13C-15N-labeled compounds. Analysis of the biosynthetic cluster provides the combination of genes for the synthesis of enduracididine and type II polyketide synthases. Enduracyclinones are active against Gram-positive pathogens (especially Staphylococcus spp.), including multi-drug-resistant strains, with minimal inhibitory concentrations in the range of 0.0005 to 4 µg mL-1 and with limited toxicity toward eukaryotic cells. The combined results from assays and macromolecular syntheses suggest a possible dual mechanism of action in which both peptidoglycan and DNA syntheses are inhibited by these molecules.

Elucidating the molecular physiology of lantibiotic NAI-107 production in Microbispora ATCC-PTA-5024.

BACKGROUND: The filamentous actinomycete Microbispora ATCC-PTA-5024 produces the lantibiotic NAI-107, which is an antibiotic peptide effective against multidrug-resistant Gram-positive bacteria. In actinomycetes, antibiotic production is often associated with a physiological differentiation program controlled by a complex regulatory and metabolic network that may be elucidated by the integration of genomic, proteomic and bioinformatic tools. Accordingly, an extensive evaluation of the proteomic changes associated with NAI-107 production was performed on Microbispora ATCC-PTA-5024 by combining two-dimensional difference in gel electrophoresis, mass spectrometry and gene ontology approaches. RESULTS: Microbispora ATCC-PTA-5024 cultivations in a complex medium were characterized by stages of biomass accumulation (A) followed by biomass yield decline (D). NAI-107 production started at 90 h (A stage), reached a maximum at 140 h (D stage) and decreased thereafter. To reveal patterns of differentially represented proteins associated with NAI-107 production onset and maintenance, differential proteomic analyses were carried-out on biomass samples collected: i) before (66 h) and during (90 h) NAI-107 production at A stage; ii) during three time-points (117, 140, and 162 h) at D stage characterized by different profiles of NAI-107 yield accumulation (117 and 140 h) and decrement (162 h). Regulatory, metabolic and unknown-function proteins, were identified and functionally clustered, revealing that nutritional signals, regulatory cascades and primary metabolism shift-down trigger the accumulation of protein components involved in nitrogen and phosphate metabolism, cell wall biosynthesis/maturation, lipid metabolism, osmotic stress response, multi-drug resistance, and NAI-107 transport. The stimulating role on physiological differentiation of a TetR-like regulator, originally identified in this study, was confirmed by the construction of an over-expressing strain. Finally, the possible role of cellular response to membrane stability alterations and of multi-drug resistance ABC transporters as additional self-resistance mechanisms toward the lantibiotic was confirmed by proteomic and confocal microscopy experiments on a Microbispora ATCC-PTA-5024 lantibiotic-null producer strain which was exposed to an externally-added amount of NAI-107 during growth. CONCLUSION: This study provides a net contribution to the elucidation of the regulatory, metabolic and molecular patterns controlling physiological differentiation in Microbispora ATCC-PTA-5024, supporting the relevance of proteomics in revealing protein players of antibiotic biosynthesis in actinomycetes.

Advancing cell wall inhibitors towards clinical applications.

Natural products represent a major source of approved drugs and still play an important role in supplying chemical diversity. Consistently, 2014 has seen new, natural product-derived antibiotics approved for human use by the US Food and Drug Administration. One of the recently approved second-generation glycopeptides is dalbavancin, a semi-synthetic derivative of the natural product A40,926. This compound inhibits bacterial growth by binding to lipid intermediate II (Lipid II), a key intermediate in peptidoglycan biosynthesis. Like other recently approved antibiotics, dalbavancin has a complex history of preclinical and clinical development, with several companies contributing to different steps in different years. While our work on dalbavancin development stopped at the previous company, intriguingly our current pipeline includes two more Lipid II-binding natural products or derivatives thereof. In particular, we will focus on the properties of NAI-107 and related lantibiotics, which originated from recent screening and characterization efforts.

Brominated Variant of the Lantibiotic NAI-107 with Enhanced Antibacterial Potency.

We identified an Actinoallomurus strain producing NAI-107, a chlorinated lantibiotic effective against multidrug-resistant Gram-positive pathogens and previously reported from the distantly related genus Microbispora. Inclusion of KBr in the production medium of either the Actinoallomurus or the Microbispora producer readily afforded brominated variants of NAI-107, which were designated as NAI-108. The other post-translational modifications naturally occurring in this lantibiotic family (i.e., hydroxylation of Pro-14 and C-terminal decarboxylation) were unaffected by the presence of a brominated tryptophan. In addition to being the first example of a bromine-containing lantibiotic, NAI-108 displayed a small but consistent improvement in antibacterial activity against all tested strains. The brominated lantibiotic maintained the same rapid bactericidal activity as NAI-107 but at reduced concentrations, consistent with its increased potency and with the role played by the hydrophobicity of the first lanthionine ring. NAI-108 thus represents an interesting addition to a promising family of potent and effective lantibiotics.

Inorganic phosphate is a trigger factor for Microbispora sp. ATCC-PTA-5024 growth and NAI-107 production.

BACKGROUND: NAI-107, produced by the actinomycete Microbispora sp. ATCC-PTA-5024, is a promising lantibiotic active against Gram-positive bacteria and currently in late preclinical-phase. Lantibiotics (lanthionine-containing antibiotics) are ribosomally synthesized and post-translationally modified peptides (RiPPs), encoded by structural genes as precursor peptides. The biosynthesis of biologically active compounds is developmentally controlled and it depends upon a variety of environmental stimuli and conditions. Inorganic phosphate (Pi) usually negatively regulates biologically-active molecule production in Actinomycetes, while it has been reported to have a positive control on lantibiotic production in Firmicutes strains. So far, no information is available concerning the Pi effect on lantibiotic biosynthesis in Actinomycetes. RESULTS: After having developed a suitable defined medium, Pi-limiting conditions were established and confirmed by quantitative analysis of polyphosphate accumulation and of expression of selected Pho regulon genes, involved in the Pi-limitation stress response. Then, the effect of Pi on Microbispora growth and NAI-107 biosynthesis was investigated in a defined medium containing increasing Pi amounts. Altogether, our analyses revealed that phosphate is necessary for growth and positively influences both growth and NAI-107 production up to a concentration of 5 mM. Higher Pi concentrations were not found to further stimulate Microbispora growth and NAI-107 production. CONCLUSION: These results, on one hand, enlarge the knowledge on Microbispora physiology, and, on the other one, could be helpful to develop a robust and economically feasible production process of NAI-107 as a drug for human use.

Characterization of the congeners in the lantibiotic NAI-107 complex.

NAI-107, a lantibiotic produced by Microbispora sp. 107891, shows potent activity against multi-drug-resistant bacterial pathogens. It is produced as a complex of related molecules, which is unusual for ribosomally synthesized peptides. Here we describe the identification, characterization, and antibacterial activity of the congeners produced by Microbispora sp. 107891 and by the related Microbispora corallina NRRL 30420. These molecules differ by the presence of two, one, or zero hydroxyl groups at Pro-14, by the presence of a chlorine at Trp-4, and/or by the presence of a sulfoxide on the thioether of the first lanthionine.

Sources of novel antibiotics--aside the common roads.

Microbial pathogens are becoming increasingly resistant to available treatments, and new antibiotics are badly needed, but the pipeline of compounds under development is scarce. Furthermore, the majority of antibiotics under development are improved derivatives of marketed compounds, which are at best only partially effective against prevailing resistance mechanisms. In contrast, antibiotics endowed with new mechanisms of action are expected to be highly effective against multi-drug resistant pathogens. In this review, examples are provided of new antibiotics classes in late discovery or clinical development, arising from three different avenues: (1) compounds discovered and never brought to market by large pharmaceutical companies; (2) old compounds reanalyzed and rejuvinated with today's tools; and (3) newly discovered molecules. For each compound, we will briefly describe original discovery, mechanism of action, any known resistance, antimicrobial profile, and current status of development.

Family of class I lantibiotics from actinomycetes and improvement of their antibacterial activities.

Lantibiotics, an abbreviation for "lanthionine-containing antibiotics", interfere with bacterial metabolism by a mechanism not exploited by the antibiotics currently in clinical use. Thus, they have aroused interest as a source for new therapeutic agents because they can overcome existing resistance mechanisms. Starting from fermentation broth extracts preselected from a high-throughput screening program for discovering cell-wall inhibitors, we isolated a series of related class I lantibiotics produced by different genera of actinomycetes. Analytical techniques together with explorative chemistry have been used to establish their structures: the newly described compounds share a common 24 aa sequence with the previously reported lantibiotic planosporicin (aka 97518), differing at positions 4, 6, and 14. All of these compounds maintain an overall -1 charge at physiological pH. While all of these lantibiotics display modest antibacterial activity, their potency can be substantially modulated by progressively eliminating the negative charges, with the most active compounds carrying basic amide derivatives of the two carboxylates originally present in the natural compounds. Interestingly, both natural and chemically modified lantibiotics target the key biosynthetic intermediate lipid II, but the former compounds do not bind as effectively as the latter in vivo. Remarkably, the basic derivatives display an antibacterial potency and a killing effect similar to those of NAI-107, a distantly related actinomycete-produced class I lantibiotic which lacks altogether carboxyl groups and which is a promising clinical candidate for treating Gram-positive infections caused by multi-drug-resistant pathogens.

Bacterial discrimination by means of a universal array approach mediated by LDR (ligase detection reaction).

BACKGROUND: PCR amplification of bacterial 16S rRNA genes provides the most comprehensive and flexible means of sampling bacterial communities. Sequence analysis of these cloned fragments can provide a qualitative and quantitative insight of the microbial population under scrutiny although this approach is not suited to large-scale screenings. Other methods, such as denaturing gradient gel electrophoresis, heteroduplex or terminal restriction fragment analysis are rapid and therefore amenable to field-scale experiments. A very recent addition to these analytical tools is represented by microarray technology. RESULTS: Here we present our results using a Universal DNA Microarray approach as an analytical tool for bacterial discrimination. The proposed procedure is based on the properties of the DNA ligation reaction and requires the design of two probes specific for each target sequence. One oligo carries a fluorescent label and the other a unique sequence (cZipCode or complementary ZipCode) which identifies a ligation product. Ligated fragments, obtained in presence of a proper template (a PCR amplified fragment of the 16s rRNA gene) contain either the fluorescent label or the unique sequence and therefore are addressed to the location on the microarray where the ZipCode sequence has been spotted. Such an array is therefore "Universal" being unrelated to a specific molecular analysis. Here we present the design of probes specific for some groups of bacteria and their application to bacterial diagnostics. CONCLUSIONS: The combined use of selective probes, ligation reaction and the Universal Array approach yielded an analytical procedure with a good power of discrimination among bacteria.

Microbial technologies for the discovery of novel bioactive metabolites.

Soil microbes represent an important source of biologically active compounds. These molecules present original and unexpected structure and are selective inhibitors of their molecular targets. At Biosearch Italia, discovery of new bioactive molecules is mostly carried out through the exploitation of a proprietary strain collection of over 50000 strains, mostly unusual genera of actinomycetes and uncommon filamentous fungi. A critical element in a drug discovery based on microbial extracts is the isolation of unexploited groups of microorganisms that are at the same time good producers of secondary metabolites. Molecular genetics can assist in these efforts. We will review the development and application of molecular methods for the detection of uncommon genera of actinomycetes in soil DNA and for the rapid dereplication of actinomycete isolates. The results indicate a substantial presence in many soils of the uncommon genera and a large diversity of isolated actinomycetes. However, while uncommon actinomycete strains may provide an increased chance of yielding novel structures, their genetics and physiology are poorly understood. To speed up their manipulation, we have developed vectors capable of stably maintaining large segments of actinomycete DNA in Escherichia coli and of integrating site specifically in the Streptomyces genome. These vectors are suitable for the reconstruction of gene clusters from smaller segment of cloned DNA, the preparation of large-insert libraries from unusual actinomycete strains and the construction of environmental libraries.

Discovering new bioactive molecules from microbial sources.

There is an increased need for new drug leads to treat diseases in humans, animals and plants. A dramatic example is represented by the need for novel and more effective antibiotics to combat multidrug-resistant microbial pathogens. Natural products represent a major source of approved drugs and still play an important role in supplying chemical diversity, despite a decreased interest by large pharmaceutical companies. Novel approaches must be implemented to decrease the chances of rediscovering the tens of thousands of known natural products. In this review, we present an overview of natural product screening, focusing particularly on microbial products. Different approaches can be implemented to increase the probability of finding new bioactive molecules. We thus present the rationale and selected examples of the use of hypersensitive assays; of accessing unexplored microorganisms, including the metagenome; and of genome mining. We then focus our attention on the technology platform that we are currently using, consisting of approximately 70,000 microbial strains, mostly actinomycetes and filamentous fungi, and discuss about high-quality screening in the search for bioactive molecules. Finally, two case studies are discussed, including the spark that arose interest in the compound: in the case of orthoformimycin, the novel mechanism of action predicted a novel structural class; in the case of NAI-112, structural similarity pointed out to a possible in vivo activity. Both predictions were then experimentally confirmed.

Draft Genome Sequence of the Microbispora sp. Strain ATCC-PTA-5024, Producing the Lantibiotic NAI-107.

We report the draft genome sequence of Microbispora sp. strain ATCC-PTA-5024, a soil isolate that produces NAI-107, a new lantibiotic with the potential to treat life-threatening infections caused by multidrug-resistant Gram-positive pathogens. The draft genome of strain Microbispora sp. ATCC-PTA-5024 consists of 8,543,819 bp, with a 71.2% G+C content and 7,860 protein-coding genes.

A glycosylated, labionin-containing lanthipeptide with marked antinociceptive activity.

Among the growing family of ribosomally synthesized, post-translationally modified peptides, particularly intriguing are class III lanthipeptides containing the triamino acid labionin. In the course of a screening program aimed at finding bacterial cell wall inhibitors, we discovered a new lanthipeptide produced by an Actinoplanes sp. The molecule, designated NAI-112, consists of 22 amino acids and contains an N-terminal labionin and a C-terminal methyl-labionin. Unique among lanthipeptides, it carries a 6-deoxyhexose moiety N-linked to a tryptophan residue. Consistently, the corresponding gene cluster encodes, in addition to the LanKC enzyme characteristic of this lanthipeptide class, a glycosyl transferase. Despite possessing weak antibacterial activity, NAI-112 is effective in experimental models of nociceptive pain, reducing pain symptoms in mice in both the formalin and the chronic constriction injury tests. Thus, NAI-112 represents, after the labyrinthopeptins, the second example of a lanthipeptide effective against nociceptive pain.

Isolation and characterization of NAI-802, a new lantibiotic produced by two different Actinoplanes strains.

Lantibiotics are biologically active peptides produced by Gram-positive bacteria. Starting from fermentation broth extracts preselected from a high-throughput screening program for discovering cell-wall inhibitors, we successfully isolated a new lantibiotic produced by Actinoplanes sp., designated as NAI-802. MS and NMR analysis together with explorative chemistry established that NAI-802 consists of 21 amino acids, 19 of which are identical to those present in the class II lantibiotic actagardine. Interestingly, NAI-802 carries one extra alanine and one extra arginine at the N- and C-termini, respectively. As expected from the overall higher positive charge, NAI-802 was slightly more active than actagardine against staphylococci and streptococci. Further improvement of its antibacterial activity was achieved by adding one additional positive charge through conversion of the C-terminal carboxylate into the corresponding basic amide. NAI-802 thus represents a novel promising candidate for treating Gram-positive infections caused by multidrug-resistant pathogens.

Orthoformimycin, a selective inhibitor of bacterial translation elongation from Streptomyces containing an unusual orthoformate.

Upon high throughput screening of 6700 microbial fermentation extracts, we discovered a compound, designated orthoformimycin, capable of inhibiting protein synthesis in vitro with high efficiency. The molecule, whose structure was elucidated by chemical, spectrometric, and spectroscopic methods, contains an unusual orthoformate moiety (hence the name) and belongs to a novel class of translation inhibitors. This antibiotic does not affect any function of the 30S ribosomal subunit but binds to the 50S subunit causing inhibition of translation elongation and yielding polypeptide products of reduced length. Analysis by fluorescence stopped flow kinetics revealed that EF-G-dependent mRNA translocation is inhibited by orthoformimycin, whereas, surprisingly, translocation of the aminoacyl-tRNA seems to be unaffected.

The genus Actinoallomurus and some of its metabolites.

In the search for novel antibiotics, natural products continue to represent a valid source of bioactive molecules. During a program aimed at identifying previously unreported taxa of actinomycetes as potential source of novel compounds, we isolated hundreds of different representatives of a new group, initially designated as 'Alpha' and independently described as Actinoallomurus. We report on a PCR-specific method for the detection of this taxon, on appropriate growth conditions and on a pilot-screening program on 78 strains. The strains produce antibacterial or antifungal compounds at a relatively high frequency. Four strains were characterized in further detail: one produced the aromatic polyketide benanomicin B and its dexylosyl derivative; a second strain produced N-butylbenzenesulfonamide; a third strain was an efficient converter of soymeal isoflavonoids from soymeal constituents; and a fourth strain produced several coumermycin-related aminocoumarins, with coumermycin A2 as the major peak, and with some new congeners as minor components of the complex. These data suggest that Actinoallomurus strains possess several pathways for secondary metabolism and represent an attractive source in the search for novel antibiotics.

Antibiotic discovery in the twenty-first century: current trends and future perspectives.

New antibiotics are necessary to treat microbial pathogens that are becoming increasingly resistant to available treatment. Despite the medical need, the number of newly approved drugs continues to decline. We offer an overview of the pipeline for new antibiotics at different stages, from compounds in clinical development to newly discovered chemical classes. Consistent with historical data, the majority of antibiotics under clinical development are natural products or derivatives thereof. However, many of them also represent improved variants of marketed compounds, with the consequent risk of being only partially effective against the prevailing resistance mechanisms. In the discovery arena, instead, compounds with promising activities have been obtained from microbial sources and from chemical modification of antibiotic classes other than those in clinical use. Furthermore, new natural product scaffolds have also been discovered by ingenious screening programs. After providing selected examples, we offer our view on the future of antibiotic discovery.