A Screen of Natural Product Extracts Identifies Moenomycin as a Potent Antigonococcal Agent

Exploring the Darobactin Class of Antibiotics: A Comprehensive Review from Discovery to Recent Advancements

The prevalence of antimicrobial resistance in Gram-negative bacteria poses a greater challenge due to their intrinsic resistance to many antibiotics. Recently, darobactins have emerged as a novel class of antibiotics originating from previously unexplored Gram-negative bacterial species such as Photorhabdus, Vibrio, Pseudoalteromonas and Yersinia. Darobactins belong to the ribosomally synthesized and post-translationally modified peptide (RiPP) class of antibiotics, exhibiting selective activity against Gram-negative bacteria. They target the β-barrel assembly machinery (BAM), which is crucial for the maturation and insertion of outer membrane proteins in Gram-negative bacteria. The dar operon in the producer's genome encodes for the synthesis of darobactins, which are characterized by a fused ring system connected via an alkyl-aryl ether linkage (C-O-C) and a C-C cross-link. The enzyme DarE, using the radical S-adenosyl-l-methionine (rSAM), facilitates the formation of these bonds. Biosynthetic manipulation of the darobactin gene cluster, along with its expression in a surrogate host, has enabled access to diverse darobactin analogues with variable antibiotic activities. Recently, two independent research groups successfully achieved the total synthesis of darobactin, employing Larock heteroannulation to construct the bicyclic structure. This paper presents a comprehensive review of darobactins, encompassing their discovery through to the most recent advancements.

Harvesting phosphorus-containing moieties for their antibacterial effects

Clinically manifested resistance of bacteria to antibiotics has emerged as a global threat to society and there is an urgent need for the development of novel classes of antibacterial agents. Recently, the use of phosphorus in antibacterial agents has been explored in quite an unprecedent manner. In this comprehensive review, we summarize the use of phosphorus-containing moieties (phosphonates, phosphonamidates, phosphonopeptides, phosphates, phosphoramidates, phosphinates, phosphine oxides, and phosphoniums) in compounds with antibacterial effect, including their use as β-lactamase inhibitors and antibacterial disinfectants. We show that phosphorus-containing moieties can serve as novel pharmacophores, bioisosteres, and prodrugs to modify pharmacodynamic and pharmacokinetic properties. We further discuss the mechanisms of action, biological activities, clinical use and highlight possible future prospects.

Bacitracin enhances ceftriaxone susceptibility of the high-level ceftriaxone-resistant gonococcal FC428 clone

IMPORTANCE

Ceftriaxone-based antimicrobial therapies for gonorrhea are threatened by waning ceftriaxone susceptibility levels and the global dissemination of the high-level ceftriaxone-resistant gonococcal FC428 clone. Combination therapy can be an effective strategy to restrain the development of ceftriaxone resistance, and for that purpose, it is important to find an alternative antimicrobial to replace azithromycin, which has recently been removed in some countries from the recommended ceftriaxone plus azithromycin dual-antimicrobial therapy. Ideally, the second antimicrobial should display synergistic activity with ceftriaxone. We hypothesized that bacitracin might display synergistic activity with ceftriaxone because of their distinct mechanisms targeting bacterial cell wall synthesis. In this study, we showed that bacitracin indeed displays synergistic activity with ceftriaxone against Neisseria gonorrhoeae. Importantly, strains associated with the FC428 clone appeared to be particularly susceptible to the bacitracin plus ceftriaxone combination, which might therefore be an interesting dual therapy for further in vivo testing.

Lessons from assembling a microbial natural product and pre-fractionated extract library in an academic laboratory

Microbial natural products are specialized metabolites that are sources of many bioactive compounds including antibiotics, antifungals, antiparasitics, anticancer agents, and probes of biology. The assembly of libraries of producers of natural products has traditionally been the province of the pharmaceutical industry. This sector has gathered significant historical collections of bacteria and fungi to identify new drug leads with outstanding outcomes-upwards of 60% of drug scaffolds originate from such libraries. Despite this success, the repeated rediscovery of known compounds and the resultant diminishing chemical novelty contributed to a pivot from this source of bioactive compounds toward more tractable synthetic compounds in the drug industry. The advent of advanced mass spectrometry tools, along with rapid whole genome sequencing and in silico identification of biosynthetic gene clusters that encode the machinery necessary for the synthesis of specialized metabolites, offers the opportunity to revisit microbial natural product libraries with renewed vigor. Assembling a suitable library of microbes and extracts for screening requires the investment of resources and the development of methods that have customarily been the proprietary purview of large pharmaceutical companies. Here, we report a perspective on our efforts to assemble a library of natural product-producing microbes and the establishment of methods to extract and fractionate bioactive compounds using resources available to most academic labs. We validate the library and approach through a series of screens for antimicrobial and cytotoxic agents. This work serves as a blueprint for establishing libraries of microbial natural product producers and bioactive extract fractions suitable for screens of bioactive compounds.

ONE-SENTENCE SUMMARY

Natural products are key to discovery of novel antimicrobial agents: Here, we describe our experience and lessons learned in constructing a microbial natural product and pre-fractionated extract library.

Structural diversity, bioactivity, and biosynthesis of phosphoglycolipid family antibiotics: recent advances

Moenomycins, such as moenomycin A, are phosphoglycolipid specialized metabolites produced by a number of actinobacterial species. They are among the most potent antibacterial compounds known to date, which drew numerous studies directed at various aspects of the chemistry and biology of moenomycins. In this review, we outline the advances in moenomycin research over the last decade. We focus on biological aspects, highlighting the contribution of the novel methods of genomics and molecular biology to the deciphering of the biosynthesis and activity of moenomycins. Specifically, we describe the structural diversity of moenomycins as well as the underlying genomic variations in moenomycin biosynthetic gene clusters. We also describe the most recent data on the mechanism of action and assembly of complicated phosphoglycolipid scaffold. We conclude with the description of the genetic control of moenomycin production by Streptomyces bacteria and a brief outlook on future developments.

Moenomycin is broadly active against multidrug-resistant Neisseria gonorrhoeae and clears an infection from a murine vaginal tract infection model

OBJECTIVES

Ceftriaxone therapy for gonorrhoea has become under increasing pressure due to waning susceptibility levels and emergence of high-level resistant strains such as the FC428 clone. Moenomycin was recently identified to display potent anti-gonococcal activity against some reference strains. Therefore, the aim of this study was to investigate moenomycin in vitro and in vivo antimicrobial activity.

METHODS

Moenomycin in vitro antimicrobial activity was investigated against 575 clinical isolates, including strains associated with the FC428 clone, using the agar dilution method. Moenomycin in vivo activity was investigated in a mouse vaginal tract gonococcal infection model.

RESULTS

The moenomycin MIC range for the strain collection was 0.004-0.06 mg/L, with a MIC50 of 0.016 mg/L and a MIC90 of 0.03 mg/L. The correlation between moenomycin and ceftriaxone susceptibility levels was poor (R = 0.13), while the fractional inhibitory concentration index (FICI) resulted in indifference for all tested strains. Therefore, development of cross-resistance between moenomycin and ceftriaxone is unlikely for N. gonorrhoeae. Determination of the moenomycin mode of activity against N. gonorrhoeae by time-kill assays showed that moenomycin is bactericidal, with over 104-fold inactivation observed after 4 h exposure. Finally, an intramuscular moenomycin dose of 10 mg/kg given on 2 consecutive days was able to clear a gonococcal infection in a mouse vaginal tract infection model within 1-3 days after the second dose, which was significantly faster than for mice treated with the vehicle control (P < 0.0001).

CONCLUSIONS

Moenomycin displays potent in vitro and in vivo antimicrobial activity against N. gonorrhoeae, warranting further exploration as alternative therapy.

Adaptive Optimization Boosted the Production of Moenomycin A in the Microbial Chassis Streptomyces albus J1074

Great efforts have been made to improve Streptomyces chassis for efficient production of targeted natural products. Moenomycin family antibiotics, represented by moenomycin (Moe) and nosokomycin, are phosphoglycolipid antibiotics that display extraordinary inhibition against Gram-positive bacteria. Herein, we assembled a completed 34 kb hybrid biosynthetic gene cluster (BGC) of moenomycin A (moe-BGC) based on a 24 kb nosokomycin analogue biosynthetic gene cluster (noso-BGC). The heterologous expression of the hybrid moe-BGC in Streptomyces albus J1074 achieved the production of moenomycin A in the recombinant strain LX01 with a yield of 12.1 ± 2 mg/L. Further strong promoter refactoring to improve the transcriptional levels of all of the functional genes in strain LX02 enhanced the production of moenomycin A by 58%. However, the yield improvement of moenomycin A resulted in a dramatic 38% decrease in the chassis biomass compared with the control strain. To improve the weak physiological tolerance to moenomycin A of the chassis, another copy of the gene salb-PBP2 (P238N&F200D), encoding peptidoglycan biosynthetic protein PBP2, was introduced into the chassis strain, producing strain LX03. Cell growth was restored, and the fermentation titer of moenomycin A was 130% higher than that of LX01. Additionally, the production of moenomycin A in strain LX03 was further elevated by 45% to 40.0 ± 3 mg/L after media optimization. These results suggested that the adaptive optimization strategy of strong promoter refactoring in the BGC plus physiological tolerance in the chassis was an efficient approach for obtaining the desired natural products with high titers.