In a recent study in Nature, researchers identified and characterized a broad-spectrum antibiotic, lariocidin (LAR).
The antibiotic inhibits bacterial growth by altering protein synthesis in ribosomes. Identifying such ribosome-targeting antibiotics could lead to the development of effective antibacterial medications to reduce the antibiotic resistance burden.
Study: A broad-spectrum lasso peptide antibiotic targeting the bacterial ribosome. Image Credit: Fahroni/Shutterstock.com
Introduction
Antibiotic resistance has caused millions of deaths worldwide and jeopardizes the capacity to treat bacterial illnesses efficiently.
Thus, discovering novel antibacterial chemicals is a primary priority, particularly those targeting Gram-negative bacteria or critical threats, as designated by the World Health Organization (WHO).
Most clinically relevant peptides demonstrate antibiotic effects without involving ribosomes. Instead, they are formed by connecting amino acids using specialized peptide synthetase-enabled protein-building machinery found in bacteria and fungi.
Lasso peptides are natural biologically active molecules having a distinctive three-dimensional form and a highly stable, structurally restricted knotted fold. They are ribosomally synthesized and post-translationally modified peptide (RiPP) molecules.
Despite ribosomes being a prime antibiotic target as they are responsible for genetically programmed protein synthesis, no study has reported lasso peptides that inhibit ribosomal activity.
About The Study
In the present study, researchers reported a potent ribosome-targeting antibiotic, lariocidin, with broad-spectrum bactericidal activity.
To find an effective antibiotic, the researchers grew several environmental bacterial strains on agar culture plates at ambient temperature for a prolonged one-year period that would reveal slow-growing and frequently overlooked strains.
They obtained fractions of bacterial colonies after reversed-phase chromatography (RPC). These fractions underwent several biochemical, microbiological, genetic, and structural analyses.
To assess their antibiotic potential, the team tested the fractions against a multidrug-resistant (MDR) isolate of Gram-negative Acinetobacter baumannii, C0286, and an antibiotic-susceptible isolate of Escherichia coli, BW25113.
Extracts of the Paenibacillus M2 strain showed potent antibacterial activity against the colistin-resistant Escherichia coli strain. Subsequent purification procedures yielded a biological compound weighing 1,870 Da (molecular mass) with antibiotic effects. The compound's molecular mass differed from that of colistins and did not match with any compound listed in antimicrobial databases.
Chemical analysis revealed that the compound is a peptide. Genomic analysis revealed a biosynthetic gene cluster (BGC) belonging to lasso peptides class II. The team named the compound lariocidin due to its lariat structure, as observed in nuclear magnetic resonance (NMR) analysis.
Mass spectrometry, X-ray crystallography, and high-performance liquid chromatography (HPLC) studies revealed two LAR derivatives, LAR-B and LAR-C, with closely related structures.
Results
The study showed that lariocidin inhibits bacterial growth by binding to the ribosome and interfering with protein synthesis. The binding occurs at a unique site in the small ribosomal subunit, where LAR interacts with the 16S ribosomal ribonucleic acid (RNA) and aminoacyl-transfer RNA (tRNA).
The LAR-binding site differs from the sites of other antibiotics targeting the small ribosomal subunit, including aminoglycosides, tetracyclines, tuberactinomycins, odilorhabdins, and negamycin.
Thus, LAR has a unique mode of interaction with ribosomes that confers minimal cross-resistance with commonly used antibiotics that target the small ribosomal subunit.
The antibiotic-ribosome binding inhibits translocation and induces miscoding. Notably, LAR binds to the sugar-phosphate backbone instead of the nucleobases of ribosomes. The findings suggest that LAR should be less likely to face resistance from binding site mutations.
The antibiotic is non-toxic to human cells and exerts potent activity against Gram-positive [B. subtilis, minimum inhibitory concentration (MIC) 1.0 µg/ml], Gram-negative (efflux-impaired E. coli, MIC 2-4 µg/ml), and mycobacterial (Mycobacterium smegmatis, MIC 1-2 µg/ml) organisms in vivo.
The study demonstrated that LAR is active in mice infected with the Acinetobacter baumannii C0286 strain that shows resistance to carbapenem, a beta-lactam antimicrobial drug. Treatment of A. baumannii-infected mice with 50 mg/kg (four doses) LAR significantly reduced bacterial counts in murine blood, spleen, and thigh by two orders of magnitude within 24 hours.
Moreover, all LAR-treated mice survived beyond two days of infection and were considerably healthier than controls. The findings suggest that LAR could be an effective antibiotic for serious drug-resistant bacterial infections.
Conclusions and Future Outlook
The study identified a lasso peptide compound that interferes with ribosomal protein synthesis to exert potent and broad-spectrum activity against several bacterial pathogens.
Identifying such ribosome-targeting bactericidal molecules with unconventional structures and novel modes of action provides attractive opportunities for developing much-needed antibacterial drugs.
Notably, common resistance mechanisms do not affect lariocidin, and the antibiotic has a low propensity to generate spontaneous resistance. Thus, such an antibiotic could expand the spectrum of medications effective against bacteria resistant to commonly used drugs. LAR could provide healthcare professionals with additional options for treating recalcitrant infections.
LAR-B, a derivative of LAR, is unique due to its double-loop structure and second chemical bond. This makes it the first example of a new class V of lasso peptides. The extra bond increases stability and resistance against body breakdown, making it valuable for developing new medicines.
Future studies could investigate mechanisms by which LAR and LAR-B interact with ribosomes and evaluate structural changes that occur in ribosomes after LAR treatment.
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