The results of research led by scientists at Harvard Medical School suggest that a natural product antibiotic first isolated back in the 1940s may be effective against potentially lethal, multidrug resistant (MDR) gram-negative bacterial infections. Development of the antibiotic, which is produced by a soil fungus and was originally known as streptothricin, had been discontinued because a human study demonstrated that it was toxic to the kidneys. Subsequent work showed that streptothricin is a mixture of components, collectively referred to as nourseothricin. Through their newly reported in vitro and preclinical in vivo study, James Kirby, PhD, and colleagues showed that purified components of the mixture demonstrate potent activity against gram negative pathogens, including pan drug-resistant Klebsiella pneumoniae Nevada strain, with minimal or no toxicity.

Their results are described in a paper in PLOS Biology, titled “Streptothricin F is a bactericidal antibiotic effective against highly drug-resistant gram-negative bacteria that interacts with the 30S subunit of the 70S ribosome.” In the paper the team concluded, “Based on unique and promising activity, we suggest that the streptothricin scaffold deserves further preclinical exploration as a potential therapeutic for drug-resistant, gram-negative pathogens.”

The rapid emergence of antimicrobial resistance presents what the authors note is “a significant challenge” for treatment of bacterial infections. Organisms such as carbapenem-resistant Enterobacterales (CRE) and Acinetobacter baumannii are of particular concern. Gram negative bacteria have a thick outer protective layer, and just about all of the approved antimicrobials that can overcome this “gram-negative permeability barrier” are natural products or synthetic or semisynthetic derivatives of natural products. The authors pointed out that  “small molecules commonly available in high-throughput screening libraries rarely share similar physicochemical properties associated with gram-negative penetrance and activity.” This means that high-throughput screening efforts to identify novel antimicrobials using synthetic chemical libraries have been largely nonproductive. “As a result, there is a significant antimicrobial discovery void.”

Nourseothricin, a natural product made by Actinomyces soil fungus, contains multiple forms of a complex molecule, streptothricin. Its discovery in the 1940s generated high hopes for it as a powerful agent against Gram-negative bacteria. But a clinical study found that nourseothricin was toxic to kidneys, so its development was dropped. With the rise of antibiotic-resistant bacterial infections spurring the search for new antibiotics, Kirby and colleagues took another look at nourseothricin.

Early studies of nourseothricin had suffered from incomplete purification of the streptothricins compounds, and large doses. “Initial studies on toxicity in animals from the 1940s used only semiquantitative activity measures and impure compound with large doses presumably exceeding 100 mg/kg and are therefore problematic to interpret,” Kirby and colleagues noted.

More recent work has shown that the multiple streptothricin forms exhibit different toxicities, with one, streptothricin-F (S-F), found to be significantly less toxic, while remaining highly active against contemporary multidrug-resistant pathogens. For their study reported in PLOS Biology, the authors characterized the antibacterial action, renal toxicity, and mechanism of action of highly purified forms of S-F and another form, streptothricins, D (S-D). “Based on potentially insufficiently explored therapeutic potential, we sought to further characterize the properties of S-F and S-D, the major constituents of nourseothricin, purified to homogeneity using modern techniques …” they wrote.

Streptothricin-F antibiotic
Streptothricin-F (yellow spheres) bound to the 16S rRNA (green) of the bacterial ribosome impinges on the decoding site where tRNA (purple) binds to the codon of the mRNA (blue). This interaction leads to translation infidelity (scrambled protein sequences), and the resulting death of the bacterial cell. The image was created by overlay of PDB 7UVX containing streptothricin-F (this manuscript) with PDB 7K00 containing mRNA and A-site tRNA (ref DOI: 10.7554/eLife.60482). [James Kirby (CC-BY 4.0,; Zoe L Watson et al., 2023, eLife, CC-BY 4.0 (]

Their experimental results showed that both were highly selective for Gram-negative bacteria. The D form was more powerful than the F form against drug-resistant Enterobacterales and other bacterial species, but caused renal toxicity at a lower dose. And while S-F was about 6-fold less potent than S-D, it still retained potent antimicrobial activity. “Importantly, S-F showed at least 10-fold lower toxicity than S-D and the natural product mixture, nourseothricin, in vitro and in vivo,” the investigators stated. “Tests with a single dose of S-F against  a pandrug-resistant Nevada CRE strain in mice showed “substantial in vivo bactericidal treatment effect … with minimal or no observable toxicity.”

Using cryo-electron microscopy, the authors showed that streptothricin-F bound extensively to a subunit of the bacterial ribosome, accounting for the translation errors  that these antibiotics are known to induce in their target bacteria. Interestingly, the binding interaction is distinct from other known inhibitors of translation, suggesting it may find use when those agents are not effective.

“We demonstrate that streptothricins, in particular S-F, S-D, and the natural product mixture, nourseothricin, are highly active against contemporary, carbapenem-resistant E. coli, Klebsiella,

Enterobacter, and A. baumannii,” the team further noted. “ …  we present data for compelling bactericidal activity of S-F against contemporary multidrug-resistant CRE and A. baumannii pathogens with in vivo confirmation of efficacy against an emblem of gram-negative antibiotic resistance … Taken together, our data establish S-F as a potently bactericidal natural product scaffold with a unique mechanism of action with what we believe are compelling properties for future medicinal chemistry exploration.”

Kirby added, “Isolated in 1942, streptothricin was the first antibiotic discovered with potent gram-negative activity. We find that not only is it activity potent, but that it is highly active [than] the hardiest contemporary multidrug-resistant pathogens and works by a unique mechanism to inhibition protein synthesis.”

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