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 with PDB 7K00 containing mRNA and A-site tRNA. Credit: James Kirby/ DOI: 10.7554/eLife.60482
In 1942, scientists discovered the antibiotic streptothricin, which they isolated from a soil bacterium. Unlike the mass-marketed penicillin, streptothricin could address gram-negative organisms. The rights to streptothricin were immediately licensed by a pharmaceutical company—but it didn’t go much further. In trials, patients developed renal or kidney toxicity. Simultaneously, several other gram-negative-targeting antibiotics were discovered. Thus, streptothricin was shelved and ultimately forgotten about.
That is until James Kirby, PI at Beth Israel Deaconess Medical Center and professor of pathology at Harvard Medical Center, stumbled upon it some 80 years later.
“As I read the literature on streptothricin and its history, I had the realization that it was not sufficiently explored,” said Kirby. “Here was this antibiotic with outstanding activity against gram-negative bacteria—and we confirmed that by testing it against a lot of different pathogens that we see in hospitals. That raised the question of whether we could get really good antibiotic activity at concentrations that are not going to cause damage to the animal or person in treatment.”
In their new study, published in PLOS Biology, Kirby and his colleagues tested that—and found great success.
The research team realized that what scientists were isolating in 1942 was not as pure as what they are working with today. In fact, what was then called streptothricin is actually a mixture of several streptothricin variants, now referred to as nourseothricin. More recent work has shown that the multiple forms have different toxicities— with one, streptothricin-F, significantly less toxic than other forms.
Kirby’s study focused on both streptothricin-F and streptothricin D. The scientists infected animal models with a strain of Klebsiella pneumoniae called the Nevada strain, which was the first pan-drug-resistant, gram-negative organism isolated in the U.S. According to the study results, the D form was more powerful than the F form, but caused renal toxicity at a lower dose. Meanwhile, a single dose of the F form cleared the gram-negative organism from the infected animal model while avoiding any toxicity. Both forms were highly selective for gram-negative bacteria.
“We’re still in the very early stages of development, but I think we've validated that this is a compound that's worth investing in further studies to find even better variants that eventually will meet the properties of a human therapeutic,” said Kirby.
Additionally, using cryo-electron microscopy, the scientists observed a distinct binding action of the F-form. According to the study, streptothricin-F binds extensively to a subunit of the bacterial ribosome, accounting for the translation errors these antibiotics are known to induce in their target bacteria. Essentially, nourseothricin works by inhibiting the ability of the organism to produce proteins—in a very sneaky way.
“When a cell makes proteins, they make them off a blueprint or message that tells the cell what amino acids to link together to build the protein. Our studies help explain how this antibiotic confuses the machinery so that the message is read incorrectly, and it starts to put together gibberish. Essentially, the cell gets poisoned because it's producing all this junk,” explained Kirby.
The researchers say the binding interaction is distinct from other known inhibitors of translation—and when dealing with multi-drug resistance, anything unique is good.
In the absence of new classes of antibiotics, scientists have been good at taking existing antibiotics and modifying them, but the resistance mechanisms “old” drugs already exist so it’s more like creating a variation of the same problem. That’s why a new class of antibiotics that act in a novel way has such powerful potential.
Continuing their research, Kirby’s lab is working closely with scientists at Northeastern University, who figured out a way to synthesize streptothricin from scratch that allows the team to cast many different variants. The scientists will then look for ones that have the ideal properties of high potency and reduced toxicity.