Back to the Future to Combat Today's "Super Bugs"

A naturally occurring antimicrobial found 80 years ago to be effective against gram- negative bacteria but too toxic for clinical use is being revisited, with new purification techniques to remove toxic components while preserving bactericidal activity.

"Isolated in 1942, streptothricin was the first antibiotic discovered with potent gram-negative activity," principle investigator James Kirby, MD, D(ABMM), Experimental Pathology Division, Beth Israel Deaconess Medical Center, Boston MA, commented in a statement.

"We find that not only is it activity potent, but that it is highly active (against) the hardiest contemporary multidrug-resistant pathogens and works by a unique mechanism to inhibit protein synthesis," Kirby said.

Multiple forms of streptothricin aminoglycosides were found to comprise the bactericidal mixture, nourseothricin, isolated from soil Actinomyces.Kirby and colleagues assessed the activity spectrum and renal toxicity of the main components, streptothricin F (S-F, 1 lysine) and stretothricin D (S-D, 3 lysines).

The investigators demonstrated that activity extended against highly drug-resistant, carbapenem-resistant Enterobacterales and Acinetobacer baumannii.Substantial treatment effect of S-F was observed (in the murine thigh model) against pan-drug-resistant, NDM-1 (New Delhi metallo-ß-lactamase) expressing Klebsiella pneumoniae.

S-F also showed at least 10-fold lower toxicity than S-D in vitro and in vivo.In single, maximal tolerated dose experiments, treatment with the nourseothricin natural product mixture caused proximal tubule kidney damage at relative low doses of 10mg/kg. In contrast,S-F exerted substantial therapeutic effect at 50 to 100mg/kg to the point of single-dose sterilization of carbapenem-resistant Enterobacterales infection with minimal to no histopathological evidence of renal toxicity.

Kirby and colleagues initially assumed that the streptothricins exerted similar mechanisms to such aminoglycosides as gentamicin and tobramycin, interacting with the 30S subunit of the ribosome target.Their structural and resistance studies suggest, however, that there may be alternative and unique mechanisms of action. Among these, they speculate, is that streptothricins cause miscoding and translation inhibition through effects on tRNA binding.

The investigators describe an interaction with helix 34 distinct from other known translation inibitors.They determined that S-F makes direct polar contact with nucleobases, C-1054 and A-1196; which contrasts with the stacking interactions of tetracyclines with C-1054, and interactions of the pseudodipeptide antibiotic negamycin with the phosphoribose backbone of helix 34.

"It (S-F) works by inhibiting the ability of the organism to produce proteins in a very sneaky way," Kirby remarked."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."

The prospect of a new class of antibiotics that act in a novel way is exciting, Kirby related, as "there's a huge environmental reservoir of resistance out there."

Kirby's group is now collaborating with colleagues at Northeastern University who have been able to synthesize streptothricin without deriving them from the natural nourseothricin source. They are also delving further into the mechanisms of action with colleagues at Case Western Reserve University Medical Center, which Kirby anticipates will facilitate designing optimal variants.

"Based on unique, promising activity," Kirby said, "we believe the streptothricin scaffold deserves further pre-clinical exploration as a potential therapeutic for the treatment of multidrug-resistant, gram-negative pathogens."