New Class of Antibiotics Disrupts Outer Membrane of Gram-Negative Bacteria

Gram-negative bacteria, such as Acinetobacter baumannii, Enterobacteriaceae, and Pseudomonas aeruginosa, are a rising concern for human health. Globally, gram-negative bacteria have displayed increasing resistance to carbapenem and cephalosporin antibiotics. New treatments for gram-negative bacteria are needed, particularly as resistance grows to the last resort antibiotic colistin.

The last new class of antibiotics introduced to the market against gram-negative bacteria was the fluroquinolones in the 1960s. According to a study published in Nature, however, a Swiss research team led by experts from the University of Zurich has discovered a new family of synthetic antibiotics with broad spectrum anti-gram-negative antimicrobial activity, including against all of the gram-negative members of the ESKAPE pathogens.

The new class of chimeric peptidomimetic antibiotics, known as OMPTA (Outer Membrane Protein Targeting Antibiotics), functions by employing a unique mechanism that targets the outer membranes of gram-negative bacteria.

"The new antibiotics interact with essential outer membrane proteins in gram-negative bacteria," John Robinson, UZH Department of Chemistry and co-head of the study, said in a statement. "According to our results, the antibiotics bind to complex fat-like substances called lipopolysaccharides and to BamA, an essential protein of the outer membrane of gram-negative bacteria.”

The outer membrane of gram-negative bacteria “comprises an asymmetric bilayer, with glycerophospholipids in the inner leaflet and lipopolysaccharide (LPS) in the outer leaflet,” the study authors wrote. “This unique permeability barrier protects the bacteria from toxic environmental factors (such as antibiotics) and contains many integral β-barrel outer membrane proteins (OMPs), which are required for biogenesis of the outer membrane.”

Investigators obtained a range of clinical isolates from the University Hospital Basel and the IHMA collection that were collected between 2012 and 2017. Eight compounds were analyzed. Chimeras 3 and 4 had a marked effect on Escherichia coli membrane structure, which showed extra membrane-like material, membrane detachment, and the appearance of vacuoles. Electron microscopy revealed that cells treated with compounds 3 and 4 showed collapsed membranes and extracellular “knob-like structures,” suggesting that both of the chimeras perturb bacterial membranes.

Compounds 3, 4, 7, and 8 were shown to be bactericidal against several gram-negative pathogens, including multidrug-resistant, extensively drug-resistant, and colistin-resistant strains. Study authors hope future research will help determine how the binding of the chimeras to BamA causes downstream bactericidal activity.

In considering how the bactericidal activity functions, investigators hypothesized that “one possibility is that binding inhibits the foldase activity of the BAM complex. The resulting incorrectly folded OMPs, when mislocated to the inner membrane, may lead to cell permeabilization and death.” Alternatively, another possibility identified was “that binding of the chimaeras to BamA provides an additional binding site in the outer membrane that enhances a permeabilizing effect mediated by the polymyxin macrocycle, and helps these antibiotics to avoid LPS-modification resistance mechanisms.”

Polyphor, a Swiss biopharmaceutical company, announced that the lead molecule of the new OMPTA class is currently in preclinical toxicology studies. Study authors said that “a lead candidate based on these derivatives has, pending future clinical studies, the potential to address life-threatening infections caused by gram-negative pathogens, and thus to resolve a considerable unmet medical need.”