Amphiphilic nebramine-based hybrids Rescue legacy antibiotics from intrinsic resistance in multidrug-resistant Gram-negative bacilli.

The inability to discover novel class of antibacterial agents, especially against Gram-negative bacteria (GNB), compel us to consider a broader non-conventional approach to treat infections caused by multidrug-resistant (MDR) bacteria. One such approach is the use of adjuvants capable of revitalizing the activity of current existing antibiotics from resistant pathogens. Recently, our group reported a series of tobramycin (TOB)-based hybrid adjuvants that were able to potentiate multiple classes of legacy antibiotics against various MDR GNB. Herein, we report the modification of TOB-based hybrid adjuvants by replacing TOB domain by the pseudo-disaccharide nebramine (NEB) through selective cleavage of the α-d-glucopyranosyl linkage of TOB. Potent synergism was found for combinations of NEB-based hybrid adjuvants with multiple classes of legacy antibiotics including fluoroquinolones (moxifloxacin and ciprofloxacin), tetracyclines (minocycline), or rifamycin (rifampicin) against both wild-type and MDR P. aeruginosa clinical isolates. We also demonstrated that a combination of the optimized NEB-CIP hybrid 1b and rifampicin protects Galleria mellonella larvae from the lethal effects of extensively drug-resistant (XDR) P. aeruginosa. Mechanistic evaluation of NEB-based hybrid adjuvants revealed that the hybrids affect the outer- and inner membranes of wild-type P. aeruginosa PAO1. This study describes an approach to optimize aminoglycoside-based hybrids to yield lead adjuvant candidates that are able to resuscitate the activity of partner antibiotics against MDR GNB.

Synergistic combinations of anthelmintic salicylanilides oxyclozanide, rafoxanide, and closantel with colistin eradicates multidrug-resistant colistin-resistant Gram-negative bacilli.

Repurposing nonantibiotic drugs for antimicrobial therapy presents a viable approach to drug discovery. Development of therapeutic strategies that overcome existing resistance mechanisms is important especially against those bacterial infections in which treatment options are limited, such as against multidrug-resistant Gram-negative bacilli. Herein, we provide in vitro data that suggest the addition of anthelmintic salicylanilides, including oxyclozanide, rafoxanide, and closantel, in colistin therapy to treat multidrug-resistant colistin-susceptible but more importantly colistin-resistant Gram-negative bacilli. As a stand-alone agent, the three salicylanilides suffered from limited outer membrane permeation in Pseudomonas aeruginosa, with oxyclozanide also susceptible to efflux. Synergy was apparent for the combinations against multidrug-resistant clinical isolates of P. aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Escherichia coli, and Enterobacter cloacae. Susceptibility breakpoints for colistin, but also with polymyxin B, were reached upon addition of 1 µg ml-1 of the corresponding salicylanilide against colistin-resistant Gram-negative bacilli. Furthermore, enhanced bacterial killing was observed in all combinations. Our data corroborate the repositioning of the three salicylanilides as adjuvants to counter resistance to the antibiotic of last resort colistin. Our findings are timely and relevant since the global dissemination of plasmid-mediated colistin resistance had been realized.