Jonathan Betts, Christopher Nagel, Ulrich Schatzschneider, Robert Poole, Robert M. La Ragione

• Antimicrobial resistance is a growing global concern in human and veterinary medicine, with an ever-increasing void in the arsenal of clinicians. Novel classes of compounds including carbon monoxoide-releasing molecules (CORMs), for example the light-activated metal complex [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br, could be used as alternatives/to supplement traditional antibacterials. Avian pathogenic \(Escherichia\) \(coli\) (APEC) represent a large reservoir of antibiotic resistance and can cause serious clinical disease in poultry, withAntimicrobial resistance is a growing global concern in human and veterinary medicine, with an ever-increasing void in the arsenal of clinicians. Novel classes of compounds including carbon monoxoide-releasing molecules (CORMs), for example the light-activated metal complex [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br, could be used as alternatives/to supplement traditional antibacterials. Avian pathogenic \(Escherichia\) \(coli\) (APEC) represent a large reservoir of antibiotic resistance and can cause serious clinical disease in poultry, with potential as zoonotic pathogens, due to shared serotypes and virulence factors with human pathogenic \(E.\) \(coli\). The \(in\) \(vitro\) activity of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br against multidrug-resistant APECs was assessed via broth microtitre dilution assays and synergy testing with colistin performed using checkerboard and time-kill assays. \(In\) \(vivo\) antibacterial activity of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br alone and in combination with colistin was determined using the \(Galleria\) \(mellonella\) wax moth larvae model. Animals were monitored for life/death, melanisation and bacterial numbers enumerated from larval haemolymph. \(In\) \(vitro\) testing produced relatively high [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br minimum inhibitory concentrations (MICs) of 1024 mg/L. However, its activity was significantly increased with the addition of colistin, bringing MICs down to \(\geq\)32 mg/L. This synergy was confirmed in time-kill assays. \(In\) \(vivo\) assays showed that the combination of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br with colistin produced superior bacterial killing and significantly increased larval survival. In both \(in\) \(vitro\) and \(in\) \(vivo\) assays light activation was not required for antibacterial activity. This data supports further evaluation of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br as a potential agent for treatment of systemic infections in humans and animals, when used with permeabilising agents such as colistin.…