Antibacterial activity of lantibiotics, alone and in combination with colistin, against Gram-negative bacteria.

In this study, the potential of the use of class I antimicrobial cationic peptides, also known as lantibiotics, nisin, warnerin and hominin, combined with subinhibitory concentrations of colistin against Gram-negative bacteria (GNB) was investigated. The minimum inhibitory concentration (MIC) and fractional inhibitory concentration index were determined via the microbroth dilution and checkerboard techniques, respectively. We showed synergistic effects of the combination of all the lantibiotics + colistin against Escherichia coli, Enterobacter aerogenes, Pseudomonas fluorescens, clinical multidrug-resistant Klebsiella pneumoniae, and multidrug-resistant Alcaligenes faecalis, whereas lantibiotics alone had no effect. In time‒kill curve studies, synergism was observed with almost all the expected combinations and was prolonged by N-ethylmaleimide, used solely as a mechanistic inhibitor of peptide degradation and not as a potential treatment. The maximum reduction exceeded 6 log CFU/mL compared to the control. Compared with no treatment, exposure to subMICs of colistin alone slightly increased membrane permeability, whereas treatment with the combination of warnerin and colistin caused more damage than did colistin alone. Energy dispersive X-ray spectroscopy revealed that pretreatment of cells with subMICs of colistin caused a decrease in the intracellular contents of Ca2+, Mg2+, phosphorus and sulfur, suggesting the possibility of implementing the antibacterial effects of lantibiotics during their subsequent action. The anti-infective activity of lantibiotics + colistin was demonstrated in Galleria mellonella larvae infected with E. aerogenes. We conclude that the synergistic antimicrobial activity of the combination of lantibiotic + colistin against GNB pathogens warrants further exploration for specific treatment of GNB infections.