Unrecognized Potent Activities of Colistin Against Clinically Important + Enterobacteriaceae Revealed in Synergy with Host Immunity.

Colistin (COL) is a cationic cyclic peptide that disrupts negatively-charged bacterial cell membranes and frequently serves as an antibiotic of last resort to combat multidrug-resistant Gram-negative bacterial infections. Emergence of the horizontally transferable plasmid-borne mobilized colistin resistance () determinant and its spread to Gram-negative strains harboring extended-spectrum β-lactamase and carbapenemase resistance genes threatens futility of our chemotherapeutic arsenal. COL is widely regarded to have zero activity against + patients based on standard antimicrobial susceptibility testing (AST) performed in enriched bacteriological growth media; consequently, the drug is withheld from patients with + infections. However, these standard testing media poorly mimic in vivo physiology and omit host immune factors. Here we report previously unrecognized bactericidal activities of COL against -+ isolates of (EC), (KP), and (SE) in standard tissue culture media containing the physiological buffer bicarbonate. Moreover, COL promoted serum complement deposition on the + Gram-negative bacterial surface and synergized potently with active human serum in pathogen killing. At COL concentrations readily achievable with standard dosing, the peptide antibiotic killed + EC, KP, and SE in freshly isolated human blood proved effective as monotherapy in a murine model of + EC bacteremia. Our results suggest that COL, currently ignored as a treatment option based on traditional AST, may in fact benefit patients with + Gram negative infections based on evaluations performed in a more physiologic context. These concepts warrant careful consideration in the clinical microbiology laboratory and for future clinical investigation of their merits in high risk patients with limited therapeutic options.