Pyocins are species-specific antibiotics that have evolved with the sole purpose of killing strains of Pseudomonas aeruginosa and as such are the most potent antibiotics known against this notoriously difficult to treat, drug-resistant, opportunistic pathogen. P. aeruginosa lung infection is the major cause of mortality in patients with cystic fibrosis (CF), who despite receiving intensive antibiotic therapy, are frequently chronically infected with this pathogen. Improved antibiotic therapy would therefore be expected to lead to decreased mortality and morbidity in patients with CF. The goal of our work is to discover and characterize novel pyocins and to ultimately develop a pyocin based therapeutic for the treatment of chronic P. aeruginosa infections.

We have recently discovered a novel lectin-like pyocin, pyocin L1, that displays potent activity against P. aeruginosa and using next generation sequencing, structural and biophysical methods, determined how this protein selectively targets Pseudomonas spp. Whole genome sequencing of pyocin L1 resistant mutants suggested that unlike other pyocins that bind specific outer membrane receptors, pyocin L1 utilizes lipopolysaccharide (LPS) as a cell surface receptor. Isothermal titration calorimetry (ITC) showed tight binding to the D-rhamnose homopolymer (CPA) component of P. aeruginosa LPS and along with NMR data, confirmed that pyocin L1 selectively binds the monosaccharide D-rhamnose. The X-ray structure of pyocin L1 in complex with D-rhamnose revealed two sugar binding pockets in the C-terminal lectin domain, the function of which was shown to be essential for cytotoxic activity by site directed mutagenesis. Analysis of CPA binding by these mutants indicated a direct correlation between the ability to bind CPA and cytotoxicity. Preliminary data indicates that pyocin L1 is highly active in P. aeruginosa infection models.

These results greatly improve our knowledge and understanding as to how this bacteriocin exerts its cytotoxic effect. This is essential for the development of pyocin based therapeutics.