Serratamolide as a novel hemolytic factor produced by Serratia marcescens

Purpose The Serratia marcescens bacterium causes vision-threatening keratitis, life-threatening hospital acquired infections and contaminates contact lens cases. The goal of this study was to identify a hemolytic factor produced by keratitis isolates and laboratory strains of S. marcescens. Methods Hemolysis was measured using sheep and mouse erythrocytes. S. marcescens was mutated with a mariner transposon. Hemolysin defective mutants were isolated on blood agar plates. Transposon insertion sites were mapped by marker rescue and sequencing. Complementation analysis was performed with plasmids. Serratamolide was extracted with ethyl acetate and purified by preparative HPLC and verified as pure with high-resolution mass spectroscopy (HR-MS) and 1H NMR analysis. Cytotoxicity to human airway and ocular epithelial cells was performed using Alamar Blue viability dye. The presence of swrW in ocular isolates was determined using PCR. Results Mutation of the swrW gene conferred a hemolysis defect that was complemented by the wild-type swrW gene on a plasmid. The SwrW protein catalyzes production of the cyclic lipopeptide serratamolide. Purified serratamolide was hemolytic to mammalian erythrocytes, and cytotoxic to human airway and ocular cells in vitro. The swrW gene was found in the majority of contact-lens associated keratitis isolates. Conclusion Serratamolide is a novel hemolysin produced by S. marcescens ocular isolates and may contribute to the ability of contact lens associated bacteria to cause infections.