Pseudomonas two-partner secretion toxin Exolysin contributes to insect killing

Abstract Pseudomonas chlororaphis is a promising biocontrol agent promoting plant-growth and providing protection against pest insects and phytopathogenic fungi. We have identified in the genome of P. chlororaphis PA23 an operon encoding the toxin Exolysin (ExlA) and its outer-membrane transporter, ExlB. We found that P. chlororaphis producing ExlA (ExlA Pch ) is cytotoxic towards murine macrophages and human epithelial cells at 30 °C. P. chlororaphis PA23 provoked shrinkage of epithelial cell, leakage of cytoplasmic components and subsequent cell death. During infection, ExlA Pch incorporated into epithelial cell membranes within detergent-resistant lipid rafts, suggesting the same mechanisms of cell destruction by pore-formation as reported for P. aeruginosa toxin. ExlA Pch was not involved in the capacity of the strain to kill fungi, amoeba or other bacteria. The contribution of ExlA in insecticidal activity of P. chlororaphis was evaluated in the wax moth larvae Galleria mallonella and in Drosophila melanogaster flies. The impact of the deletion of a gene encoding exlA homologue was tested in the natural fly pathogen P. entonomophila . In both models, the ExlA absence delayed killing, suggesting the contribution of the toxin in bacteria-insect pathogenic interactions.