Pseudomonas aeruginosa utilises its type III secretion system to kill the free-living amoeba Acanthamoeba castellanii

Pseudomonas aeruginosa is a free-living and common environmental bacterium. It is an opportunistic and nosocomial pathogen causing serious human health problems. To overcome its predators, such as macrophages and environmental phagocytes, it utilises different survival strategies, such as the formation of microcolonies and the production of toxins mediated by a type III secretion system (TTSS). The aim of this study was to examine interaction of TTSS effector proteins of P. aeruginosa PA103 with Acanthamoeba castellanii by co-cultivation, viable count, eosin staining, electron microscopy, apoptosis assay, and statistical analysis. The results showed that P. aeruginosa PA103 induced necrosis and apoptosis to kill A. castellanii by the effects of TTSS effector proteins ExoU, ExoS, ExoT, and ExoY. In comparison, Acanthamoeba cultured alone and co-cultured with P. aeruginosa PA103 lacking the known four TTSS effector proteins were not killed. The results are consistent with P. aeruginosa being a strict extracellular bacterium that needs TTSS to survive in the environment, because the TTSS effector proteins are able to kill its eukaryotic predators, such as Acanthamoeba.