Interactions of Pseudomonas aeruginosa and Corynebacterium spp. with non-phagocytic brain microvascular endothelial cells and phagocytic Acanthamoeba castellanii.

Several lines of evidence suggest that Acanthamoeba interact with bacteria, which may aid in pathogenic bacterial transmission to susceptible hosts, and these interactions may have influenced evolution of bacterial pathogenicity. In this study, we tested if Gram-negative Pseudomonas aeruginosa and Gram-positive Corynebacterium spp. can associate/invade and survive inside Acanthamoeba castellanii trophozoites and cysts, as well as non-phagocytic human brain microvascular endothelial cells. The results revealed that both Corynebacterium spp. and P. aeruginosa were able to associate as well as invade and/or taken up by the phagocytic A. castellanii trophozoite. In contrast, P. aeruginosa exhibited higher association as well as invasion of non-phagocytic HBMEC compared with Corynebacterium spp. Notably, P. aeruginosa remained viable during the encystment process and exhibited higher levels of recovery from mature cysts (74.54 bacteria per amoebae) compared with Corynebacterium spp. (2.69 bacteria per amoeba) (P < 0.05). As Acanthamoeba cysts can be airborne, these findings suggest that Acanthamoeba is a potential vector in the transmission of P. aeruginosa to susceptible hosts. When bacterial-ridden amoebae were exposed to favourable (nutrient-rich) conditions, A. castellanii emerged as vegetative trophozoites and remained viable, and likewise viable P. aeruginosa were also observed but rarely any Corynebacterium spp. were observed. Correspondingly, P. aeruginosa but not Corynebacterium spp. exhibited higher cytotoxicity to non-phagocytic HBMEC, producing more than 75% cell death in 24 h, compared to 20% cell death observed with Corynebacterium spp. Additionally, it was observed that the bacterial conditioned medium had no negative effect on A. castellanii growth. Further characterization of amoebal and bacterial interactions will assist in identifying the role of Acanthamoeba in the transmission and evolution of pathogenic bacteria.