Xanthomonas citri

Xanthomonas citri is a Gram-negative, rod-shaped bacterium. Although it is harmless for humans, it is a phytopathogen, known for being the causing agent of citrus canker.

Bacterial killing via a secretion system
Xanthomonas citri uses its type IV secretion system to kill other Gram-negative bacterial species in a contact-dependent manner. The secretion of the effector proteins requires a conserved C-terminal domain, and its bacteriolytic activity is neutralized by a cognate immunity protein.

Defense against predators
Xanthomonas citri also uses type VI secretion system in defense against the predatory amoeba Dictyostelium discoideum.