xanthomonas citri 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. xanthomonas citri scientific classification domain: bacteria phylum: pseudomonadota class: gammaproteobacteria order: xanthomonadales family: xanthomonadaceae genus: xanthomonas species: x. citri binomial name xanthomonas citri(hasse 1915) gabriel et al. 1989 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.