legionellales legionellales the legionellales are an order of pseudomonadota. like all pseudomonadota, they are gram-negative. they comprise two families, typified by legionella and coxiella, both of which include notable pathogens. for example, q fever is caused by coxiella burnetii and legionella pneumophila causes legionnaires' disease and pontiac fever. legionellales legionella sp. under ultraviolet illumination scientific classification domain: bacteria phylum: pseudomonadota class: gammaproteobacteria order: legionellales families and genera legionellaceae legionella coxiellaceae aquicella coxiella diplorickettsia rickettsiella "ca. berkiella" "ca. fiscibacter" "ca. ovatusbacter" "ca. pokemonas" members of the order legionellales can be molecularly distinguished from other gammaproteobacteria by the presence of four conserved signature indels (csis) in the proteins trna-guanine(34) transglycosylase, lipoprotein-releasing system transmembrane protein lole and trna (guanosine(37)-n1)-methyltransferase trmd.