Summary
The X‐bacteria which initiated organismic association with the D strain of
Amoeba proteus
in 1966 as parasites have changed to obligate endosymbionts on which the host depends for survival. Owing to the difficulty in cultivating the bacteria
in vitro
, the identity of X‐bacteria has not been determined. As the life cycle of X‐bacteria is similar to that of
Legionella
spp. in soil amoebae, we applied the polymerase chain reaction method with specific primers aimed at
Legionella
spp. for the detection and cloning of 16S rRNA gene. The identity and intracellular localization of the endosymbiont were confirmed by the application of a specific fluorescently labelled 16S rRNA‐targeted probe. In addition we cloned RNA polymerase β‐subunit gene (
rpoB
) of X‐bacteria by genomic library tagging. A phylogenetic analysis of the 16S rRNA gene placed the bacterium within a unique monophyletic group containing all other members of the genus
Legionella
. Phylogeny from
rpoB
and
mip
genes further confirmed the taxonomic context of X‐bacteria to be a
Legionella
sp. In all three phylogenic analyses, X‐bacterium was placed apart from
Legionella
‐like amoebal pathogens present in soil amoebae. Thus, we propose the name ‘
Candidatus
Legionella jeonii’ sp. nov. for the endosymbiotic X‐bacteria in
Amoeba proteus
.