Phylogenetic diversity of intra-amoebal legionellae as revealed by 16S rRNA gene sequence comparison

Following a recent phylogenetic analysis of all 39 species of the genus Legionella and related organisms, the name Legionella lytica has been proposed for bacteria previously considered either as Sarcobium lyticum or as 'Legionella-like amoebal pathogens' (LLAPs). To investigate the phylogenetic integrity of this newly proposed species, we determined the 16S rRNA gene sequences of 10 LLAPs isolated from various environmental sources. All 10 isolates clustered within a monophyletic group containing all other members of the genus Legionella. Eight of the 10 isolates formed a monophyletic subgroup within the genus which also included the two previously characterized L. lytica strains. Four of these 10 isolates shared a specific and very close relationship with the L. lytica type strain (> 99% sequence similarity). However, although clearly legionellae, the remaining two LLAP strains bore no specific evolutionary relationship to either L. lytica or any other Legionella species (< 96% sequence similarity). Both isolates lay on their own relatively deep-rooted branches within the radiation of the Legionella cluster. LLAPs do not, therefore, represent a unique species or even a single line of descent within the genus, and investigation of more isolates may reveal them to be as evolutionarily diverse as the other presently recognized Legionella species.