Unexpected genomic features in widespread intracellular bacteria: evidence for motility of marine chlamydiae

Chlamydiae are obligate intracellular bacteria comprising important human pathogens and symbionts of protists. Molecular evidence indicates a tremendous diversity of chlamydiae particularly in marine environments, yet our current knowledge is based mainly on terrestrial representatives. Here we provide first insights into the biology of marine chlamydiae representing three divergent clades. Our analysis of single-cell amplified genomes revealed hallmarks of the chlamydial lifestyle, supporting the ancient origin of their characteristic developmental cycle and major virulence mechanisms. Surprisingly, these chlamydial genomes encode a complete flagellar apparatus, a previously unreported feature. We show that flagella are an ancient trait that was subject to differential gene loss among extant chlamydiae. Together with a chemotaxis system, these marine chlamydiae are likely motile, with flagella potentially playing a role during host cell infection. This study broadens our view on chlamydial biology and indicates a largely underestimated potential to adapt to different hosts and environments.