A study on the presence of flagella in the order Rickettsiales: the case of 'Candidatus Midichloria mitochondrii'.

According to Bergey's Manual of Systematic Bacteriology, the Rickettsiales are '…bacteria with typical gram-negative cell walls and no flagella'. The recently sequenced genome of 'Candidatus Midichloria mitochondrii', a divergent lineage within the order Rickettsiales capable of invading mitochondria in ixodid ticks, revealed the presence of 26 putative flagellar genes. Open questions in relation to this observation are whether these genes are expressed and whether they possess the domains expected for the flagellar function. Here we show that: (a) the putative flagellar proteins of 'Ca. M. mitochondrii' actually possess the conserved domains and structural features required for their function in a model bacterium; (b) the seven flagellar genes of 'Ca. M. mitochondrii' that have been tested are expressed at the RNA level; and (c) the putative flagellar cap gene of this bacterium (FliD) is expressed at the protein level, and can be stained within the bacterium and at its surface. Beside the specific questions that we have addressed that relate to the first evidence, to our knowledge, for a flagellar apparatus in a member of the order Rickettsiales, we present here novel tools (recombinant protein and antibodies) that will facilitate the study of 'Ca. M. mitochondrii'.