Sassera, et al (2006): ‘Candidatus M. mitochondrii’ appears in EM observations (Lewis, 1979; Zhu et al., 1992; Sacchi et al., 2004) as a Gram-negative bacterium with a bacillus shape of ≈ 0.45 μm in diameter and ≈1.2 μm in length. This bacterium is observed within various cell types (lumenal cells, funicular cells and oocytes) of the ovary of the hard tick I. ricinus (Ixodidae). In all of the above cell types, the bacteria are observed free in the cytoplasm or included in a host-derived membrane. In addition, in luminal cells and oocytes, the bacterium is also observed within the mitochondria, in the periplasmic space between the two membranes of these organelles. As the development of the oocyte proceeds, the bacteria appear to consume the inner part of the mitochondria and multiply therein. The mitochondrial matrix is reduced as a result and some mitochondria appear as sacs full of bacteria (Sacchi et al., 2004). Different numbers of bacteria have been observed within the mitochondria, from a single bacterium to over 20. Despite the high number of mitochondria consumed by the bacterium, the eggs of the tick develop normally. In situ hybridization with probes designed to target specific 16S rRNA gene regions resulted in the staining of only ovarian cells in female ticks (Beninati et al., 2004). In male ticks, there is only PCR evidence for the presence of the bacterium (Lo et al., 2006). The symbiont appears to be ubiquitous in the females of I. ricinus across its distribution (prevalence, 100 %), while a significantly lower prevalence is observed in males (44 %). In males that test positive by PCR, the bacterial load also appears lower compared with females (Lo et al., 2006). Evidence for efficient vertical transmission of the bacterium has been reported based on PCR screening of eggs. Sequencing of the 16S rRNA and gyrB genes from ticks from 11 different countries from across the distribution of I. ricinus showed a low level of variability in both genes. One substitution in 380 bases was found in the 16S rRNA gene sequence and two substitutions in 519 bases were found in the gyrB gene sequence (Lo et al., 2006). [...] ‘Candidatus M. mitochondrii’ belongs to the phylum Proteobacteria, to the class Alphaproteobacteria and to the order Rickettsiales. ‘Candidatus M. mitochondrii’ is assigned on the basis of the 16S rRNA (AJ566640) and gyrB gene sequences (AM159536).