Plot of the first Principal Coordinate Analysis (PCO) axis of COGs content distances. Multivariate analysis graphics of the COGs content of all studied bacteria using R ade4 package. Each bacterium is represented by a symbol linked by a line to the gravity center of the group it belongs to (M, obligate intracellular mutualists, red triangles; P, obligate intracellular parasites, purple triangles; FHA, facultative host- associated, green asterisks; and FL, free-living, blue squares). An ellipse was also drawn for each class, which size increases with the coordinates' dispersion in the class. It is computed such that it would contain 68% of the individuals in the studied class if the distribution were Gaussian. Otherwise, it is just a way to compare dispersion between classes. 1 represents Treponema pallidum; 2 represents Mycobacterium leprae; 3 represents Candidatus Protochlamydia amoebophila UWE25; 4 represents Coxiella burnetii. These species with larger genome sizes and gene contents than the other obligate intracellular bacteria undergo reductive evolution [28, 30]. Some of these bacteria have high number of pseudogenes [27, 29, 31]. The presence of pseudogenes displays an ongoing process of gene degradation.