1. Rickettsias. This subgroup multiplies by binary fisinon not by a complex developmental cycle most live in an obligate intracellular association parasitic or mutualistic, with eukaryotic hosts ( vertebrates or arthropods) ; a few can be grown on moderately complex bacteriological media containing blood. The cell walls contain muramic acid Glutamate is oxidized with generation of ATP. Rickettsias are mainly rod-shaped, coccoid, and often pleomorphic bacteria that stain Gram negative and lack flagella; exceptions may occur for each of these features, as follows:
Some appear ring-shaped in staind preparations.
Some have a flagellum.
Some may stain Gram positive.
The parasitic species are associated with the reticuloendothelial and vascular endothelial cells or erythrocytes of vertebrates and often with various organs of arthropods, which may act as vectors or primary hosts. Some cause disease in humans or other vertebrate and invertebrate hosts. The mutualistic species occur in insects and are regarded as essential for development and reproduction of the host.
The are three genera that are able to grow on bacteriological media, Bartonella, Grahamella, and Rochalimaea, but which have been traditionally classified with the rickettsias because of their intracellular parasitic growth habit Phylogenetically, Bartonella and Rochalimaea fall intro the α-2 subgroup of the Proteobacteria and are discussed in Group of this manual.