Light and electron microscopic study of a Rickettsiella species from the cockroach Blatta orientalis

Abstract

An infection with Rickettsiella sp. was responsible for an illness causing heavy body swelling in the Oriental cockroach Blatta orientalis. Reproduction of the colony stagnated. Vacuoles with parasitic bacteria occurred mainly in the fat body, but also in nearly all other organs, such as gut epithelium, Malpighian tubules, blood cells, and ovarioles. The parasites clearly differed from the symbiotic bacteria of the genus Blattabacterium, which regularly occur in the mycetocytes of B. orientalis. The vacuoles contained four stages of Rickettsiella: (1) infectious, electron-dense, rod-like elementary bodies (mean size 300 x 145 nm); (2) an electron-dense, flat intermedium stage, called flat body (mean size 515 x 255 x 125 nm); (3) an electron-light, spherical intermedium stage, called condensing sphere (mean size 340 nm); portions of cytoplasm condensed crescent-like at the border or in the center of the cell; and (4) large, spherical, electron-light initial bodies that multiplied by binary fission (mean size 600 nm). The initial bodies had a three-layered cell boundary, but all other stages had a five-layered cell boundary. Elementary and flat bodies contained an electron-light, oblique lamella and an oval structure with an array of ribosome-like granules, respectively. In contrast to other species of Rickettsiella, crystal formation or multiple division did not occur. The described species of Rickettsiella is different from "R. blattae," which belongs to the R. popilliae group. Instead, it shares more similarities with the R. chironomi group. To avoid confusion, it was provisionally named "R. crassificans."