Germs within Worms: Localization of Neorickettsia sp. within Life Cycle Stages of the Digenean Plagiorchis elegans

Stephen E. Greiman, Yasuko Rikihisa, Jacob Cain, Jefferson A. Vaughan, Vasyl V. Tkach

Neorickettsia are bacterial endosymbionts of parasitic flukes (Digenea) that also have the potential of infecting and causing disease in the vertebrate hosts of the fluke (e.g., sennetsu fever). One of the largest gaps in our knowledge of Neorickettsia biology is the very limited information available regarding the localization of the bacterial endosymbiont within its digenean host. In this study we used indirect immunofluorescence microscopy to visualize Neorickettsia sp. within several life cycle stages of the digenean Plagiorchis elegans. Individual sporocysts, cercariae, metacercariae, and adults of P. elegans naturally infected with Neorickettsia were obtained from our laboratory-maintained life cycle, embedded, sectioned and prepared for indirect immunofluorescence microscopy using anti-N. risticii horse serum as the primary antibody. Neorickettsia were found within the tegument of sporocysts, throughout cercarial embryos (germ balls) and fully formed cercariae (within the sporocysts), throughout metacercariae, and within the tegument, parenchyma, vitellaria, uteri, testes, cirrus sacs, and eggs of adults. Interestingly, Neorickettsia were not found within the ovarian tissue. This suggests that vertical transmission of Neorickettsia within adult digeneans occurs via the incorporation of infected vitelline cells into the egg rather than direct infection of the ooplasm of the oocyte, as has been described for other bacterial endosymbionts of invertebrates (e.g., Rickettsia, Wolbachia).