ABSTRACT
Holospora obtusa, an alpha-proteobacterium, is an obligate endonuclear pathogen of the ciliate Paramecium caudatum. It is engulfed by the host cell in the course of phagocytosis but soon escapes from the phagosome and is transported across the host cell cytoplasm to the paramecium macronucleus. Electron microscopy reveals a comet-like tail resembling that of Listeria trailing after H. obtusa in the host cytoplasm. In this study we investigated the role of the host cell actin and Arp3 in the process of infection with Holospora. Cytochalasin D treatment significantly reduced the rate of nuclear infection. Using immunocytochemistry and experimental infection of GFP-actin-transfected paramecia we demonstrated that the Paramecium actin1-1 took part in the bacterial escape from the phagosome, its trafficking in the cytoplasm and entry into the host macronucleus. Rapid assembly/disassembly of actin filaments in P. caudatum led to quick loss of actin1-1 from the trails left by H. obtusa. Immunocytochemistry using anti-bovine Arp3 antibodies demonstrated the presence of Arp3 in these trails. Our data indicate that details of H. obtusa infection are rather similar to those of Listeria and Rickettsia.
