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Endonuclear bacteria were observed within Euglena hemichromata cells isolated from a bloom in a small pond in South Korea. Ultrastructural features of the host cell and resident bacteria are presented and a route of infection pathway into host cells proposed. The bacteria are rod shaped, with a thick outer wall and an inner osmiophilic layer, as in gram-negative forms. Endonuclear bacteria are suggested to enter the host cytoplasm from the euglenoid reservoir, where they are taken up by a phagocytotic process, resulting in cytoplasmic bacteria surrounded by a single host membrane. The host cytoplasmic membrane subsequently makes contact and fuses with the host outer nuclear envelope and the bacterium is apparently taken into the nucleus by an invagination of the inner nuclear envelope. Within the host nucleus, endonuclear bacteria are initially surrounded by a single host membrane. Later, the bacteria grow, divide, and lose the host membrane. Bacterial growth and reproduction eventually consume the entire host nuclear region, filling it with bacterial cells. The large number of bacterial cells presumably then break out of the host cell and are free to infect other euglenoids. This type of endonucleobiosis may be involved in the control of euglenoid blooms in nature.
INTRODUCTION
The colonization of nuclei by prokaryotic or eukaryotic microbes is frequently observed and has been reported in numerous host cells since the last century (e.g. see Gortz 1986). Endonuclear bacteria have been previously reported in several protozoan and algal species (Ball 1969; Leedale 1969; Silva 1978; Preer J.R. & Preer L.B. 1984) and extensive studies of Paramecium Ehrenberg have described the bacterium-host relationship (Ossipov et al. 1975; Fujishima & Gortz 1983; Fujishima & Fujita 1985; Schmidt et al. 1987; Gortz 1988; Gortz & Wiemann 1989). Gortz (1986) has introduced the term of 'endonucleobiosis' to describe symbiotic relationships involving host nuclei with specific 'guest' organisms, whether the relationships are based on mutualism or parasitism.
Endonucleobiosis (as well as endocytobiosis) has been reported previously in euglenoids (Roth 1959; Ueda 1960; Leedale 1969; Gerola & Bassi 1978; Surek & Melkonian 1983; Walne et al. 1986; Dragos et al. 1990; Schnepf et al. 2002). Heterotrophic euglenoids, such as Peranema trichophorum (Ehrenberg) Stein, Entosiphon sulcatum Stein, and Petalomonas sphagnophila Christen, have a feeding apparatus that functions in the uptake...