Summary: The ultrastructure of the infectious and reproductive forms of , a bacterium colonizing the macronucleus of , was analysed with the aid of ultrathin sections, freeze-fracture, freeze-etching and cytochemical electron microscopical techniques. The infectious form differed considerably from the reproductive form. The bacterial cytoplasm of the infectious form was confined to less than half of the cell, the rest being occupied by a voluminous periplasmic space. The periplasmic space contained a highly osmiophilic fine granular material. At the end of the cell distal from the cytoplasm, a tip containing less osmiophilic fine granular material was observed. Freeze-fracture and freeze-etching studies revealed differences in the patterns of intramembranous particles between the two forms. It is suggested that some of the structures characterizing the infectious form have a function in the infection process.


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