Bacterial endocytobionts of the ciliate Paramecium calkinsi

Five species of gram-negative bacteria inhabiting the cytoplasm and the macronucleus of the ciliate Paramecium calkinsi are described. Two species of endocytobionts have been found in the cytoplasm. Both of them were represented by only a vegetative form. In the case of the first species, numerous endocytobiotic bacteria (length, 2 μm; diameter, 0.3-0.4 μm) were enclosed in individual endobiontophoral vacuoles. The second one was not as abundant in its host cells and the bacteria lie freely in the host cytoplasm. These bacteria (length, 2-3 μm; diameter, 0.6-0.9 μm) often contained viral particles. Three species of endocytobionts have been discovered in the macronucleus. The first one (1-2 μm long, 0.3-0.4 μm in diameter) resembled Nonospora macronucleata. However, it differed from N. macronucleata in having specific electron dense zones in its cytoplasm and stained differently by Ag-NOR. The second one represents a new species of the Holospora genus: H. curvata sp. nov. The infectious form of this endocytobiont (15-20 μm long, 0.7-0.9 μm in diameter) is curved, the vegetative form (2-3 μm long, the same diameter) was rod-like or slightly curved. The host specificity of these endocytobionts is well expressed. The third species of intranuclear bacteria, H. bacillata, had been previously described in P. woodruffi. The infective capacity of H. curvata and two "stocks" of H. bacillata from different hosts was compared.