Paramecium calkinsi and P-putrinum (Ciliophora, Protista) harboring alpha-subgroup bacteria in the cytoplasm

New intracellular bacteria were detected in the cytoplasm of Paramecium cal kinsi and P. putrinum. Some of the bacteria were not evenly distributed in the cytoplasm of the host but were found in the center of the cell, eventua lly near the nuclei, but not in the cortex area, whereas another species wa s found in the cortex area. These peculiarities of intracellular bacteria l ocalization in the host suggest that the conditions in various parts of the cytoplasm favor bacterial maintenance to different extent. Due to the resu lts obtained by transmission electron microscopy and in situ hybridization using appropriate oligonucleotide probes, the bacteria, three or possibly f our species, are Gram-negative and belong to the alpha-subgroup of proteoba cteria. Bacteria from one stock of Fl calkinsi were found to be infectious for bacteria-free cells of P. calkinsi and P. nephridiatum.