Recovery of the ciliate Paramecium multimicronucleatum following bacterial infection with Holospora obtusa

The macronucleus (Ma) of Paramecium multimicronucleatum can be experimentally infected with bacteria of the species Holospora obtusa, a macronuclear-specific parasite of P. caudatum. However, usually all bacteria disappear from the nucleus within 1-2 days after infection. The results of infecting several different stocks of P. multimicronucleatum with several different isolates of H. obtusa from P. caudatum were studied during some days after infection to investigate this disappearance. Using light, fluorescence and electron microscopy at different stages of bacterial disappearance, it was shown that what we call “cleaning” of the nucleus is a fast and active process. P. multimicronucleatum can be infected with the infectious forms of H. obtusa within 2 h, but the majority of the bacteria were lost from the Ma by 10-19 h. They were released from the host nucleus into the cytoplasm and then to the surrounding medium. At first, the infected Ma shortened and became almost rounded. Before leaving the Ma, the majority of the bacteria somehow assembled into groups and these aggregates, coated with material that appears to be nuclear chromatin, protruded into the cytoplasm and were finally separated from the Ma. Sometimes single bacteria were extruded in the same manner. Bacterial release from infected Ma can be stopped by low temperature and is delayed and reduced by nocodazole treatment suggesting that intra-macronuclear microtubules may be involved.