• endosymbiosis;
  • heat shock resistance;
  • host–parasite;
  • osmotic stress;
  • virulence;
  • tolerance


Under certain conditions, otherwise parasitic organisms may become beneficial to their host. Parasite-mediated heat and osmotic stress resistance have been demonstrated for Paramecium caudatum, infected by several species of parasitic bacteria of the genus Holospora. Here, using the micronucleus-specific bacterium Holospora undulata, we investigate how infection mediates the response of two genotypes (clones ‘K8’ and ‘VEN’) of P. caudatum to heat (35 °C) and osmotic (0.24% NaCl) stress. In contrast to previous findings, we find no evidence for heat stress protection in infected individuals. We do, however, show an effect of symbiont-mediated osmotic stress resistance for the K8 clone, with infected individuals having higher survival than their uninfected counterparts up to 24 h after the onset of salt exposure. Despite this, both infected and uninfected individuals of the VEN clone showed higher survival rates than clone K8 individuals under osmotic stress. Thus, it would seem that parasite-mediated stress protection is restricted to certain combinations of host genotypes and types of stress and does not represent a general phenomenon in this system.