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Legionella pneumophila is a gram-negative bacterium prevalent in fresh water which accidentally infects humans and is responsible for the disease called legionellosis. Intracellular growth of L. pneumophila in Tetrahymena is inconsistent; in the species Tetrahymena tropicalis stationary-phase forms (SPFs) of L. pneumophila differentiate into mature intracellular forms (MIFs) without apparent bacterial replication and are expelled from the ciliate as pellets containing numerous MIFS. In the present work, we tested the impact of L. pneumophila passage through T. tropicalis. We observed that MIFs released from T. tropicalis are more resistant to various stresses than SPFs. Under our conditions, MIFs harboured a higher gentamicin resistance, maintained even after 3 months as pellets. Long-term survival essays revealed that MIFs survived better in a nutrient-poor environment than SFPs, as a reduction of only about 3 logs was observed after 4 months in the MIF population, whereas no cultivable SPFs were detected after 3 months in the same medium, corresponding to a loss of about 7 logs. We have also observed that MIFs are significantly more infectious in human pneumocyte cells compared with SPFs. These results strongly suggest a potential role of ciliates in increasing the risk of legionellosis.