Impaired stress tolerance of the L. pneumophila Lp ΔclpP mutant during stationary phase. Overnight cultures of different strains were inoculated into fresh medium and grew to stationary phase (OD600 from 3.5 to 4.5), and the cells were then treated with (A) 1 mM H2O2 for 30 minutes. * p < 0.05, (B) pH 4.0 citric acid for 30 minutes. * p < 0.01, (C) 57°C heat shock for 20 minutes. * p < 0.05, or (D) 0.3 M KCl for 1 hour. * p < 0.05. The experiments were carried out in triplicate.