Viability and infectivity of viable but nonculturable Legionella pneumophila strains induced at high temperatures

Thermal disinfection is commonly used to prevent the proliferation of culturable Legionella in engineered water systems (EWS). In response to such stress, culturable Legionella populations can switch into a viable but nonculturable (VBNC) state. The importance of such VBNC Legionella cells is currently hotly debated. Here, we investigated the stress response patterns and transitions of the bacteria to the VBNC state at 55 °C, 60 °C and 70 °C on two L. pneumophila strains for >80 days using a combination of cell-based viability indicators. Complete loss of culturability at 55 °C, 60 °C and 70 °C occurred after 3–8 h, 60 min and 85 days (55, 60 °C) and 8 days (70 °C).•VBNC L. pneumophila infect amoeba and THP-1 cells with low efficiency.•Outer-membrane epitopes are very persistent over several weeks.•Culture-based methods detect the most virulent L. pneumophila subpopulations.