Ligionella pneumophila: Avirulent to virulent conversion through passage in cultured human embryonic lung fibroblasts

Legionella pneumophila isolated in guinea pigs from human lung tissues was highly virulent as determined by its infectivity and lethality in guinea pigs. Repeated passages of the bacteria on agar media resulted in the loss of virulence in guinea pigs. Virulence, however, was restored by cultivating the avirulent bacteria in cell cultures of human embryonic lung fibroblasts. Death of the host animals was the result of infection; no lethal toxin was detected in the cultural filtrate. These findings indicate that the virulent form ofL. pneumophilia is capable of surviving inside the host cells either through its endogenous resistance to environmental factors within the host cells or by host cell selection. Intracellular multiplication of the virulent bacteria followed by destruction of host cells appears to be an important pathogenic mechanism of Legionnaires' disease.