The progeny of L egionella pneumophila in human macrophages shows unique developmental traits

The Gram‐negative bacterium L egionella pneumophila is an intracellular parasite of amoebae and an accidental human pathogen that causes a noncommunicable atypical pneumonia known as L egionnaires' disease ( LD ). In some mammalian cells (e.g. H e L a), L . pneumophila follows a biphasic developmental cycle, differentiating between a replicative form that actively multiplies intracellularly, and a mature infectious form ( MIF ) that emerges as progeny. To date, it is not known whether the L . pneumophila progenies that emerge from amoebae and human macrophages reach similar developmental stages. Here, we demonstrate that in relation to the fully differentiated and highly infectious MIF s that emerge from amoebae, the L . pneumophila progeny that emerges from macrophages is morphologically undifferentiated, less resistant to antibiotics and less able to initiate infections. However, the L . pneumophila progeny from macrophages did not show any defects in intracellular growth. We thus concluded that macrophage infection with L . pneumophila yields a low number of bona fide MIF s. Because MIF s are the transmissive forms of L . pneumophila produced in vivo , our results showing that they are not efficiently produced in cultured macrophages provide an initial insight into why LD is not communicable.