Intracellular growth in Acanthamoeba castellanii affects monocyte entry mechanisms and enhances virulence of Legionella pneumophila

Since Legionella pneumophila is an intracellular pathogen, entry into and r eplication within host cells are thought to be critical to its ability to c ause disease. L, pneumophila grown in one of its environmental hosts, Acant hamoeba castellanii, is phenotypically different from L. pneumophila grown on standard laboratory medium (BCYE agar). Although amoeba-grown L. pneumop hila displays enhanced entry into monocytes compared to BCYE-grown bacteria , the mechanisms of entry used and the effects on virulence have not been e xamined. To explore whether amoeba-grown L. pneumophila differs from BCYE-g rown L. pneumophila in these characteristics, we examined entry into monocy tes, replication in activated macrophages, and virulence in mice. Entry of amoeba-grown L. pneumophila into monocytes occurred more frequently by coil ing phagocytosis, was less affected by complement opsonization, and was Les s sensitive to microtubule and microfilament inhibitors than was entry of B CYE-grown bacteria. In addition, amoeba-grown L. pneumophila displays incre ased replication in monocytes and is more virulent in A/J, C57BL/6 Beige, a nd C57BL/6 mice. These data demonstrate for the first time that the intra-a moebal growth environment affects the entry mechanisms and virulence of L. pneumophila.