GROWTH OF LEGIONELLA-PNEUMOPHILA IN ACANTHAMOEBA-CASTELLANII ENHANCESINVASION

Legionella pneumophila is considered to be a facultative intracellular parasite. Therefore, the ability of these bacteria to enter, i.e., in vade, eukaryotic cells is expected to be a key pathogenic determinant. We compared the invasive ability of bacteria grown under standard lab oratory conditions with that of bacteria grown in Acanthamoeba castell anii, one of the protozoan species that serves as a natural host for L . pneumophila in the environment. Amoeba-grown L. pneumophila cells we re found to be at least 100-foId more invasive for epithelial cells an d 10-fold more invasive for macrophages and A. castellanii than were L . pneumophila cells grown on agar. Comparison of agar- and amoeba-grow n L. pneumophila cells by light and electron microscopy demonstrated d ramatic differences in the morphology and structure of the bacteria. A nalyses of protein expression in the two strains of bacteria suggest t hat these phenotypic differences may be due to the expression of new p roteins in amoeba-grown L. pneumophila cells. In addition, the amoeba- grown bacteria were found to enter macrophages via coiling phagocytosi s at a higher frequency than agar-grown bacteria did. Replication of L . pneumophila in protozoans present in domestic water supplies may be necessary to produce bacteria that are competent to enter mammalian ce lls and produce human disease.