Localization of Legionella bacteria within ribosome-studded phagosomes is not restricted to Legionella pneumophila

In this report, we investigate the intracellular fate of selected members o f the genus Legionella within the monocytic cell line Mono Mac 6 cells. By means of electron microscopy and immunocytochemistry, we could show that Le gionella pneumophila as well as Legionella longbeachae are able to induce r ibosome-studded phagosomes which associate with the rough endoplasmic retic ulum (RER), whereas Legionella micdadei remains to be located within smooth phagosomes but also shows signs of RER association. In addition, we could demonstrate a remarkable correlation between the phagosome type and the mor phological phenotype of intracellular bacteria: within ribosome-studded pha gosomes, bacteria generally lacked the outer coat of low electron density w hereas bacteria within the smooth phagosomes still possessed this outer coa t. The virulence factors responsible for inhibition of phagosome maturation and their distribution within the genus Legionella as well as the biologic al significance of the morphological difference of bacteria within smooth a nd ER-associated phagosomes remain to be investigated. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. A ll rights reserved.