An electron microscopy study of Legionella pneumophila after in vitro and in vivo culture.

The ultrastructure of Legionella pneumophila serogroup 1 (Philadelphia Strain) was studied by Scanning Transmission Electron Microscopy (STEM) following in vitro and in vivo culture. The results obtained were identical for all the samples tested; the structure belonged to the prokaryotic type, with features consistent with the known structure of Gram-negative rods. A characteristic feature is the loose undulated outer membrane, a peptidoglycan layer which was difficult to visualize and was better seen in lysed bacteria. The outermost layer was stained by ruthenium red for polysaccharides. In guinea-pig lung tissue and in chick embryo yolk sac membranes, the bacteria were more often intracellular in intracytoplasmic ribosome-studded vacuoles.