ABSTRACT

On the basis of a number of laboratory criteria, including DNA homology studies, the causal organism, named Legionella pneumophila, was shown to be a newly recognized bacterial genus and species. Transmission electron microscopy of L. pneumophila in thin-sectioned material showed that the organisms possessed the typical morphological properties previously ascribed to Gram-negative bacteria. The ultrastructure of L. pneumophila has been studied by freeze-etching electron microscopy. In these reports it was shown that the enveloping layers enclosing the bacterial cytoplasm appeared as four short ridges and corresponded to the two "unit" membranes seen in thin-sectioned material. The most widely used laboratory method for detecting serological evidence of legionellosis and identification of legionella organisms in tissues is the immunofluorescent antibody test using direct or indirect techniques. The intensive investigations that followed the initial descriptions of Legionella have led to a more precise understanding of the nature and properties of these organisms.