. A 55-year-old man who is being treated for adenocarcinoma of the lung is admitted to a hospital because of a temperature of 38.9degC (102degF), chest pain, and a dry cough. Sputum is collected. Gram's stain of the sputum is unremarkable and culture reveals many small Gram-negative rods able to grow only on a charcoal yeast extract agar. This organism most likely is

The symptoms of Legionnaires' disease are similar to those of mycoplasmal pneumonia and influenza. Affected persons are moderately febrile, complain of pleuritic chest pain, and have a dry cough. Unlike Klebsiella and Staphylococcus, Legionella pneumophila exhibits fastidious growth requirements. Charcoal yeast extract agar either with or without antibiotics is the preferred isolation medium. While sputum may not be the specimen of choice for Legionella, the discovery of small Gram-negative rods by direct fluorescent antibody (FA) technique should ceainly heighten suspicion of the disease. L. pneumophila is a facultative intracellular pathogen and enters macrophages without activating their oxidizing capabilities. The organisms bind to macrophage C receptors, which promote engulfment. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition