ABSTRACT

Following the isolation and characterization of Legionella pneumophila, seven additional species of bacteria belonging to the genus Legionella have been discovered. These fastidious Gram-negative bacteria resemble L. pneumophila in cultural characteristics, but possess distinct antigenic and genetic properties. Disease due to any of the Legionella species may collectively be referred to as "legionellosis", reserving the term "Legionnaires' disease" for cases due specifically to L. pneumophila. Diagnosis, especially serologic diagnosis, has been complicated by the realization that patients may have pneumonia caused by more than one species of Legionella. A number of patients have been described who had dual infections with L. micdadei and L. pneumophila. The clinical response of patients with L. micdadei pneumonia treated with erythromycin is quite variable. Some patients become afebrile within 24 hr, while others continue to have fever for a number of days. As in Legionnaires' disease, resolution of the radiographic infiltrates is characteristically slow.