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A widely publicized outbreak of pneumonia in persons attending an American Legion convention in Philadelphia in 1976 prompted investigations that defined L. pneumophila and the legionellae. Other outbreaks of respiratory illness caused by related organisms since 1947 have been diagnosed retrospectively. Several dozen species of Legionella exist, some with multiple serogroups. L. pneumophila is the major cause of disease in humans; Legionella micdadei and a few other species sometimes cause pneumonia. The other legionellae are rarely isolated from patients or have been isolated only from the environment.

Morphology and Identification

L. pneumophila is the prototype bacterium of the group. Legionellae of primary medical importance are listed in Table 22-1.

TABLE 22-1Legionella Species Isolated from Humans

A. Typical Organisms

Legionellae are fastidious, aerobic Gram-negative bacteria that are 0.5–1 μm wide and 2–50 μm long (Figure 22-1). They often stain poorly by Gram’s method and are not easily seen in stains of clinical specimens. Gram-stained smears should be made for suspect Legionella growth on agar media. Basic fuchsin (0.1%) should be used as the counterstain because safranin stains the bacteria very poorly. Silver stains, such as Warthin-Starry and Dieterle, can be used to detect legionellae in embedded tissues. Of note, L. micdadei may be acid-fast stain positive.


A: Gram-stain of a L. pneumophila; the legionellae stain faintly with basic fuchsin and poorly with safranin. Original magnification ×1000. (Courtesy CDC Public Health Image Library.) B: Direct fluorescent antibody stain of Legionella of mixed species using antibodies against legionellae genus antigens conjugated with fluorescein. Original magnification ×1000. (Courtesy R Nadarajah.)

B. Culture and Growth Characteristics

Legionellae can be grown on complex media such as buffered charcoal yeast extract (BCYE) agar with α-ketoglutarate, L-cysteine, and iron at a pH of 6.9, temperature of 35°C, and 90% humidity. Antibiotics can be added to make the medium selective for Legionella species. The charcoal acts as a detoxifying agent. Legionellae grow slowly; visible colonies are usually present after 3 days of incubation. Colonies that appear after overnight incubation are not Legionella species. Colonies are round or flat with entire edges. They vary in color from ...

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