1. A genus of aerobic, motile, nonacid-fast, nonencapsulated, gram-negative bacilli (family Legionellaceae) that have a nonfermentative metabolism and require l-cysteine HCl and iron salts for growth; they dwell in water, spread in air, and are pathogenic for humans. Over 40 species have been identified; the type species is Legionella pneumophila.
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