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The death toll in the outbreak of the mysterious respiratory disease in Philadelphia rose by two to 25 as medical detectives accelerated efforts today to seek a chemical or poison as the possible cause.

—The New York Times, August 7, 1976

Legionella is a genus of Gram-negative bacilli that takes its name from the outbreak at the American Legion convention where it was first discovered. The name of the type species, Legionella pneumophila, reflects its propensity to cause the necrotizing pneumonia known as Legionnaires disease. Legionella species are now known to be widespread in the environment in ponds, amoebas, and the plumbing of large buildings. Coxiella, a cause of pneumonia known long before Legionella, shares many pathogenic, epidemiologic, and clinical features with Legionella.




Legionella pneumophila is a thin, pleomorphic, Gram-negative rod that may show elongated, filamentous forms up to 20 μm long. In clinical specimens, the organism stains poorly or not at all by Gram stain or the usual histologic stains; however, it can be demonstrated by certain silver impregnation methods (Dieterle stain) and by some simple stains without decolorization steps. Polar, subpolar, and lateral flagella may be present. Most species of Legionella are motile. Spores are not found.

Gram-negative rod that stains with difficulty

Structurally, L pneumophila has features similar to those of Gram-negative bacteria with a typical outer membrane, thin peptidoglycan layer, and cytoplasmic membrane. The toxicity of L pneumophila lipopolysaccharide (LPS) is significantly less than that of other Gram-negative bacteria such as Neisseria and the Enterobacteriaceae. This has been attributed to chemical makeup of the LPS side chains which renders the cell surface highly hydrophobic, a property which may promote distribution in aerosols.

LPS is less toxic than that of most Gram-negative species

Side chains are hydrophobic


Legionella is a facultative intracellular pathogen multiplying to high numbers inside free- living amoebas, other protozoa, and macrophages. In human-made water systems the organisms persist in a low metabolic state imbedded in biofilms. In vitro L pneumophila fails to grow on common enriched bacteriologic media such as blood agar due to requirements for certain amino acids (l-cysteine), ferric ions, and slightly acidic conditions (optimal pH 6.9). Even when these requirements are met, growth under aerobic conditions is slow, requiring 2 to 5 days to produce colonies that have a distinctive surface resembling ground glass. Although a few enzymatic actions (catalase, oxidase, β-lactamase) are demonstrable, the classification of Legionella depends largely on antigenic features, chemical analysis, and nucleic acid homology comparisons. The closest relative among pathogenic bacteria is Coxiella burnettii (see below).

Intracellular parasite of protozoa

Biofilms form in water systems

Growth requires l-cysteine, ferric ions, and low pH

Classification ...

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