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Legionella pneumophila is a thin (0.5 to 0.7μm), pleomorphic, Gram-negative rod thatmay show elongated, filamentous forms up to 20μm long. In clinical specimens, the or-ganism 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 that omit decolorization steps. Polar, subpolar, and lateral flagella may be present. Most species of Legionella are motile. Spores are not found.
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 are a homopolymer of an unusual sugar (legionaminic acid), which renders the cell surface highly hydrophobic. It has been postulated that this hydrophobicity may promote adherence of bacterial cells to membranes or their concentration in aerosols.
Legionella species fail to grow on common enriched bacteriologic media such as bloodagar. This is due to unusual 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.
Due to the difficulty in growing Legionella there are few phenotypic properties to use in its classification. It is possible to directly demonstrate some enzymatic actions (cata-lase, oxidase, β-lactamase), but the other cultural and metabolic taxonomic tests used to classify other bacteria cannot be applied to Legionella. Thus, the classification depends largely on antigenic features, chemical analysis, and nucleic acid homology comparisons.
L. pneumophila has 14serogroups andtherearemorethan 30 other Legionella species(eg, L.bozemanii, L. dumoffii, L. micdadei). The original Philadelphia strain (serogroup 1) is still the most common and a limited number of L. pneumophila serogroups (1 to 4) account for 80 to 90% of cases. Not all of the non – L. pneumophila species have been isolated from human infections.
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