Legionella pneumophila

Legionella pneumophila is a thin, pleomorphic, flagellated Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogen in this group and is the causative agent of legionellosis or Legionnaires' disease.

Characterization
L. pneumophila is non-acid-fast, non-sporulating, endochronogimalational and morphologically a non-capsulated rod-like bacteria. Aerobic and unable to hydrolyse gelatin or produce urease, they are also non-fermentative. L. pneumophila is neither pigmented nor does it autofluoresce. It is oxidase- and catalase-positive, and produces beta-lactamase.

Cell membrane structure
While L. pneumophila is categorized as a Gram-negative organism, it stains poorly due to its unique lipopolysaccharide-content in the outer psuedospamodulating leaflet of the outer cell membrane. On the side-chains of the cell wall are carried the bases for the somatic antigen specificity of these organisms. The chemical composition of these side chains both with respect to components as well as arrangement of the different sugars determines the nature of the somatic or O-antigen determinants, which are important means of serologically classifying many Gram-negative bacteria. At least 35 different serovars of L. pneumophila have been described as well as several other species being subdivided into a number of serovars.

Sera have been used both for slide agglutination studies as well as for direct detection of bacteria in tissues using fluorescent-labelled antibody. Specific antibody in patients can be determined by the indirect fluorescent antibody test. ELISA and microagglutination tests have also been successfully applied.

Pathogenesis
L. pneumophila is a facultative intracellular parasite that can invade and replicate inside amoebae and, in humans, in macrophages. The internalisation of the bacteria can be enhanced by the presence of antibody and complement but is not absolutely required. A pseudopod coils around the bacterium in this unique form of phagocytosis. Once internalised, the bacteria surround themselves in a membrane-bound vacuole that does not fuse with lysosomes that would otherwise degrade the bacteria. In this protected compartment the bacteria multiply. The bacteria use a Type IVB Secretion System known as Icm/Dot to inject effector proteins into the host. These effectors are involved in increasing the bacteria's ability to survive inside the host cell. They also secrete a 39kDa metalloprotease into culture fluids, which is cytotoxic for some cultured tissue culture cells.

The pathogenic nature of L. pneumophila was first recognized after a 1976 outbreak among a group of elderly men attending an American Legion convention in Philadelphia, Pennsylvania (hence the name Legionaires' disease). This outbreak affected over 200 individuals, with 34 fatalities. It is worth noting that person-to-person transmission of L. pneumophila has not been demonstrated.