Everything about Legionella Pneumophila totally explained
Legionella pneumophila is a thin,,
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, and morphologically a non-capsulated rod-like bacteria.
Aerobic and unable to
hydrolyse gelatin or produce
urease, they're 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 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 intracellular bacteria 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 isn't absolutely required. A
pseudopod coils around the bacterium in this unique form of
phagocytosis. Once internalized, the bacteria surround themselves in a membrane-bound vacuole that doesn't 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 have a type II secretion system that secretes a 39kDa
metalloprotease into culture fluids, which is
cytotoxic for some cultured tissue culture cells. The type II secretion system is also required for full virulence
(External Link
). 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
Legionnaires' disease). This outbreak affected over 200 individuals, with 34 fatalities. It is worth noting that person-to-person transmission of
L. pneumophila hasn't been demonstrated.
Genomics
The determination and publication of the complete genome sequences of three clinical
L. pneumophila isolates in 2004 paved the way for the understanding of the molecular biology of
L. pneumophila in particular and
Legionella in general. In depth comparative genome analysis using DNA arrays to study the gene content of 180
Legionella strains revealed the high genome plasticity and frequent horizontal gene transfer. Further insight in the
L. pneumophila life cycle was gained by investigating the gene expression profile of
L. pneumophila in
Acanthamoeba castellanii, its natural host.
L. pneumophila exhibits a biphasic life cycle and defines transmissive and replicative traits according to gene expression profiles.
Further Information
Get more info on 'Legionella Pneumophila'.
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