Legionella pneumophila

Taxobox
color = lightgrey
name = "Legionella pneumophila"

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image_width = 240px
image_caption = TEM image of "L. pneumophila"
regnum = Bacteria
phylum = Proteobacteria
classis = Gamma Proteobacteria
ordo = Legionellales
familia = Legionellaceae
genus = "Legionella"
species = "L. pneumophila"
binomial = "Legionella pneumophila"
binomial_authority = Brenner DJ, Steigerwalt AG, McDade JE 1979

"Legionella pneumophila" is a thin, , flagellated
Gram-negative bacterium of the genus "Legionella".cite book | author = Madigan M; Martinko J (editors). | title = Brock Biology of Microorganisms | edition = 11th ed. | publisher = Prentice Hall | year = 2005 | id = ISBN 0-13-144329-1 ] cite book | author = Heuner K; Swanson M (editors). | title = Legionella: Molecular Microbiology | publisher = Caister Academic Press | year = 2008 | url=http://www.horizonpress.com/leg | id = [http://www.horizonpress.com/leg ISBN 978-1-904455-26-4 ] ] "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 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 leaflet of the outer cell membrane.cite book | author = Ryan KJ; Ray CG (editors) | title = Sherris Medical Microbiology | edition = 4th ed. | publisher = McGraw Hill | year = 2004 | id = ISBN 0-8385-8529-9 ] 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. Ameoba can serve as a reservoir for "L. pneumophila" as well as provide protection from environmental stresses such as chlorination. [Greub G, Raoult D. Morphology of Legionella pneumophila according to their location within Hartmanella vermiformis. Res. Microbiol. 2003 Nov;154(9):619-21. [http://dx.doi.org/10.1016/j.resmic.2003.08.003 DOI:10.1016/j.resmic.2003.08.003 ] ] 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 internalized, 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. Survival is enhanced by these effector proteins (Ank proteins) because they interfere with fusion of the "Legionella"-containing vacuole with the host's degradation endosomes. [Pan X, Lührmann A, Satoh A, Laskowski-Arce MA, Roy CR. Ankyrin repeat proteins comprise a diverse family of bacterial type IV effectors. "Science". 2008 Jun 20;320(5883):1651-4. PMID 18566289. [http://dx.doi.org/10.1126/science.1158160 DOI:10.1126/science.1158160] ]

They have a type IV secretion system that secretes a 39kDa metalloprotease into culture fluids, which is cytotoxic for some cultured tissue culture cells. The type IV secretion system is also required for full virulence [http://www.ncbi.nlm.nih.gov/pubmed/11254562?ordinalpos=15&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum] . 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" has not been demonstrated.cite book | author = Winn WC Jr | title = Legionella. "In:" Baron's Medical Microbiology "(Baron S "et al", eds.)| edition = 4th ed. | publisher = Univ of Texas Medical Branch | year = 1996 | id = [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.2229 (via NCBI Bookshelf)] ISBN 0-9631172-1-1 ]

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.cite book | author = Heuner K; Swanson M (editors). | title = Legionella: Molecular Microbiology | publisher = Caister Academic Press | year = 2008 | url=http://www.horizonpress.com/leg | id = [http://www.horizonpress.com/leg ISBN 978-1-904455-26-4 ] ]

References