Legionella pneumophila
Encyclopedia
Legionella pneumophila is a thin, ærobic, pleomorphic, flagellated, non-spore forming, Gram-negative
bacterium of the genus Legionella
. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of legionellosis
or Legionnaires' disease.
acid-fast
, non-sporulating
, and morphologically a non-capsulated rod-like bacteria, often characterized as being a coccobacillus
. Ærobic
and unable to hydrolyse
gelatin
or produce urease
, they are also non-fermentative
. L. pneumophila is neither pigment
ed nor does it autofluoresce
. It is oxidase
- and catalase
-positive, and produces beta-lactamase
. L. pneumophila has a colony morphology that is gray-white with textured cut-glass appearance; it also requires cysteine
and iron
to thrive.
-content in the outer leaflet of the outer cell membrane. The bases for the somatic antigen
specificity of these organisms are located on the side-chains of its cell wall. 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.
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.
.
are caused by L. pneumophila.
s. 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 endosome
s.
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.
) or fluoroquinolones (moxifloxacin
) are the standard treatment for Legionella pneumonia in humans.
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
bacterium of the genus Legionella
Legionella
Legionella is a pathogenic Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. It may be readily visualized with a silver stain....
. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of legionellosis
Legionellosis
Legionellosis is a potentially fatal infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella. Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in temperatures between , with an optimum temperature...
or Legionnaires' disease.
Characterization
L. pneumophila is anacid-fast
Acid-fast
Acid-fastness is a physical property of certain bacteria, specifically their resistance to decolorization by acids during staining procedures.Acid-fast organisms are difficult to characterize using standard microbiological techniques Acid-fastness is a physical property of certain bacteria,...
, non-sporulating
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
, and morphologically a non-capsulated rod-like bacteria, often characterized as being a coccobacillus
Coccobacillus
A coccobacillus is a type of rod-shaped bacteria. The word coccobacillus reflects an intermediate shape between coccus and bacillus . Coccobacilli rods are so short and wide that they resemble cocci. Haemophilus influenzae and Chlamydia trachomatis are coccobacilli...
. Ærobic
Aerobic organism
An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment.Faculitative anaerobes grow and survive in an oxygenated environment and so do aerotolerant anaerobes.-Glucose:...
and unable to hydrolyse
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
gelatin
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
or produce urease
Urease
Urease is an enzyme that catalyzes the hydrolysis of urea into carbon dioxide and ammonia. The reaction occurs as follows:In 1926, James Sumner showed that urease is a protein. Urease is found in bacteria, yeast, and several higher plants. The structure of urease was first solved by P.A...
, they are also non-fermentative
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
. L. pneumophila is neither pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
ed nor does it autofluoresce
Autofluorescence
Autofluorescence is the natural emission of light by biological entities such as mitochondria and lysosomes, and is used to distinguish the light originating from artificially added fluorescent markers...
. It is oxidase
Oxidase
An oxidase is any enzyme that catalyzes an oxidation-reduction reaction involving molecular oxygen as the electron acceptor. In these reactions, oxygen is reduced to water or hydrogen peroxide ....
- and catalase
Catalase
Catalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen...
-positive, and produces beta-lactamase
Beta-lactamase
Beta-lactamases are enzymes produced by some bacteria and are responsible for their resistance to beta-lactam antibiotics like penicillins, cephamycins, and carbapenems . These antibiotics have a common element in their molecular structure: a four-atom ring known as a beta-lactam...
. L. pneumophila has a colony morphology that is gray-white with textured cut-glass appearance; it also requires cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
to thrive.
Cell membrane structure
While L. pneumophila is categorized as a Gram-negative organism, it stains poorly due to its unique lipopolysaccharideLipopolysaccharide
Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
-content in the outer leaflet of the outer cell membrane. The bases for the somatic antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
specificity of these organisms are located on the side-chains of its cell wall. 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.
Detection
SeraSera
Sera may refer to:- Music :* Sera, a Symphonic rock/electronica singer/composer/producer* Sera, the well known bass player in Greece- Places :* Sera, Hiroshima, a town in Japan* Sera District, Hiroshima, a district located in Hiroshima, Japan...
have been used both for slide agglutination studies as well as for direct detection of bacteria in tissues using fluorescent
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
-labelled antibody
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
. Specific antibody in patients can be determined by the indirect fluorescent antibody test. ELISA
ELISA
Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
and microagglutination tests have also been successfully applied.
Reservoir
L. pneumophila is a facultative intracellular bacterium that can invade and replicate inside amoebae in the environment. Ameoba can thus serve as a reservoir for L. pneumophila as well as provide protection from environmental stresses such as chlorinationChlorination
Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water...
.
Frequency of occurrence
In the USA about 30 infections with L. pneumophila appear per 100.000 residents per year. The infections peak within the summer months. Within endemic regions, about 4% to 5% of PneumoniaPneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...
are caused by L. pneumophila.
Pathogenesis
In humans, L. pneumophila invades and replicates in macrophageMacrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s. The internalisation of the bacteria can be enhanced by the presence of antibody
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
and complement
Complement system
The complement system helps or “complements” the ability of antibodies and phagocytic cells to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime...
but is not absolutely required. A pseudopod
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
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 endosome
Endosome
In biology, an endosome is a membrane-bound compartment inside eukaryotic cells. It is a compartment of the endocytic membrane transport pathway from the plasma membrane to the lysosome. Molecules internalized from the plasma membrane can follow this pathway all the way to lysosomes for...
s.
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 LegionellaLegionella
Legionella is a pathogenic Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. It may be readily visualized with a silver stain....
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
Acanthamoeba
Acanthamoeba is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in fresh water and other habitats. The cells are small, usually 15 to 35 μm in length and oval to triangular in shape when moving. The pseudopods form a clear hemispherical lobe at the anterior,...
castellanii, its natural host. L. pneumophila exhibits a biphasic life cycle and defines transmissive and replicative traits according to gene expression profiles.
Treatment
Macrolides (azithromycinAzithromycin
Azithromycin is an azalide, a subclass of macrolide antibiotics. Azithromycin is one of the world's best-selling antibiotics...
) or fluoroquinolones (moxifloxacin
Moxifloxacin
Moxifloxacin is a fourth-generation synthetic fluoroquinolone antibacterial agent developed by Bayer AG . It is marketed worldwide under the brand names Avelox, Avalox, and Avelon for oral treatment. In most countries, the drug is also available in parenteral form for intravenous infusion...
) are the standard treatment for Legionella pneumonia in humans.