''Legionella pneumophila'' is a thin,
aerobic,
pleomorphic,
flagellated, non-spore-forming,
Gram-negative
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...
bacterium of the genus ''
Legionella''.
''L. pneumophila'' is the primary human
pathogenic bacterium in this group and is the causative agent of
Legionnaires' disease, also known as legionellosis.
In nature, ''L. pneumophila'' infects freshwater and soil
amoebae of the genera ''
Acanthamoeba'' and ''
Naegleria''. The mechanism of infection is similar in amoeba and human cells.
Characterization
''L. pneumophila'' is a Gram-negative, non-encapsulated, aerobic bacillus with a single, polar flagellum often characterized as being a
coccobacillus. It is
aerobic and unable to
hydrolyse
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
gelatin or produce
urease. It is also non-
fermentative. ''L. pneumophila'' is neither
pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
ed nor does it
autofluoresce. It is
oxidase- and
catalase-positive, and produces
beta-lactamase. ''L. pneumophila'' colony morphology is gray-white with a textured, cut-glass appearance; it also requires
cysteine and
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
to thrive. It grows on
yeast extract agar in "opal-like" colonies.
Cell membrane structure
While ''L. pneumophila'' is categorized as a Gram-negative organism, it stains poorly due to its unique
lipopolysaccharide
Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O- antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the out ...
content in the outer leaflet of the outer cell membrane.
The bases for the somatic
antigen
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respon ...
specificity of this organism are located on the side chains of its cell wall. The chemical composition of these side chains both with respect to components and arrangement of the different sugars, determines the nature of the somatic or O-
antigenic 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
Sera have been used both for slide
agglutination
In linguistics, agglutination is a morphological process in which words are formed by stringing together morphemes, each of which corresponds to a single syntactic feature. Languages that use agglutination widely are called agglutinative la ...
studies and for direct detection of bacteria in tissues using
immunofluorescence
Immunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on microbiological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to spe ...
via
fluorescent
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, ...
-labelled
antibody
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of t ...
. Specific antibody in patients can be determined by the indirect fluorescent antibody test.
ELISA
The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay uses a solid-phase type of enzyme immunoassay (EIA) to detect the presen ...
and microagglutination tests have also been successfully applied.
''Legionella'' stains poorly with Gram stain, stains positive with silver, and is cultured on charcoal yeast extract with iron and cysteine.
Ecology and reservoirs
''L. pneumophila'' is a
facultative intracellular parasite that can invade and replicate inside
amoebae in the environment, especially species of the genera ''
Acanthamoeba'' and ''
Naegleria'', which can thus serve as a reservoir for ''L. pneumophila.'' These
hosts
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
People
*Jim Host (born 1937), American businessman
*Michel Host ...
also provide protection from environmental stresses, such as
chlorination. Legionella has been shown to proliferate on the walls of pipes in biofilms.
Sloughed legionella from biofilms in plumbing systems can be aerosolized through faucets, showers, sprinklers, and other fixtures which can lead to infection after prolonged exposure.
Frequency of occurrence
In the United States, approximately 3 infections with ''L. pneumophila'' appear per 100,000 people per year. The infections peak in the summer. Within endemic regions, about 4% to 5% of
pneumonia
Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severit ...
cases are caused by ''L. pneumophila''.
Pathogenesis
In humans, ''L. pneumophila'' invades and replicates inside
macrophages. The internalization of the bacteria can be enhanced by the presence of
antibody
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of t ...
and
complement, but is not absolutely required. Internalization of the bacteria appears to occur through
phagocytosis. However, ''L. pneumophila'' is also capable of infecting non-phagocytic cells through an unknown mechanism. A rare form of phagocytosis known as coiling phagocytosis has been described for ''L. pneumophila'', but this is not dependent on the Dot/Icm (intracellular multiplication/defect in organelle trafficking genes)
bacterial secretion system and has been observed for other pathogens. Once internalized, the bacteria surround themselves in a membrane-bound
vacuole
A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...
that does not fuse with
lysosomes that would otherwise degrade the bacteria. In this protected compartment, the bacteria multiply.
Dot/Icm type IV secretion system and effector proteins
The bacteria use a type IVB secretion system known as Dot/Icm to inject
effector proteins into the host. These effectors are involved in increasing the bacteria's ability to survive inside the host cell. ''L. pneumophila'' encodes for over 330 "effector" proteins, which are secreted by the Dot/Icm translocation system to interfere with host cell processes to aid bacterial survival. It has been predicted that the genus ''Legionella'' encodes more than 10,000 and possibly up to ~18,000 effectors that have a high probability to be secreted into their host cells.
One key way in which ''L. pneumophila'' uses its effector proteins is to interfere with fusion of the ''Legionella''-containing vacuole with the host's
endosomes, and thus protect against lysis. Knock-out studies of Dot/Icm translocated effectors indicate that they are vital for the intracellular survival of the bacterium, but many individual effector proteins are thought to function redundantly, in that single-effector knock-outs rarely impede intracellular survival. This high number of translocated effector proteins and their redundancy is likely a result of the bacterium having evolved in many different
protozoan hosts.
''Legionella''-containing vacuole
For ''Legionella'' to survive within macrophages and protozoa, it must create a specialized compartment known as the ''Legionella''-containing vacuole (LCV). Through the action of the Dot/Icm secretion system, the bacteria are able to prevent degradation by the normal endosomal trafficking pathway and instead replicate. Shortly after internalization, the bacteria specifically recruit
endoplasmic reticulum-derived vesicles and
mitochondria to the LCV while preventing the recruitment of endosomal markers such as
Rab5a and
Rab7a. Formation and maintenance of the vacuoles are crucial for pathogenesis; bacteria lacking the Dot/Icm secretion system are not pathogenic and cannot replicate within cells, while deletion of the Dot/Icm effector SdhA results in destabilization of the vacuolar membrane and no bacterial replication.
Nutrient acquisition
Legionella is
auxotrophic for seven amino acids: cysteine, leucine, methionine, valine, threonine, isoleucine, and arginine. Once inside the host cell, ''Legionella'' needs nutrients to grow and reproduce. Inside the vacuole, nutrient availability is low; the high demand of
amino acids is not covered by the transport of free amino acids found in the host
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
. To improve the availability of amino acids, the parasite promotes the host mechanisms of
proteasomal degradation. This generates an excess of free amino acids in the cytoplasm of ''L. pneumophila''-infected cells that can be used for intravacuolar proliferation of the parasite.
To obtain amino acids, ''L. pneumophila'' uses the
AnkB F-Box effector, which is farnesylated by the activity of three host enzymes localized in the membrane of the LCV:
farnesyltransferase,
Ras-converting enzyme-1 protease, and ''
ICMT''.
Farnesylation allows AnkB to get anchored into the cytoplasmic side of the vacuole.
Once AnkB is anchored into the LCV membrane, it interacts with the
SCF1 ubiquitin ligase complex and functions as a platform for the docking of K48-linked polyubiquitinated proteins to the LCV.
The K48-linked polyubiquitination is a marker for proteasomal degradation that releases two- to 24-amino-acid-long peptides, which are quickly degraded to amino acids by various
oligopeptidases and
aminopeptidase
Aminopeptidases are enzymes that catalyze the cleavage of amino acids from the amino terminus ( N-terminus) of proteins or peptides (exopeptidases). They are widely distributed throughout the animal and plant kingdoms and are found in many sub ...
s present in the cytoplasm. Amino acids are imported into the LCV through various amino acid transporters such as the
neutral amino acid transporter B(0).
The amino acids are the primary carbon and energy source of ''L. pneumophila'', that have almost 12 classes of
ABC-transporters
The ATP-binding cassette transporters (ABC transporters) are a transport system superfamily that is one of the largest and possibly one of the oldest gene families. It is represented in all extant phyla, from prokaryotes to humans. ABC tran ...
,
amino acid permeases, and many
proteases, to exploit it. The imported amino acids are used by ''L. pneumophila'' to generate energy through the
TCA cycle
The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and prote ...
(Krebs cycle) and as sources of carbon and nitrogen.
However, promotion of proteasomal degradation for the obtention of amino acids may not be the only
virulence strategy to obtain carbon and energy sources from the host. Type II–secreted degradative enzymes may provide an additional strategy to generate carbon and energy sources.
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 high genome plasticity and frequent
horizontal gene transfer. Further insight in the ''L. pneumophila''
lifecycle
Life cycle, life-cycle, or lifecycle may refer to:
Science and academia
*Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring
*Life-cycle hypothesis, ...
was gained by investigating the gene expression profile of ''L. pneumophila'' in ''Acanthamoeba castellanii'', its natural host. ''L. pneumophila'' exhibits a biphasic lifecycle and defines transmissive and replicative traits according to gene expression profiles.
Genetic transformation
Transformation is a bacterial adaptation involving the transfer of DNA from one bacterium to another through the surrounding liquid medium. Transformation is a bacterial form of
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
.
In order for a bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must enter a special physiological state referred to as "
competence".
To determine which molecules may induce competence in ''L. pneumophila'', 64 toxic molecules were tested.
Only six of these molecules, all DNA-damaging agents, caused strong induction of competence. These were
mitomycin C (which introduces DNA inter-strand crosslinks),
norfloxacin,
ofloxacin, and
nalidixic acid (inhibitors of
DNA gyrase
DNA gyrase, or simply gyrase, is an enzyme within the class of topoisomerase and is a subclass of Type II topoisomerases that reduces topological strain in an ATP dependent manner while double-stranded DNA is being unwound by elongating RNA-poly ...
that cause double-strand breaks),
bicyclomycin (causes double-strand breaks), and
hydroxyurea (causes oxidation of DNA bases). These results suggest that competence for transformation in ''L. pneumophilia'' evolved as a response to
DNA damage
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
.
Perhaps induction of competence provides a survival advantage in a natural host, as occurs with other pathogenic bacteria.
Drug Targets
Several enzymes in the bacteria have been proposed as tentative drug targets. For example, enzymes in the iron uptake pathway have been suggested as important drug targets. Further, a cN-II class of IMP/GMP specific
5´-nucleotidase which has been extensively characterized kinetically. The tetrameric enzyme shows aspects of positive homotropic
cooperativity, substrate activation and presents a unique
allosteric site that can be targeted to design effective drugs against the enzyme and thus, the organism. Moreover, the enzyme is distinct than its human counterpart making it an attractive target for drug development.
Treatment
Macrolides (
azithromycin or
clarithromycin) or
fluoroquinolones (
levofloxacin
Levofloxacin, sold under the brand name Levaquin among others, is an antibiotic medication. It is used to treat a number of bacterial infections including acute bacterial sinusitis, pneumonia, H. pylori (in combination with other medications), ...
or
moxifloxacin
Moxifloxacin is an antibiotic, used to treat bacterial infections, including pneumonia, conjunctivitis, endocarditis, tuberculosis, and sinusitis. It can be given by mouth, by injection into a vein, and as an eye drop.
Common side effects ...
) are the standard treatment for ''Legionella'' pneumonia in humans, with levofloxacin being considered first line with increasing resistance to azithromycin. Two studies support superiority of levofloxacin over macrolides, although not FDA approved.
[The Sanford Guide to Antimicrobial Therapy 2013]
References
External links
Major topic "Legionella pneumophila": free-full text articles in PubMedLegionella pneumophila, causative agent of Legionnaire's disease and Pontiac fever at MetaPathogenType strain of ''Legionella pneumophila'' at Bac''Dive'' - the Bacterial Diversity Metadatabase
{{Taxonbar, from=Q147885
Legionellales
Gram-negative bacteria
Bacteria described in 1979