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Legionella pneumophila
cellular organisms
Legionella
- Bacteria - Proteobacteria - Gammaproteobacteria - Legionellales - Legionellaceae -
- Legionella pneumophila
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Brief facts
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Regular phagocytosis vs. Legionella intracellular parasitism
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Legionella life cycles
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References
Brief facts
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Legionellae are Gram-negative aerobic rods, isolated from surface water,
mud, or thermally polluted lakes or streams. It is pathogenic for man and it
has no known soil or animal sources. Legionella species are widespread in
natural and man-made fresh water reservoirs. One of interesting
characteristics of these bacteria is the ability to parasitize intracellularly on
eukariotic cells. Majority of Legionella species infect free-living ameobae such
as Hartmanella sp. and Acanthamoeba castellanii. The species grow best at
temperatures from 20°C to 42°C. However, some of Legionella species are
able to infect mammalian alveolar macrophages and cause a disease.
Legionella pneumophila are Gram-negative aerobic coccobacilii, isolated
from surface water, mud, or thermally polluted lakes or streams. It is
pathogenic for man and it has no known soil or animal sources. Legionella
pneumophila is a main causative agent of Legionnaires' Disease. The
species is responsible for more than 98% of the Legionnaires' Disease cases,
and about 95% are due to serogroup 1. It has been isolated from numerous
environmental sites as well as from human lung tissue, respiratory secretions,
and blood.
Legionnaires' Disease (LD) is an acute, sometimes fatal, pneumonia-like
bacterial infection characterized by high fever, malaise, muscle aches,
respiratory disorders and headache. It is named for an outbreak at the 1976
Philadelphia convention of the American Legion. Legionnaires' disease can be
very serious and can cause death in up to 5% to 30% of cases. Most cases
can be treated successfully with antibiotics. The incubation period is about 7
days (from 2 to 19 days). Sporadic cases occur year around, but most cases
of outbreaks happen in the warmer periods of the year. It is impossible to
clinically distinguish patients with LD from patients with pneumococcal
pneumonia. Culturing bacteria from biological samples, detection of antibodies
in blood, detection of Legionella antigens in urine, and specific amplification of
Legionella DNA by Polymerase Chain Reaction (PCR) are methods that can be
applied to diagnose the disease with various degree of certainty depending on
time and reagents' availability, and specific needs.
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Pontiac fever is a milder infection caused by the same type of Legionella
bacteria. The symptoms of Pontiac Fever (called after Pontiac in Michigan,
where it was first documented) usually last for 2 to 5 days and may also
include fever, headaches, and muscle aches (myalgia; however, there is no
pneumonia. Symptoms usually go away without treatment.
Transmission
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Legionella come from natural fresh water reservoirs, such as lakes, ponds,
and puddles, where they parasitize on a broad range of protozoan species as
hosts. The availability of the hosts plays a major role in the reproduction and
mass release of highly infectious Legionella forms into environments where
they can be spread by airborn water caplets and inhaled by people.
Legionella thrive in man-made aquatic environments where the water
temperature is higher than ambient temperature: whirlpool spas, cooling
towers (air-conditioning units from large buildings), water used for drinking
and bathing, water fountains, humidifiers, ice machines, and vegetable
misters, etc. In fact, most cases of legionellosis can be traced to using such
systems. It is a major concern for health professionals and construction and
water systems maintenance workers.
Legionella bacilii enter the human body through inhalation of contaminated
aerosols. They can reach the alveolae in the lungs where they are attacked
and engulfed by alveolar macrophages. Instead of being killed the bacilii
hijack the phagosomes, convert them into specialized membrane-bound
compartments, and start living and multiplying in a manner indistinguishable
from that found during growth within amoebae, their usual hosts.
Infection of the human macrophages appears to be a dead end in spreading
the bacteria because transmission of Legionella pneumophila among humans
has not been observed so far.
Intracellular parasitism
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Interest in this bacterium stems from its ability to manipulate host cell
vesicular-trafficking pathways and establish a membrane-bound replication
vacuole, making it model for intracellular or, more specifically,
intravacuolar pathogens.
Intracellular lifestyle of these microorganisms allows them to gain the
following competitive advantages relative to other microorganisms: (1)
evasion of predators in the environment and (2) evasion of host's humoral
and cellular immune responses while having easy access to all necessary
nutrients.
To be successful these pathogens need to solve the following important
problems: (1) prevent the newly formed vacuole from merging (fusing) into
antimicrobial lysosomal network; (3) manage nutrients's acquisition through
the vacuolar membrane; (3) deal with space limitations during proliferative
stage.
Regular phagocytosis vs. Legionella intracellular parasitism
Life cycle of Legionella
Life Cycle Stages
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Laboratory in vitro culture in broth
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Exponential
phase bacteria
Replicative (non-infectious)forms of bacteria
in exponential growth stage, unflagellated and
sodium resistant
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Post-exponential
(stationary)
phase bacteria
Infectious forms of bacteria, flagellated and
motile; they appear when nutrients become
sparse and the pathohens become overcrowded
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Natural life cycle
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Intracellular form
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Non-infectious
replicating form
Bacteria actively replicating in LCV
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Differentiating form
Bacteria at last rounds of replication
when daughter cells acquire virulent
traits (flagellae, resistance to
environmental stresses, etc.)
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Free-swimming form
Transmissive virulent form of bacteria that
escaped from the previous host and are ready to
penetrate next one
References
PubMed articles
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Isberg RR, O'Connor TJ, Heidtman M. The Legionella pneumophila replication vacuole:
making a cosy niche inside host cells. Nat Rev Microbiol. 2009 Jan;7(1):13-24. PMID:
19011659
Diederen BM. Legionella spp. and Legionnaires' disease. J Infect. 2008 Jan;56(1):1-12.
PMID: 17980914
Jules M, Buchrieser C. Legionella pneumophila adaptation to intracellular life and the host
response: clues from genomics and transcriptomics. FEBS Lett. 2007 Jun 19;581
(15):2829-38. PMID: 17531986
Albert-Weissenberger C, Cazalet C, Buchrieser C. Legionella pneumophila - a human
pathogen that co-evolved with fresh water protozoa. Cell Mol Life Sci. 2007 Feb;64
(4):432-48. PMID: 17192810
Brüggemann H, Cazalet C, Buchrieser C. Adaptation of Legionella pneumophila to the
host environment: role of protein secretion, effectors and eukaryotic-like proteins. Curr
Opin Microbiol. 2006 Feb;9(1):86-94. PMID: 16406773
Molofsky AB, Swanson MS. Differentiate to thrive: lessons from the Legionella
pneumophila life cycle. Mol Microbiol. 2004 Jul;53(1):29-40. PMID: 15225301
Major topic "Legionella": free full text articles in PubMed
Websites
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Life cycle, growth cycles and developmental cycle of Legionella pneumophila
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Wikipedia: Legionella
Last updated 04/07/09
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