Legionella genomospecies 1
Candidatus Legionella jeonii
Candidatus Legionella jeonii
Legionella is a pathogenic group of Gram-negative bacteria
that includes the species L. pneumophila, causing legionellosis
(all illnesses caused by Legionella) including a pneumonia-type
Legionnaires' disease and a mild flu-like illness
called Pontiac fever.
Legionella may be visualized with a silver stain or cultured in
cysteine-containing media such as buffered charcoal yeast extract
agar. It is common in many environments, including soil and aquatic
systems, with at least 50 species and 70 serogroups identified. The
bacterium, however, is not transmissible from person to person:
furthermore, most people exposed to the bacteria do not become ill.
The side chains of the cell wall carry the bases responsible for the
somatic antigen specificity of these organisms. 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 antigen determinants, which are essential means of
serologically classifying many Gram-negative bacteria.
Legionella acquired its name after an outbreak of a then-unknown
"mystery disease" sickened 221 persons, causing 34 deaths. The
outbreak was first noticed among people attending a convention of the
American Legion—an association of U.S. military veterans. The
convention occurred in
Philadelphia during the U.S. Bicentennial year
in July 21–24, 1976. This epidemic among U.S. war veterans,
occurring in the same city as—and within days of the 200th
anniversary of—the signing of the Declaration of Independence, was
widely publicized and caused great concern in the United States.
On January 18, 1977, the causative agent was identified as a
previously unknown bacterium subsequently named Legionella. See
Legionnaires' disease for full details.
2.1 Sources of Legionella
2.2 Airborne transmission from cooling towers
2.3 Vaccine research
3 Molecular biology
4.2 Copper-silver ionization
4.4 Moist heat sterilization
4.5 European standards
6 See also
8 External links
Legionella is traditionally detected by culture on buffered charcoal
yeast extract agar.
Legionella requires the presence of cysteine and
iron to grow, so does not grow on common blood agar media used for
laboratory-based total viable counts or on-site dipslides. Common
laboratory procedures for the detection of
Legionella in water
concentrate the bacteria (by centrifugation and/or filtration through
0.2-μm filters) before inoculation onto a charcoal yeast extract agar
containing selective agents (e.g. glycine, vancomycin, polymixin,
cyclohexamide, GVPC) to suppress other flora in the sample. Heat or
acid treatment are also used to reduce interference from other
microbes in the sample.
After incubation for up to 10 days, suspect colonies are confirmed as
Legionella if they grow on buffered charcoal yeast extract agar
containing cysteine, but not on agar without cysteine added.
Immunological techniques are then commonly used to determine the
species and/or serogroups of bacteria present in the sample.
Although the plating method is quite specific for most species of
Legionella, one study has shown that a coculture method that accounts
for the close relationship with amoebae may be more sensitive since it
can detect the presence of the bacteria even when masked by its
presence inside the amoeba. Consequently, the clinical and
environmental prevalence of the bacteria is likely to be
underestimated due to the current lab methodology.
Many hospitals use the
Legionella urinary antigen test for initial
Legionella pneumonia is suspected. Some of the
advantages offered by this test are that the results can be obtained
in hours rather than the several days required for culture, and that a
urine specimen is generally more easily obtained than a sputum
specimen. Disadvantages are that the urine antigen test only detects
Legionella pneumophila serogroup 1 (LP1); only a culture
will detect infection by non-LP1 strains or other
and that isolates of
Legionella are not obtained, which impairs public
health investigations of outbreaks.
New techniques for the rapid detection of
Legionella in water samples
have been developed, including the use of polymerase chain reaction
and rapid immunological assays. These technologies can typically
provide much faster results.
Government public health surveillance has demonstrated increasing
proportions of drinking water–associated outbreaks, specifically in
Legionella pneumophila bacterium (green) caught by a Vermamoeba
vermiformis amoeba (orange)
In the natural environment,
Legionella lives within amoebae such as
Naegleria spp., and Vermamoeba vermiformis, or
other protozoa such as Tetrahymena pyriformis.
Upon inhalation, the bacteria can infect alveolar macrophages, where
the bacteria can replicate. This results in
Legionnaires' disease and
the less severe illness Pontiac fever.
Legionella transmission is via
inhalation of water droplets from a contaminated source that has
allowed the organism to grow and spread (e.g., cooling towers).
Transmission also occurs less commonly via aspiration of drinking
water from an infected source. Person-to-person transmission has not
been demonstrated; though, it could be possible in rare cases.
Once inside a host, the incubation period may be up to two weeks.
Prodromal symptoms are flu-like, including fever, chills, and dry
cough. Advanced stages of the disease cause problems with the
gastrointestinal tract and the nervous system and lead to diarrhea and
nausea. Other advanced symptoms of pneumonia may also present.
However, the disease is generally not a threat to most healthy
individuals, and tends to lead to severe symptoms more often in
immunocompromised hosts and the elderly. Consequently, the water
systems of hospitals and nursing homes should be periodically
monitored. The Texas Department of State Health services provides
recommendations for hospitals to detect and prevent the spread of
hospital acquired disease due to
Legionella infection. According
to Infection Control and Hospital Epidemiology, hospital-acquired
Legionella pneumonia has a fatality rate of 28%, and the source is the
water distribution system.
Legionella species typically exist in nature at low concentrations, in
groundwater, lakes, and streams. They reproduce after entering
man-made equipment, given the right environmental conditions.[citation
needed] In the United States, the disease affects between 8,000 and
18,000 individuals a year.
Sources of Legionella
Documented sources include cooling towers, swimming pools
(especially in Scandinavian countries), domestic water systems and
showers, ice-making machines, refrigerated cabinets, whirlpool
spas, hot springs, fountains, dental equipment,
soil, automobile windshield washer fluid, and industrial
Airborne transmission from cooling towers
The largest and most common source of Legionnaires' disease
outbreaks are cooling towers (heat rejection equipment used in air
conditioning and industrial cooling water systems) primarily because
of the risk for widespread circulation. Many governmental agencies,
cooling tower manufacturers, and industrial trade organisations have
developed design and maintenance guidelines for controlling the growth
and proliferation of
Legionella within cooling towers.
Research in the Journal of Infectious Diseases (2006) provided
evidence that L. pneumophila, the causative agent of Legionnaires'
disease, can travel at least 6 km from its source by airborne
spread. It was previously believed that transmission of the bacterium
was restricted to much shorter distances. A team of French scientists
reviewed the details of an epidemic of
Legionnaires' disease that took
place in Pas-de-Calais, northern France, in 2003–2004. Of 86
confirmed cases during the outbreak, 18 resulted in death. The source
of infection was identified as a cooling tower in a petrochemical
plant, and an analysis of those affected in the outbreak revealed that
some infected people lived as far as 6–7 km from the plant.
No vaccine is available for legionellosis. Vaccination studies using
heat-killed or acetone-killed cells have been carried out in guinea
pigs, which were then given
Legionella intraperitoneally or by
aerosol. Both vaccines were shown to give moderately high levels of
protection. Protection was dose-dependent and correlated with antibody
levels as measured by enzyme-linked immunosorbent assay to an outer
membrane antigen and by indirect immunofluorescence to heat-killed
cells. However, a licensed vaccine for people in the
US is most probably still many years away.
Legionella has been discovered to be a genetically diverse species
with 7-11% of genes strain-specific. The molecular function of some of
the proven virulence factors of
Legionella have been discovered.
Legionella growth can occur through chemical or thermal
methods. The more expensive of these two options is
temperature control—i.e., keeping all cold water below 25 °C
(78 °F) and all hot water above 51 °C (124 °F). The
high cost incurred with this method arises from the extensive
retrofitting required for existing complex distribution systems in
large facilities and the energy cost of chilling or heating the water
and maintaining the required temperatures at all times and at all
distal points within the system.
Temperature affects the survival of
Legionella as follows:
Above 70 °C (158 °F) –
Legionella dies almost instantly
At 60 °C (140 °F) – 90% die in 2 minutes (Decimal
reduction time (D) = 2 minutes)
At 50 °C (122 °F) – 90% die in 80–124 minutes,
depending on strain (Decimal reduction time (D) = 80–124 minutes)
48 to 50 °C (118 to 122 °F) – can survive but do not
32 to 42 °C (90 to 108 °F) – ideal growth range
25 to 45 °C (77 to 113 °F) – growth range
Below 20 °C (68 °F) – can survive, even below freezing,
but are dormant
Other temperature sensitivity
60 to 70 °C (140 to 158 °F) to 80 °C (176 °F)
– Disinfection range
66 °C (151 °F) –
Legionella dies within 2 minutes
60 °C (140 °F) –
Legionella dies within 32 minutes
55 °C (131 °F) –
Legionella dies within 5 to 6 hours
A very effective chemical treatment is chlorine. For systems with
marginal issues, chlorine provides effective results at 0.5
ppm residual in the hot water system. For systems
Legionella problems, temporary shock
chlorination—where levels are raised to higher than 2 ppm for a
period of 24 hours or more and then returned to 0.5 ppm may be
effective. Hyperchlorination can also be used where
the water system is taken out of service and the chlorine residual is
raised to 50 ppm or higher at all distal points for 24 hours or more.
The system is then flushed and returned to 0.5 ppm chlorine prior to
being placed back into service. These high levels of chlorine
penetrate biofilm, killing both the
Legionella bacteria and the host
organisms. Annual hyperchlorination can be an effective part of a
Legionella preventive action plan.
Industrial-size copper-silver ionization is recognized by the U.S.
Environmental Protection Agency and WHO for
Legionella control and
prevention. Copper and silver ion concentrations must
be maintained at optimal levels, taking into account both water flow
and overall water usage, to control Legionella. The disinfection
function within all of a facility's water distribution network occurs
within 30 to 45 days. Key engineering features such
as 10 amps per ion chamber cell and automated variable voltage outputs
having no less than 100 VDC are but a few of the required features for
Legionella control and prevention, using a specific,
nonreferenced copper-silver system. Swimming pool ion generators are
not designed for potable water treatment.
Questions remain whether the silver and copper ion concentrations
required for effective control of symbiotic hosts could exceed those
allowed under the U.S. Safe Drinking Water Act's Lead and Copper Rule.
In any case, any facility or public water system using copper-silver
for disinfection should monitor its copper and silver ion
concentrations to ensure they are within intended levels – both
minimum and maximum. Further, no current standards for silver in the
EU and other regions allow use of this technology.
Copper-silver ionization is an effective process to control Legionella
in potable water distribution systems found in health facilities,
hotels, nursing homes, and most large buildings. However, it is not
intended for cooling towers because of pH levels greater than 8.6,
that cause ionic copper to precipitate. Furthermore, tolytriazole, a
common additive in cooling water treatment, could bind the copper
making it ineffective. In 2003, researchers who heavily support
ionization developed a validation process that supports their research
on ionization. Ionization became the first such hospital disinfection
process to have fulfilled a proposed four-step modality evaluation; by
then it had been adopted by over 100 hospitals. Additional studies
indicate ionization is superior to thermal eradication.
Chlorine dioxide has been approved by the U.S. Environmental
Protection Agency as a primary disinfectant of potable water since
Chlorine dioxide does not produce any carcinogenic byproducts
like chlorine when used in the purification of drinking water that
contains natural organic compounds such as humic and fulvic acids,
chlorine tends to form halogenated disinfection by-products such as
trihalomethanes. Drinking water containing such disinfection
by-products has been shown to increase the risk of cancer. ClO2 works
differently to chlorine; its action is one of pure oxidation rather
than halogenation, so these halogenated by-products are not
Chlorine dioxide is not a restricted heavy metal like
copper. It has proven excellent control of
Legionella in cold and hot
water systems and its ability as a biocide is not affected by pH, or
any water corrosion inhibitors such as silica or phosphate.
Monochloramine is an alternative. Like chlorine and chlorine dioxide,
monochloramine is approved Environmental Protection Agency as a
primary potable water disinfectant. Environmental Protection Agency
registration requires a biocide label which lists toxicity and other
data required for all registered biocides. If the product is being
sold as a biocide, then the manufacturer is legally required to supply
a biocide label, and the purchaser is legally required to apply the
biocide per the biocide label. When first applied to a system,
chlorine dioxide can be added at disinfection levels of 2 ppm for 6
hours to clean up a system. This will not remove all biofilm, but will
effectively remediate the system of Legionella.
Moist heat sterilization
Moist heat sterilization (superheating to 140 °F (60 °C)
and flushing) is a nonchemical treatment that typically must be
repeated every 3–5 weeks.
Several European countries established the European Working Group for
Legionella Infections to share knowledge and experience about
monitoring potential sources of Legionella. The working group has
published guidelines about the actions to be taken to limit the number
of colony-forming units (that is, live bacteria that are able to
Legionella per litre:
Legionella bacteria CFU/litre
Action required (35 samples per facility are required, including 20
water and 10 swabs)
1000 or less
System under control
more than 1000
up to 10,000
Review program operation: The count should be confirmed by immediate
resampling. If a similar count is found again, a review of the control
measures and risk assessment should be carried out to identify any
more than 10,000
Implement corrective action: The system should immediately be
resampled. It should then be "shot dosed" with an appropriate biocide,
as a precaution. The risk assessment and control measures should be
reviewed to identify remedial actions. (150+ CFU/ml in healthcare
facilities or nursing homes require immediate action.)
Monitoring guidelines are stated in Approved Code of Practice L8 in
the UK. These are not mandatory, but are widely regarded as so. An
employer or property owner must follow an Approved Code of Practice,
or achieve the same result. Failure to show monitoring records to at
least this standard has resulted in several high-profile prosecutions,
e.g. Nalco + Bulmers – neither could prove a sufficient scheme to be
in place whilst investigating an outbreak, therefore both were fined
about £300,000GBP. Important case law in this area is R v Trustees of
the Science Museum 3 All ER 853, (1993) 1 WLR 1171
Employers and those responsible for premises within the UK are
required under Control of Substances Hazardous to Health to undertake
an assessment of the risks arising from Legionella. This risk
assessment may be very simple for low risk premises, however for
larger or higher risk properties may include a narrative of the site,
asset register, simplified schematic drawings, recommendations on
compliance, and a proposed monitoring scheme.
The L8 Approved Code of Practice recommends that the risk assessment
should be reviewed at least every 2 years and whenever a reason exists
to suspect it is no longer valid, such as water systems have been
amended or modified, or if the use of the water system has changed, or
if there is reason to suspect that
Legionella control measures are no
It has been suggested that
Legionella could be used as a weapon, and
indeed genetic modification of
Legionella pneumophila has been shown
where the mortality rate in infected animals can be increased to
nearly 100%. A former Soviet bioengineer, Sergei Popov,
stated in 2000 that his team experimented with genetically enhanced
bioweapons, including Legionella. Popov worked as a lead
researcher at the Vector Institute from 1976 to 1986, then at Obolensk
until 1992, when he defected to the West. He later divulged much of
Soviet biological weapons program and settled in the United
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