What is Legionella?

Definition

Legionella pneumophila, the bacterium that causes
Legionnaires’
disease, leads to severe pneumonia and
is the most commonly known form of Legionella disease worldwide.

Natural Habitat and Features

Legionella are gram-negative rod-shaped bacteria that proliferate in water, mud, streams, and within some aquatic devices, such as decorative water fountains. Whirlpool spas have also been found to harbor Legionella, as have air conditioning systems and water heaters. Some forms of Legionella are known to infect such amebas as Acanthamoeba castellanii or Hartmanella species. Several cases of L. longbeachae have been identified in potting compost in Scotland. The transmission was believed to occur through the inhalation of droplets from the compost.

When not infecting humans or animals, Legionella reside within
fresh-water protozoa and amebas, and within biofilm, which is an aggregate of different types of
bacteria that link together, as on a water surface. Legionellae have been proven
to reproduce with fourteen different types of amebas, two species of protozoa, and
one species of slime mold. Legionellae alternate between two states, the
reproductive state and the transmissive state.

L. pneumophila thrives best at temperatures ranging from 60° to 107.6° Fahrenheit (20° to 42° Celsius). The incubation period for Legionella ranges from two to nineteen days. Most outbreaks occur in warm weather.

Pathogenicity and Clinical Significance

Legionella invades the host lungs, and the microbes are immediately attacked by the host
macrophages. However, Legionella converts these macrophages into
compartments in which the bacteria then multiply. This is similar to the means
Legionella employ when they grow within protozoa, which are
usually their hosts. Each year up to eighteen thousand people are
hospitalized for Legionnaires’ disease in the United States. Up to 30 percent of these
cases are fatal.

The presence of this pathogen in humans can be detected with sputum cultures or
with urinary antigen assays. A second test may be needed several days later if the
first test is negative and patients still present with symptoms that are
indicative of Legionella infection, such as vomiting, high fever,
chills, cough, diarrhea, and confusion. Legionella infection is
more likely to occur in persons who have already been hospitalized for another
illness and who then contract Legionella; it is also common in
those persons with compromised immune systems, such as persons with
cancer or with human immunodeficiency virus (HIV)
infection. In addition, persons who take immunosuppressive drugs, such as those
who have had organ transplants, have an elevated risk of
Legionella infection.

Legionella infection may present in hospitals in which patients live in close quarters. All patients diagnosed with pneumonia while in the hospital should be tested for Legionella.

Guinea pigs were the first species in which Legionella was
isolated in 1943, but the genus was not established until 1979, three years after
the 1976 outbreak, when more than two hundred persons attending an American Legion
conference at a hotel in Philadelphia became severely ill; twenty-nine attendees
died. The pathogen was traced to the air conditioning system of the hotel. A
milder form of this infection is referred to as Pontiac
fever, named after the city in Michigan where an outbreak of
Legionella infection occurred in 1968, infecting more than
sixty county health workers. This discovery was made retrospectively after the
1976 identification of Legionella. The source of the infection in
the Pontiac case was found to be the air conditioning system. In contrast to
Legionnaires’ disease, Pontiac fever may develop in healthy persons.

Legionella can be isolated in the blood, lung tissue, sputum, and stool. It has also been cultured from bone marrow.

Drug Susceptibility

Erythromycin is a common drug of choice for eradicating infection with
Legionella, although other macrolide
antibiotics, such as azithromycin or levofloxacin, have been
approved by the U.S. Food and Drug Administration for the
treatment of this infection. Legionella infection may initially
be misdiagnosed as Streptococcus pneumoniae infection, the most
common cause of pneumonia, if laboratory testing is not performed.

Bibliography

Bitar, Dina M., et al. “Legionnaires’ Disease and Its Agent Legionella pneumophila.” In Community-Acquired Pneumonia, edited by Norbert Suttorp, Tobias Welte, and Reinhard Marre. Boston: Birkhäuser, 2007. Covers Legionnaires’ disease while the rest of the book covers other forms of pneumonia that can develop in personsliving in communities other than nursing homes or hospitals.

Dirven, Kristien, et al. “Comparison of Three Legionella Urinary Antigen Assays During an Outbreak of Legionellosis in Belgium.” Journal of Medical Microbiology 54 (2005): 1213-1216. Compares and contrasts the use of urinary antigen tests to identify the presence of Legionella.

Fields, Barry S., Robert F. Benson, and Richard E. Besser. “ Legionella and Legionnaires’ Disease: Twenty-five Years of Investigation.” Clinical Microbiology Reviews 15 (2002): 506-526. This article discusses the clinical presentation, life cycle, diagnosis, and epidemiologic trends of Legionella.

Pravinkumar, S. J., et al. “A Cluster of Legionnaires’ Disease Caused by Legionella longbeachae Linked toPotting Compost in Scotland, 2008-2009.” European Surveillance 15 (2010). Available at http://www. eurosurveillance.org/viewarticle.aspx?articleid= 19496. Discusses the finding of a form of Legionella that was identified in potting soil rather than in the aquatic environment with which it was usually associated.