Legionella: A Stealthy Bacteria Hiding in Plain Sight
By Roberto Juarez Jr. BS, M(ASCP)CM
📌 In this post, we talk about how Legionella bacteria was discovered, where it is most likely to be found, Legionnaire’s and Pontiac fever, and the best testing method Microbiology Specialists Inc. use to isolate Legionella bacteria from clinical and environmental sample sources.
The Discovery of Legionella
In 1976, during an American Legion (an association of US military veterans) convention in Philadelphia, 221 attendees became ill with pneumonia and 21 died as a result of a mysterious disease. Further studies concluded the bacteria Legionella pneumophila, the first named member of the family Legionellaceae, to be the causative agent. The bacteria caused a type of pneumonia that eventually became known as Legionnaires’ disease.
The first cases of Pontiac fever were found at Pontiac, Michigan in 1968 among people who worked at and visited the city’s health department. Health department officials were able to later prove Pontiac fever and Legionnaires’ disease were both caused by exposure to Legionella bacteria.
Legionnaires’ Disease Explained
Legionnaires’ disease is a lung infection that typically manifests as pneumonia, high fever, and occasionally abdominal pain and diarrhea. Other symptoms may include:
- Muscle aches
- Headache
- Fatigue
- Loss of appetite
- Cough
About 70% of Legionnaires’ cases are caused by Legionella pneumophila while 20% to 25% of cases are associated with travel.
Host factors such as suppressed immune system, chronic lung disease, alcoholism, and heavy smoking, predispose individuals to Legionnaire’s infection.
What is Pontiac Fever?
Pontiac fever is also caused by exposure to Legionella bacteria but is a milder “flu-like” illness compared to Legionnaires’. Some symptoms include fever, chills, headache, muscle aches and loss of appetite. It often does not require treatment and most people recover in 2 to 5 days.
Antimicrobial Susceptibility Testing for Legionella
Susceptibility testing is not standardized or normally performed for samples containing Legionella. If infections are diagnosed early, a macrolide such as azithromycin or a fluoroquinolone can be used to successfully treat patients.
Where is Legionella found?
Legionella are aerobic, gram-negative bacteria that can be found worldwide, occurring naturally in lakes, rivers, hot springs, and soil. They are generally not present in sufficient numbers in natural settings to cause disease.
Legionella can be transmitted to people when they inhale small droplets of water that contain the bacteria. Transmission between humans is rare, as reported by one 2016 study.1
Legionella can thrive in human-made environmental sources such as potable water distribution systems, hot water systems, cooling towers, shower heads and sink faucets, ornamental fountains, whirlpool spas, respiratory therapy equipment, and large, complex plumbing systems. Since Legionella species can tolerate chlorine concentrations of 3 mg/L, they resist water treatment and eventually colonize human-made water distribution systems. One notable species is Legionella longbeachae because it has been associated with human infections who have been exposed to gardening materials, such as compost and potting soil.
Some factors that contribute to Legionella colonizing these water sources include:
- Can multiply between 20℃ to 43℃ and survive for varying periods between 40℃ to 60℃.
- Ability to adhere to pipes, rubber, plastics, and persist in piped water systems even when flushed.
- Can survive and multiply within free-living protozoa and algae.
Isolation and Identification of Legionella Bacteria
While several methods are available for isolating and identifying Legionella, the most important test is culture of the organism. After processing the specimen, the sample is inoculated on specialized solid media and incubated at 35℃ for up to 7 days. Suspect colonies are subcultured on special media and identification is confirmed using Matrix-Assisted Laser Desorption Ionization–Time of Flight (MALDI-ToF).
Potable and nonpotable water samples are sufficient for detecting Legionella bacteria in environmental samples. Clinical samples for culture include sputum, bronchoalveolar lavage, and bronchial washings.
Microbiology Specialists Inc. is a full service clinical and environmental microbiology laboratory in Houston, Texas and a CDC E.L.I.T.E.* certified lab that uses the culture method to isolate and MALDI-ToF to identify Legionella from a variety of sources including clinical samples, water cooling towers, hospital sinks, and other potable and non-potable water sources. Our experienced team ensures accurate results in a timely fashion. If your facility suspects or needs to rule out the presence of Legionella, trust the specialists at Microbiology Specialists Inc. for your Legionella testing needs. Call us today at (713) 663-6888 for more information.
*Environmental Legionella Isolation Techniques Evaluation
Sources
- Ana Correia, M.D et. al, “Probable Person-to-Person Transmission of Legionnaires’ Disease.” The New England Journal of Medicine, 4 Feb. 2016,
- “Legionnaires Disease History, Burden, and Trends.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 25 Mar. 2021,
- Mahon, Connie R., and Donald C. Lehman. “Legionella.” Textbook of Diagnostic Microbiology, Elsevier, Maryland Heights, Missouri, 2015, pp. 408–414.