Legionella pneumophila

Legionella pneumophila in a lung fibroblasts

Legionella pneumophila is a gram-negative, unbekapseltes and spore -free rod bacterium. It is the trigger of Legionnaires' disease and Pontiac fever.

Microbiological properties

Legionella pneumophila is an obligate aerobic, gram-negative rod with mostly monopolar flagellum. In wet mount usually find short rods, whereas in culture prevail mostly different sized rod bacteria (2-20 microns ). Legionella mainly use amino acids as an energy source. Sugar can not be metabolized to acids. Legionella are dependent on cysteine, and iron ( III) ions in the nutrient medium. Therefore, they are found in nature usually in combination with autotrophic microorganisms (eg, iron -manganese bacteria) or those found in amoeba (eg Acanthamoeba, Naegleria ). Amoebae appear to play an important role in the spread of Legionella pneumophila. Legionella do not form urease and nitratase, but are catalase positive. Legionella are sensitive to dehydration. At a temperature of 60 ° C, they survive for about five minutes.


Legionella pneumophila is found worldwide in soil and water samples. As a source of infection for people to find it in hot water pipes with insufficiently heated water (< 70 ° C), air conditioning systems and cooling towers.


Legionellosis usually occur sporadically, rarely in small epidemics. In the U.S. it is estimated that about 30 cases of illness per year and 100,000 inhabitants occur. The legionellosis have a peak in the summer months. In endemic areas, it is estimated that about 5% of pneumonia due to Legionella. For Germany it is estimated that nearly 4 % of all outpatient occurring pneumonias are caused by Legionella.


Legionnaires' disease is an atypical interstitial pneumonia with fever, diarrhea, headache and disorientation. It frequently affects immunocompromised people (eg, alcoholics, HIV sufferers ).

Pontiac fever is an acute, self-limited respiratory illness with cough and runny nose, which heals spontaneously usually after seven days.


The transmission is airborne usually via inhalation of aerosols containing pathogens from air - conditioning or water taps. It is estimated that only 10 % of infections really lead to disease. A human-to -human transmission does not occur.

Incubation period

The incubation period for Legionnaires' disease is two to ten days of Pontiac fever but only one to two days.


Legionella pneumophila adheres to host cells via a pilus and induces a special form of phagocytosis (called coiling phagocytosis ) but are missing in the intracellular killing and multiplies in non-activated phagocytic cells. Legionella pneumophila induces a partial necrotizing inflammation of the alveoli. The defense of Legionella pneumophila as an intracellular pathogen is likely dependent on T lymphocytes. These activate the macrophages containing pathogens, so that the pathogens can be destroyed inside the phagocytes. Legionella pneumophila produces exotoxins (eg, hemolysin ) and enzymes. The role in the pathological process of these exotoxins is not yet fully understood. The protein MIP ( macrophage infectivity potentiator ), a 24 kDa protein appears to be involved in phagocytosis induction.


The treatment of Legionnaires' disease usually occurs with macrolides (eg, erythromycin ), in combination with rifampicin or a fluoroquinolone. Pathogen detection is notifiable under § 7 of the Infection Protection Act.


  • Antigen detection: A Legionella pneumophila antigen detection can be carried out in the urine ( ELISA) and is the recommended guideline according to diagnostic method of choice in cases of suspected Legionella infection.
  • Microscopy: The Gram stain can be Legionella bad color. Therefore, usually one uses fluorescence - labeled monoclonal antibodies.
  • Cultivation: The cultivation may succeed from lung biopsies, bronchial and broncho- alveolar lavage. It is important that the laboratory is the suspected diagnosis of legionellosis telling as the pathogen is not to be put on routine culture media. The growth can succeed on buffered charcoal - yeast agar with addition of alpha- ketoglutarate ( BCYE alpha agar). Colonies appear at kapnophiler incubation in two to seven days.
  • PCR: The PCR allows rapid detection of excitation samples as bronchoalveolar lavage fluid, lung tissue, tracheal and sputum, but not in the urine. The sensitivity is very high, the results are sometimes better than culture methods.
  • Serology: In the blood, antibodies against L. pneumophila can be detected after about ten days.


Legionella pneumophila was discovered only in 1976, when at a meeting of the American Legion in Phildadelphia occurred an epidemic among the participating veterans. 29 of 182 sick veterans died at that time. At that time the Legionnaires' disease got its name.