Legionella pneumophila in a pulmonary fibroblast
|Brenner et al. 1979|
Legionella pneumophila is a gram-negative, unencapsulated and spore-free rod bacterium. It is the cause of Legionnaires' disease and Pontiac fever . The most important (human pathogenic) serotypes of Legionella pneumophilafor humansare serotypes 1 (70 to 80 percent of all legionnaires in humans), 4 and 6.
Legionella pneumophila is an obligatory aerobic , gram-negative rod with mostly monopolar flagellation . In the native preparation, mostly short rods are found, while rod bacteria of different sizes (2–20 µm) predominate in culture. Legionella mainly use amino acids as an energy source. Sugars cannot be metabolized into acids. Legionella depend on cysteine and iron (III) ions in the nutrient medium. Therefore, they are usually found in the wild in combination with autotrophic microorganisms (e.g. iron-manganese bacteria) or they are found in amoebas ( e.g. Acanthamoeba , Naegleria ). Amoebas appear to play an important role in the spread of Legionella pneumophila. Legionella do not form urease and nitrase , 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 humans, it can be found in hot water pipes with insufficiently heated water (<60 ° C), air conditioning systems and cooling towers.
Legionnaires usually occur sporadically, less often in small epidemics . In the United States, it is estimated that around 30 cases occur per year for every 100,000 population. The legionnaires have a peak in the summer months. In endemic areas, it is estimated that around 5% of pneumonia can be traced back to Legionella. For Germany, it is estimated that almost 4% of all outpatient pneumonia can be traced back to Legionella.
The transmission is usually airborne, mostly by inhaling aerosols containing pathogens from air conditioning systems or water taps. It is estimated that only 10% of infections actually lead to disease. There is no person-to-person transmission.
The incubation period for Legionnaires' disease is two to ten days, but that of Pontiac fever is only one to two days.
Legionella pneumophila adheres to host cells via a pilus and induces a special form of phagocytosis (so-called coiling phagocytosis ), but escapes intracellular destruction and multiplies in non-activated phagocytic cells. Legionella pneumophila induces a partially necrotizing inflammation of the alveoli . The defense against Legionella pneumophila as an intracellular pathogen is probably dependent on T lymphocytes . These activate the macrophages containing the pathogen so that the pathogens inside the phagocytes can be destroyed. Legionella pneumophila produces exotoxins (such as hemolysin ) and enzymes . The role of these exotoxins in the disease process has not yet been conclusively clarified. The protein MIP (macrophage infectivity potentiator), a 24 kDA protein, appears to be involved in the induction of phagocytosis.
Legionnaires' disease is usually treated with macrolides ( e.g. erythromycin ) in combination with rifampicin or a fluoroquinolone . The pathogen detection is notifiable according to § 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 diagnostic method of choice in cases of suspected Legionella infection.
- Microscopy : Legionella is difficult to stain with the Gram stain. Therefore, fluorescence- labeled monoclonal antibodies are mostly used .
- Cultivation : The cultivation can be achieved from lung biopsies , bronchial secretions and a broncho-alveolar lavage . It is important that you inform the laboratory of the suspected diagnosis of legionnaires' disease, as the pathogen cannot be attracted to routine culture media. It can be grown on buffered charcoal yeast agar with the addition of alpha-ketoglutarate (alpha BCYE agar). Colonies appear after two to seven days with capnophilic incubation.
- PCR : PCR enables pathogens to be detected quickly from samples such as bronchoalveolar lavage, lung tissue, tracheal secretions and sputum, but not from urine. The sensitivity is very high, the results are partly better than with culture methods.
- Serology : Antibodies against L. pneumophila can be detected in the blood after about ten days.
Legionella pneumophila was not discovered until 1976 when an epidemic emerged among participating veterans at a meeting of the American Legion in Philadelphia. 29 of the 182 sick veterans died at the time. It was then that Legionnaires' disease got its name.
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