Morphology of Legionella pneumophila:
- Legionellae are thin, non-capsulated bacilli, 25 µm x 0.3-1 µm, coccobacillary in clinical material and assuming longer forms in culture.
- Most are motile with polar or subpolar flagella.
- They are gram-negative but stain poorly, particularly in smears from clinical specimens.
- They stain better by silver impregnation but are best visualized by direct fluorescent antibody (DFA) staining with monoclonal or polyclonal sera.
Cultural characteristics of Legionella pneumophila
- Legionellae are nutritionally fastidious.
- Their growth is enhanced with iron salts and depends on the supplementation of media the L-cysteine.
- They have fastidious requirements and grow on complex media such as buffered charcoal, yeast extract (BCYE) agar, with L-cysteine and antibiotic supplements, with 5 percent CO2, at pH 6.9, 35°C and 90 percent humidity.
- Growth is slow and colonies take 3 to 6 days to appear.
Biochemical characteristics of Legionella pneumophila:
- The organisms are non-fermentative and derive energy from the metabolism of amino acids.
- Most species are motile and catalase-positive, liquefy gelatin and do not reduce nitrate or hydrolyze urea.
Pathogenesis of Legionella pneumophila:
- Respiratory tract disease caused by LegionelIa species develops in susceptible people who inhale infectious aerosols.
- Legionellae are facultative intracellular parasites that can multiply in alveolar macrophages and monocytes following entry into the alveoli through aerosols.
- Dissemination occurs by endobronchial, hematogenous, lymphatic and contiguous spread.
- Because of their intracellular location, humoral antibodies are ineffective. Cellular immunity is responsible for recovery.
Diseases caused by Legionella pneumophila:
- Asymptomatic Legionella infections are relatively common.
- Symptomatic infections generally affect the lungs and are present in one of two forms
- Pontiac fever- An influenza-like illness.
- Legionnaires’ disease - A severe form of pneumonia
- Pontiac Fever:
- Pontiac fever is a milder, nonfatal ‘influenza like’ illness with fever, chills, myalgia malaise, and headache but no clinical evidence of pneumonia. Outbreaks with high attack rates may occur.
- Legionnaire’s Disease
- The incubation time is 2 to 10 days.
- The disease presents with fever, non-productive cough and dyspnea, rapidly progressing, if untreated, to pneumonia.
- The primary manifestation is pneumonia.
- Multiorgan dis ease involving the gastrointestinal tract, central nervous system, liver and kidneys is common.
- Case fatality may be 15 to 20 percent but can be much higher in patients with severely depressed cell-mediated immunity, the cause of death being progressive respiratory failure and shock.
- All age groups are susceptible, though more cases have occurred in the elderly.
- Legionnaires’ disease (legionellosis) is characteristically more severe and causes considerable morbidity, leading to death unless therapy is initiated promptly.
- Legionnaire’s disease may be either epidemic or sporadic.
Laboratory Diagnosis of Legionella pneumophila:
1. Microscopy:
- Legionellae stain poorly with gram stain. Nonspecific staining methods, such as those using Dieterle’s silver or Gimenez’s stain, can be used to visualize the organisms.
- The most sensitive way of detecting legionellae microscopically in clinical specimens is to use the direct fluorescent antibody (DFA) test, in which fluorescein-labelled monoclonal or polyclonal antibodies directed against Legionella species are used .
- The test is specific, with false-positive reactions observed only rarely if monoclonal antibody preparations are used.
2. Culture:
- Although legionellae were difficult to grow initially, commercially available media now make growth easy.
- Legionellae require L-cysteine and their growth is enhanced with iron (supplied in hemoglobin or ferric pyrophosphate).
- The medium most commonly used for the isolation of legionellae is buffered charcoal yeast extract (BCYE) agar, although other supplemented media have also been used.
- Antibiotics can be added to suppress the growth of rapidly growing contaminating bacteria.
- Legionellae grow in air or 3 to 5 percent carbon dioxide at 35°C after 3 to 5 days. Their small (1 to 3 mm) colonies have a ground-glass appearance
3. Antigen Detection:
- Enzyme-linked immunoassays, radio-immunoassays, the agglutination of antibody coated latex particles, and nucleic acid analysis studies have all been used to detect legionellae in respiratory specimens and urine.
- If an antigen test is used, culture should always be performed.
- Nucleic acid analyses have been disappointing to date.
4. Serology:
- Detection of serum antibody is done by ELISA or indirect immunofluorescent assay.
Treatment of Legionella pneumophila :
- For treatment of disease caused by Legionella pneumophila, the newer macrolides, ciprofloxacin, and tetracyclines are effective. Rifampicin is employed in severe cases.
- Beta lactamase antibiotics and amino glycosides are ineffective in treatment for Legionella pneumophila.
Prevention of Legionellosis:
- Prevention of legionellosis can be carried out by the identification of the environmental source of the organism and decrease in the microbial burden.
- Hyperchlorination of the water supply and the continuation of elevated water temperatures have proved relatively successful.
- However, complete elimination of Legionella organisms from a water supply is often difficult or impossible.
- Because the organism has a low potential for causing disease, reducing the number of organisms in the water supply is frequently an adequate control measure.
- Hospitals with patients at high risk for disease should monitor their water supply on a regular basis for the presence of Legionella and their hospital population for disease.
- Continuous copper-silver ionization of the water supply may be necessary if hyperchlorination or superheating of the water does not eradicate disease (complete eradication of the organisms in the water supply is probably not feasible).
Epidemiology of Legionella pneumophila:
- Sporadic and epidemic legionellosis has a worldwide distribution. Legionellae are widely distributed in natural water sources, such as stagnant waters, mud and hot springs, where the nutritional and growth requirements for these fastidious bacteria are provided by some types of algae.
- The bacteria are commonly present in natural bodies of water, such as lakes and streams, as well as in air conditioning cooling towers and condensers and in water systems (e.g. showers, hot tubs).
- Legionellae survive and multiply inside free-living amoebae and other protozoa.
- They also multiply in some artificial aquatic environments, which serve as amplifiers.
- Human infection is typically by inhalation of aero sols produced by cooling towers, air conditioners and shower heads which act as disseminators.
- Aerosolized legionellae can survive for long and can be carried over long distances. No animal reservoir exists and infection is limited to human beings.
- No carrier state is established. Man to man transmission does not occur.
- The outcome of inhalation of legionellae depends on the size of the infecting dose, virulence of the strain and resistance of the host.
- Known risk factors are smoking, alcohol, advanced age, intercurrent illness, hospitalization and immunodeficiency.
- Men are more often affected than women.
- In the developed countries, legionellosis accounts for 1 to 3 percent of community acquired, and 10 to 30 percent of hospital acquired pneumonias.
- Its prevalence in the developing countries is not adequately known.