Mircrobiology Lecture -17 Legionella

Legionella

Taxonomy

Family:

Legionellaceae

Genus:

Legionella

Species:

L. pneumophila Legionella

Morphology  Slender, pleomorphic, gram-negative rods, coccobacilli in tissue and clinical material, filamentous on artificial media;  Motile by means of one or more polar flagella;  Strains poorly with common technique (Gram stain). Stained non-specifically using a silver impregnation method (Dieterle’s silver stain)

Legionella

Legionella pneumophila with many lateral flagella

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Gram stain of Legionella pneumophila

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Gram stain of Legionella pneumophila

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Immune fluorescence of Legionella pneumophila

Legionella

Biochemical and cultural properties  Biochemically inert, nonfermentative;  Derive energy from a metabolism of amino acids;  Culture on specialized medium, such as BCYE (buffered charcoal yeast-extract agar). Contain L-cysteine and iron salts as an essential growth factors (optimal pH of 6.9);  Grow in air of 3-5% carbon dioxide at 35oC after 3-5 days. Colonies are small (1-3 mm) have ground-glass appearance;  Weakly oxidase positive, produce catalase, gelatinase and betalactamase;

Colonies of L.pneumophila on buffered charcoal-yeast extract agar Legionella

Growth on the selective media

Colonies are small with a crystalline-like, ground glass appearance

Legionella

Virulence  Facultative, intracellular parasite;  Outer membrane protein;  Type IV pili;  Capable of replication in alveolar macrophages and monocytes (and amoeba in nature);  Prevents phagolysosome fusion;  Produce cytotoxin, a small peptide, interfering with oxygen-dependent process of phagocytosis;  Produce proteolytic enzymes, phosphatase, lipase and nuclease;  Activates the alternate complement pathway Legionella

Legionella inside macrophages

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Electron Micrograph

Uptake of L.pneumophila by a macrophage

Dividing Legionella in endosome Legionella

Clinical significance Legionella cause primarily respiratory tract infections •

Legionnaires disease – atypical, acute lobar pneumonia with multisystem symptoms. - Incubation period is 2-10 days - Epidemic and sporadic - Mortality is about 15-20%

•

Pontiac fever – is a mild disease - Incubation period is 1-2 days - Only epidemic - Influenza-like illness - is a self limited form - nonfatal respiratory infection

Nonrespiratory infections - wound abscesses - hematogenous spread infection - encephalitis - endocarditis

Legionella

Comparison of Diseases Caused by Legionella Legionnaire’s Disease

Pontiac Fever

Epidemiology Presentation

Epidemic, sporadic

Epidemic

Attack rate (%)

<5

>90

Person-to-person spread

No

No

Underlying pulmonary disease

Yes

No

Epidemic disease in late summer or autumn; endemic disease throughout year

Throughout year

Time of onset

Clinical manifestations Incubation period (days)

2 - 10

1-2

Yes

No

Course

Requires antibiotic therapy

Self-limited

Mortality (%)

15-20; higher if diagnosis is delayed

<1

Pneumonia

Legionella

Epidemiology  Commonly found in natural bodies of water, cooling towers, condensers, and water systems (including hospital systems);  Patients at high risk for symptomatic disease include patients with compromised pulmonary function and patients with decreased cellular immunity (particularly transplant patients);  Transmitted by Inhalation of infectious aerosols;

Legionella

Sources of Legionella for human

Legionella

Immunity  Immunity to the disease is primarily cell mediated with humoral immunity playing a small role;  The bacteria are not killed until sensitized T-cells activate parasitized macrophages;

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Treatment, Control and Prevention Treatment  Severe disease treated with macrolides, such as azithromycin or erythromycin. Fluoroquinolones (levofloxacin) are also effective;  Pontiac fever is usually treated symptomatically without antibiotics. It is a self-limited disease

Control and prevention  No vaccine or preventive drug is available;  Decrease environmental exposure to reduce risk of the disease;  For environmental sources associated with disease, treat with hyperchlorination, superheating, or copper-silver ionization Legionella

Diagnosis Direct detection methods • Organisms can be observed on histologic examination of tissue section using silver or Gimsa stains;

Fluorescent antibody-stained L.pneumophila

•

Direct immune fluorescent antibody test of respiratory secretions for detection antigens;

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ELISA - identification of legionella antigen in urine by latex agglutination;

•

PCR for detection nucleic acid; Legionella

Diagnosis Cultivation 2. Clinical specimen: sputum, bronchial aspirates and lung biopsy; 3. Culture on BCYE agar is the diagnostic test of choice;  Require L-cysteine and iron salts  Antibiotics might be used to help inhibit the growth of fastidious bacteria  Grown aerobically or  Grow using 3-5% CO2 at 35° for 3 to 5 days  pH 6.9 for optimum growth  Colonies are small with a crystalline-like, ground glass appearance Legionella

Diagnosis Serology Diagnosed by indirect immunofluorescent assay  To test serological response to infection  Pair serums are used for detection of four fold or greater rise in antibody titer in  typical clinical test Histology

Legionella

Thank you for your attention! Questions?