Legionellae
LegionellaMorphology:Fastidious, aerobic G- rods stain poorly by Gram’s methodThere are several species of Legionella:-Legionella pneumophila: the most important, cause Legionnaire’s disease & pontiac fever.-L. micdadei: sometimes causes pneumonia.Culture:• Grown on complex media BCYE + α-ketoglutarate at Ph 6.9, 35C˚ & humidity 90% →grow slowly, visible colonies after 3 days (in blood culture require 2 weeks or more) → variation in colony, round or flat with entire edges, vary in color from colorless to iridescent pink or blue & translucent.• Catalase +,oxidase +
Antigenic Structure:• There is more than 10 serogroups of L. pneumophila, serogroup 1 the most common (complex antigenicstructure).• Legionellae produce: proteases, phosphatase, lipase,DNase & RNase
pathogenesis:• legionellae found in warm moist environments & infection of immunocompromised humans follows inhalation of the bacteria from aerosols (contaminated air-conditioning systems, showerheads & similar sources).
L. pneumophila causes a lobar, segmental or patchy pulmonary infiltration, acute purulent pneumonia involving the alveoli with dense intra-alveolar exudate of macrophages, PMNs, RBCs & proteinaceous material. There is interstitial infiltration & little or no inflammation of the bronchioles & upper airways.
L. pneumophila enters & grows within human alveolar macrophages & monocytes (phagocytosed but not killed),so L. pneumophila in lesions found within phagocytic cells. Ribosomes, mitochondria & small vesicles accumulate around phagocytic vacuoles → the cells destroyed → the bacteria released & infected other macrophages
Clinical Findings:• Legionnaire’s disease: asymptomatic infection is common in all ages, highest in men over 55 years. Risk factors:smoking, chronic bronchitis & emphysema, steroid & other immunosuppressive drugs, cancer chemotherapy & diabetes mellitus.• Infection may be febrile illness of short duration or sever, rapidly progressive illness with high fever,chills, malaise, nonproductive cough, hypoxia, diarrhea. Chest X-rays reveal patchy & multilobar consolidation.
There are leukocytosis, hyponatremia, hematuria (even renal failure) or abnormal liver function.• Pontiac fever: fever, chills, myalgia, malaise, headache (6-12 hours), dizziness, photophobia, neck stiffness & confusion. Respiratory symptoms are much less in Pontiac fever than Legionnaire’s disease & include mild cough & sore throat.
Diagnostic Lab.:• Specimens: bronchial washings, pleural fluid, lung biopsy & blood.• Smear: Direct Ab tests (low sensitivity).• Culture: BCYE agar then IF staining. • Specific tests: Legionellae Ags in patient’s urine by immunologic method.• Serologic tests: detection Abs to Legionellae (outbreaks).
Treatment:•The drug of choice is erythromycin. Rifampin whentreatment delayed.•Ventilation & management of shock is essential
Epidemiology & Control:• The natural habitats for Legionellae are lakes, streams,rivers & thermally heated bodies of water & soil (survivesup to 63C˚).• Contamination of residential water systemscommunity acquired inf.• Contamination of hospital water systems hospitalacquired or nosocomial infection.• Control of Legionellae in water & air-conditioningsystems by hyperchlorination & superheating of water.