What is the role of gram stains and cultures in the diagnosis of Legionella infection?

Updated: Nov 13, 2018
  • Author: Mobeen H Rathore, MD, CPE, FAAP, FIDSA; Chief Editor: Russell W Steele, MD  more...
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Gram stains and cultures of sputum, lower respiratory tract secretions, tissue, or blood

  • In children, the best samples are obtained by bronchoscopy. [16] Samples obtained by bronchoalveolar lavage (BAL) are better than those from bronchial washings. In addition, fluid or pus from normally sterile sites (eg, cerebrospinal fluid [CSF], pleural fluid, peritoneal fluid) should be cultured.

  • Gram stain may show small, pleomorphic, weakly staining, gram-negative bacilli or no organisms with a large number of polymorphonuclear leukocytes. L micdadei may stain with acid-fast stain.

  • Culturing, considered the criterion standard, requires the use of special media (buffered charcoal yeast extract [BCYE] agar with L-cysteine and ferric ions to support growth, antibiotics to prevent overgrowth of other organisms, and dyes to impart a distinctive color to the organisms).

  • Specialized techniques may require 2-7 days to isolate Legionella organisms. Routine sputum cultures provide 80% sensitivity and 100% specificity. Culture from BAL specimens has a sensitivity of 90% or greater.

  • Since co-infection with other pathogens often occurs, isolation of other pathogens does not exclude the possibility of concomitant Legionella infection.

  • Legionella species can also be isolated from blood cultures; yield can be optimized by blind subculture (before sample results turn positive) onto BCYE agar from radiometric culture bottles.


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