Question 13

With regards to infection with Legionella pneumophila and Legionella longbeachae:
a) Outline the mode of transmission of each species. (3 marks)
b) List four methods for diagnosis. (2 marks)
c) Outline the antimicrobial therapy. (2 marks)
d) Outline strategies for the prevention of infection. (3 marks)

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College Answer

Syllabus topic/section:

2.1.3 Sepsis and Infections – L1.

To allow candidate to demonstrate familiarity with an atypical infection.
andidates performed well if they were able to distinguish between the species of Legionella and were precise with modes of transmission and transmission sources for patients including water and soil. Combination therapy was rarely mentioned in discussion of antimicrobial therapy.

Contact precautions were incorrectly mentioned by many candidates, and candidates who performed best in outlining infection prevention strategies included broad/public health strategies as well as strategies for individuals. Candidates are again reminded that the term “outline” indicates the requirement for brief details in the answer, rather than a simple list (e.g. how (dose and duration) and why for antimicrobials (clinical considerations and side effects) rather than simply listing the antimicrobial).


Mode of transmission for three marks would have to be more than just "aeroloised, water coolers for L.pneumophila and potting mix for L.longbeachae".So:

  • L.pneumophila:
    • Aerosol transmission
    • Mostly from contaminated water sources
    • Sources in the built environment include:
      • Water heaters/coolers
      • Humidifiers
      • Air conditioners and their ducts
      • Showers, plumbing fixtures, water features
  • L.longbeachae:
    • Aerosol/dust transmission
    • Usually not found in water sources
    • Most commonly associated with exposure to
      • Potting mix
      • Garden soil
      • Compost


  • Urinary antigen (but L.longbeachae is not detected by this method).
  • PCR detects both L.pneumophila and L.longbeachae 
  • Paired serology has a 99% specificity and 90% sensitivity, but has a delay inherent in the process
  • Culture may take up to seven days, but is the gold standard

Antimicrobial therapy

  • Azithromycin
  • Ciprofloxacin
  • Doxycycline
  • L.longbeachae may not respond to tetracyclines


  • Prevention of transmission to staff
    • No need for isolation
    • Standard contact precautions apply
  • Prevention of transmission in the community
    • Notification to public health officials to help identify source and trace other potentially exposed individuals
  • Prevention of future infection or outbreaks
    • For L.longbeachae, instructions re. safe handling of potting mix/gardening soil (gloves, handwashing, masks, taking care not to inhale dust and to minimise exposure to aerosol)
    • For L.pneumophila, control of water supply contamination:
      • Chlorination of water
      • Regular audit and regulation for maintenance of water systems
      • Copper-silver ionisation
    • Information for susceptible individuals who have greater risk (eg. people on long term steroids) to help them mitigate their personal exposure risk


Edelstein, Paul H., and Christian Lück. "Legionella." Manual of clinical microbiology (2015): 887-904.

Chambers, Stephen T., et al. "Legionellosis caused by non-Legionella pneumophila species, with a focus on Legionella longbeachae." Microorganisms 9.2 (2021): 291.

Jomehzadeh, Nabi, et al. "Legionella and legionnaires' disease: An overview." Journal of Acute Disease 8.6 (2019): 221-232.

Lindsay, D. S. J., et al. "Legionella longbeachae serogroup 1 infections linked to potting compost." Journal of medical microbiology 61.2 (2012): 218-222.

Nisar, Muhammad Atif, et al. "Legionella pneumophila and protozoan hosts: implications for the control of hospital and potable water systems.Pathogens 9.4 (2020): 286.

McKINNEY, ROGER M., et al. "Legionella longbeachae species nova, another etiologic agent of human pneumonia." Annals of internal medicine 94.6 (1981): 739-743.

Agulló-Ortuño, Ma Teresa, et al. "Biochemical and immunologic features of an outbreak of Legionnaires disease: comparative study between community-acquired pneumonias." Diagnostic microbiology and infectious disease 56.1 (2006): 7-11.