Legionellosis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Ogechukwu Hannah Nnabude, MD

Overview

Legionella is often transmitted by aerosolized droplets produced from contaminated whirlpool spa, river, cruise ships, cooling towers, air conditioners, water supply systems . L. pneumophila usually invades the host cells and replicates intracellularly. Legionella is internalized using pseudopods and protects itself in a membrane-bound vacuole that does not fuse with lysosomes.

Pathophysiology

Transmission

  • Legionella is acquired by inhaling droplets from contaminated water sources such as faucets, showers, humidifiers, whirlpool spa, river, cruise ships, cooling towers, air conditioners, water supply systems [1]. L. longbeachae is thought to be transmitted by compost during gardening activity[2]. However, it has also been seen in industrial coolers [3].
  • Although is is believed that Legionella is not transmitted from one person to another person [4], there have been reports of person to person transmission [5].
  • Healthcare-associated transmission of Legionella is common and has a higher mortality rate.

Pathogenesis

  • Legionella are able to live within amoebic cells. Free-living amoebae may be responsible for enabling Legionella to better survive environmental hazards, adapt to living within macrophages, and may be a significant reservoir of Legionella. These amoebae may assist Legionella to remain undetected within the human environments [6].
  • Legionella invades the host cells and replicates intracellularly.
  • The internalisation of the bacteria can be enhanced by the presence of antibody and complement but is not absolutely required.
  • A pseudopod coils around the bacterium in this unique form of phagocytosis.
  • Once internalised, the bacteria surround themselves in a membrane-bound vacuole that does not fuse with lysosomes that would otherwise degrade the bacteria. In this protected compartment the bacteria multiply.
  • The bacteria use a Type IVB secretion system known as Icm/Dot to inject effector proteins into the host.
  • These effectors are involved in increasing the bacteria's ability to survive inside the host cell. They also secrete a 39kDa metalloprotease into culture fluids, which is cytotoxic for some cultured tissue culture cells.

References

  1. Prussin AJ, Schwake DO, Marr LC (2017). "Ten Questions Concerning the Aerosolization and Transmission of Legionella in the Built Environment". Build Environ. 123: 684–695. doi:10.1016/j.buildenv.2017.06.024. PMC 5665586. PMID 29104349.
  2. Graham FF, White PS, Harte DJ, Kingham SP (2012). "Changing epidemiological trends of legionellosis in New Zealand, 1979-2009". Epidemiol Infect. 140 (8): 1481–96. doi:10.1017/S0950268811000975. PMID 21943591.
  3. Thornley CN, Harte DJ, Weir RP, Allen LJ, Knightbridge KJ, Wood PRT (2017). "Legionella longbeachae detected in an industrial cooling tower linked to a legionellosis outbreak, New Zealand, 2015; possible waterborne transmission?". Epidemiol Infect. 145 (11): 2382–2389. doi:10.1017/S0950268817001170. PMID 28625225.
  4. Boamah DK, Zhou G, Ensminger AW, O'Connor TJ (2017). "From Many Hosts, One Accidental Pathogen: The Diverse Protozoan Hosts of Legionella". Front Cell Infect Microbiol. 7: 477. doi:10.3389/fcimb.2017.00477. PMC 5714891. PMID 29250488.
  5. Correia AM, Ferreira JS, Borges V, Nunes A, Gomes B, Capucho R; et al. (2016). "Probable Person-to-Person Transmission of Legionnaires' Disease". N Engl J Med. 374 (5): 497–8. doi:10.1056/NEJMc1505356. PMID 26840151.
  6. Segovia Hernández M (2005). "[The journey of Legionella pneumophila from amoebae to macrophage. Reflections on the largest outbreak of legionnaire's disease]". An R Acad Nac Med (Madr). 122 (3): 489–98, discussion 498-504. PMID 16524240.


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