LEGIONELLA - World Health Organization

Legionellae can multiply in 14 species of protozoa, including:
• Acanthamoeba, Naegleria and Hartmanella spp.
• the ciliates Tetrahymena pyriformis, Tetrahymena vorax (Rowbotham, 1980; Tyndall & Domingue,
1982; Fields et al., 1984; Rowbotham, 1986; Wadowsky et al., 1991)
• one species of slime mould (Rowbotham, 1980; Fields et al., 1993; Fields, Benson &
Besser, 2002).
Protozoa are an important vector for the survival and growth of Legionella within natural and
artificial environments, and have been detected in environments implicated as sources of
legionellosis. However, not all amoebae are acceptable hosts, indicating that a degree of host
specificity is involved. In the natural environment, L. pneumophila proliferates in protozoa
within intracellular phagosomes, possibly producing proteases with cytotoxic activity, and
thus causing localized tissue destruction (Quinn, Keen & Tompkins, 1989).
Once it has been ingested by an amoeba, the survival of L. pneumophila depends on the temperature
of the water. At 22 °C, the bacteria are digested by the amoeba (Nagington & Smith, 1980),
whereas at 35 °C they can proliferate inside the amoeba (Rowbotham, 1980). Temperature
also affects the expression of flagella, with a larger number of flagellated bacteria present at
30 ºC than at 37 ºC (Ott et al., 1991). Flagella have an important role in the pathogenicity of
many organisms, including Salmonella and Pseudomonas aeruginosa. Heuner & Steinert (2003)
found that nonflagellated legionellae were less capable of infecting protozoa and macrophages
than wild-type flagellated strains.
Protozoa help to protect Legionella from the effects of biocides (Barker et al., 1992) and thermal
disinfection (Storey, Ashbolt & Stenstrom, 2004a). Legionellae can survive in encysted
amoebal cells (Skinner et al., 1983; Harf & Monteil, 1988) and it has been postulated that
this can be a mechanism by which L. pneumophila is able to survive adverse environmental conditions
and survive within airborne aerosols (Berendt, 1980; Hambleton et al., 1983; Tully, 1991).
Phagocytic cells
The virulence of Legionella is linked to its capacity to proliferate in humans, where it infects
phagocytic cells opportunistically (i.e. taking advantage of certain conditions to cause disease).
However, these studies preceded the recognition of serological cross-reaction between L.
pneumophila and Campylobacter spp. Infection of susceptible animals such as guinea pigs, rats,
mice and hamsters has shown that the pattern of growth in macrophages is similar to that in
protozoa. The bacterium has been isolated from the lungs of calves, and serological conversion
has been observed in many animals, including horses, antelope and sheep (Boldur et al.,
1987). Therefore, infection is not solely caused by the virulence of L. pneumophila, but can
also depend on the susceptibility of the host. Attempts to infect birds (quails and pigeons)
with L. pneumophila were unsuccessful (Arata et al., 1992).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS