LEGIONELLA - World Health Organization

phagosome from its endocytic pathway (Fields, Benson & Besser, 2002). Genes such as pilE
(coding for the pilin protein) and pilD (coding for prepilin peptidase) are important for unrestricted
intracellular growth. Other loci involved in intracellular multiplication are mak (macrophage
killing), mil (macrophage-specific infectivity loci), and pmi (protozoan and macrophage infectivity).
Defects in any of these loci obstruct or interrupt intracellular growth (Sadosky, Wiater &
Shuman, 1993; Gao, 1997; Gao, Harb & Kwaik, 1998; Fields, Benson & Besser, 2002).
Tissue-destructive protease is another important factor in the ability of Legionella to cause
infection (Baskerville et al., 1986). Other factors that may increase virulence include several
cytotoxins, heat shock proteins and compounds associated with iron uptake. The stationary
phase response and the iron acquisition functions of L. pneumophila also play key roles in
pathogenesis, as do a number of other loci, including the pts and enh genes (Cianciotto, 2001).
Virulence factors affect the ability of legionellae to grow within protozoa, as seen from studies
showing the effect of incubating a virulent L. pneumophila strain and the corresponding
avirulent strain with an Acanthamoebae polyphaga from a source implicated in an outbreak of
Legionnaires’ disease (Surman, Morton & Keevil, 1999; Surman et al., 2002). Figure 1.2 shows
the organism after overnight incubation at 37 ºC.
F gure . Acanthamoebae polyphaga solated from a source mpl cated n an outbreak
a)
c)
of Leg onna res’ d sease
b)
d)
Photograph courtesy of Dr S Surman-Lee
a) No legionellae (control).
b) An avirulent L. pneumophila strain when viewed
by transmission electron microscopy (TEM).
Some of the amoebae contain vacuoles with
L. pneumophila inside; others contain degenerate
material, including what appears to be the remains
of Legionella. The amoebae are motile, with no
signs of infection, and none has burst. These
amoebae, apart from the presence of Legionella
in the vacuoles, do not differ from the control.
c) An avirulent L. pneumophila strain incubated with
the corresponding virulent L. pneumophila strain.
The legionellae have infected the Acanthamoeba
and replicated within it, with many intracellular
L. pneumophila.
d) Damage caused to the amoeba by the cytotoxic
activity of Legionella, which caused death of the
amoeba.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS