General information:
 Non-spore-forming, Gram-negative bacilli, a member of the γ-proteobacteria, and so far, 42 species and 65 serogroups of Legionellae have been described
Intracellular replication within selected host cells is the primary and perhaps sole means of proliferation in the environment
Many of these species are reported to be pathogenic for humans, but L. pneumophila is the most frequently isolated species associated with disease
Characteristics:
Dual host system
 allowing intrcellular growth in protozoa, such as Acanthamoeba castellanii, Hartmanella sp. or Naeglaria sp., and in human alveolar macrophages. Protozoa are essential for the growth of Legionella and the interaction with amoeba seems to generate a pool of virulence traits during evolution
Intracellular pathogen
invades and replicates within a protective phagosome inside alveolar macrophages
Two phase of growth
replicative phase -- sodium resistance, non-flagellated, low cytotoxicity
infectious phase -- short, thick, flagellated and highly cytoxicity
Killing and lysis of macrophages require two steps
trigger apoptosis at early stage
induce pore formation later
The key to L. pneumophila's virulence is its ability to prevent phagosome-lysosome fusion
The genome sequences of L. pneumophila contains a large number of genes encoding proteins that show high similarity to eukaryotic proteins or motifs known to be involved in protein-protein interactions, which are present either solely or primarily in eukaryotes, suggesting that L. pneumophila may have acquired specific mechanisms of cross-talk with its eukaryotic hosts
A particularly high number and wide variety of secretion systems presented in L. pneumophila genome
Disease:
Legionnaires' Disease
L. pneumophila Corby, 3576470 bp, NC_009494
L. pneumophila Lens, 3345687 bp, NC_006369
L. pneumophila Paris, 3503610 bp, NC_006368
L. pneumophila Philadelphia 1, 3397754 bp, NC_002942
Publications:
 Chien M, et al., 2004. The genomic sequence of the accidental pathogen Legionella pneumophila. Science 305(5692):1966-1968.
Cazalet C, et al., 2004. Evidence in the Legionella pneumophila genome for exploitation of host cell functions and high genome plasticity. Nat. Genet. 36(11):1165-1173.
Glockner G, et al., 2008. Identification and characterization of a new conjugation/type IVA secretion system (trb/tra) of Legionella pneumophila Corby localized on two mobile genomic islands. Int J Med Microbiol 298(5):411-428.
Figures:
Schema for L. pneumophila pathway in macrophages (Reproduced from: Swanson MS, Hammer BK, 2000. Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages. Annu. Rev. Microbiol. 54:567-613.)
Major virulence factors in Legionella:
 Adherence
Hsp60
MOMP
Type IV pili
Endotoxin
LPS
Enzyme
Mip
Iron uptake
CcmC
FeoAB
FrgA
IraAB
Legiobactin
Motility
Flagella
Nutrient acquisition
Pht
Regulation
CsrA
LetA/S
RelA
RpoS
Secretion system
lsp T2SS
Dot/Icm
Stress protein
GspA
KatAB
SodB
SodC
Toxin
RtxA
Unclassified
LigA
MilA
eml loci
enh loci
pmi loci
Location of virulence-associated genes in L. pneumophila:
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