Infect Immun, July 1998, p. 3029-3034, Vol. 66, No. 7
Department of Microbiology and Immunology,
The University of Michigan Medical School, Ann Arbor, Michigan
Received 30 October 1997/Returned for modification 6 January
1998/Accepted 10 February 1998
In nature, Legionella pneumophila replicates
exclusively as an intracellular parasite of amoebae, but it also
persists in the environment as a free-living microbe. Studies of how
this opportunistic pathogen recognizes and responds to distinct
extracellular and intracellular environments identified a link between
the growth phase and expression of traits previously correlated with
virulence. When cultured in broth, only post-exponential-phase L. pneumophila was sodium sensitive, cytotoxic, osmotically
resistant, competent to evade macrophage lysosomes, infectious, and
motile. Likewise, the L. pneumophila phenotype changed
during growth in macrophages. During the intracellular replication
period, this bacterium was sodium resistant and lacked flagella;
concomitant with macrophage lysis, L. pneumophila
became sodium sensitive and flagellated. Expression of the virulent
phenotype was a response to starvation, since exponential-phase
L. pneumophila became cytotoxic, sodium sensitive, and
motile after incubation in broth from stationary-phase cultures, except
when it was supplemented with amino acids. Together, these data
indicate that while nutrients are plentiful, intracellular L. pneumophila organisms are dedicated to replication;
when amino acids become limiting, the progeny express virulence factors
to escape the spent host, to disperse and survive in the aquatic environment, and to reestablish a protected intracellular niche favorable for growth.
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Expression of Legionella pneumophila
Virulence Traits in Response to Growth Conditions
*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, The University of Michigan Medical School, 6734 Medical Sciences Building II, Ann Arbor, MI 48109-0620. Phone: (734) 647-7295. Fax: (734) 764-3562. E-mail:
mswanson{at}umich.edu.
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