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Infect Immun, March 1998, p. 883-892, Vol. 66, No. 3
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Identification of Macrophage-Specific Infectivity
Loci (mil) of Legionella pneumophila That Are Not
Required for Infectivity of Protozoa
Lian-Yong
Gao,
Omar S.
Harb, and
Yousef Abu
Kwaik*
Department of Microbiology and Immunology,
University of Kentucky Chandler Medical Center, Lexington, Kentucky
40536-0084
Received 2 October 1997/Returned for modification 24 November
1997/Accepted 8 December 1997
We have recently shown that many mutants of Legionella
pneumophila exhibit similar defective phenotypes within both U937
human-derived macrophages and the protozoan host
Acanthamoeba (L.-Y. Gao, O. S. Harb, and Y. Abu Kwaik,
Infect. Immun. 65:4738-4746, 1997). These observations have suggested
that many of the mechanisms utilized by L. pneumophila to
parasitize mammalian and protozoan cells are similar, but our data have
not excluded the possibility that there are unique mechanisms utilized
by L. pneumophila to survive and replicate within
macrophages but not protozoa. To examine this possibility, we screened
a bank of 5,280 miniTn10::kan transposon
insertion mutants of L. pneumophila for potential mutants that exhibited defective phenotypes of cytopathogenicity and
intracellular replication within macrophage-like U937 cells but not
within Acanthamoeba polyphaga. We identified 32 mutants
with various degrees of defects in cytopathogenicity,
intracellular survival, and replication within human macrophages,
and most of the mutants exhibited wild-type phenotypes
within protozoa. Six of the mutants exhibited mild defects in protozoa.
The defective loci were designated mil (for macrophage-specific infectivity loci). Based on their intracellular growth defects within macrophages, the mil mutants were
grouped into five phenotypic groups. Groups I to III included the
mutants that were severely defective in macrophages, while members of the other two groups exhibited a modestly defective phenotype within
macrophages. The growth kinetics of many mutants belonging to groups I
to III were also examined, and these were shown to have a similar
defective phenotype in peripheral blood monocytes and a wild-type
phenotype within another protozoan host, Hartmannella vermiformis. Transmission electron microscopy of A. polyphaga infected by three of the mil mutants
belonging to groups I and II showed that they were similar to the
parent strain in their capacity to recruit the rough endoplasmic
reticulum (RER) around the phagosome. In contrast, infection of
macrophages showed that the three mutants failed to recruit the RER
around the phagosome during early stages of the infection. None of the
mil mutants was resistant to NaCl, and the dot
or icm NaClr mutants are severely defective
within mammalian and protozoan cells. Our data indicated that in
addition to differences in mechanisms of uptake of L. pneumophila by macrophages and protozoa, there were also genetic
loci required for L. pneumophila to parasitize mammalian
but not protozoan cells. We hypothesize that L. pneumophila has evolved as a protozoan parasite in the environment but has acquired
loci specific for intracellular replication within macrophages. Alternatively, ecological coevolution with protozoa has allowed L. pneumophila to possess multiple redundant mechanisms to
parasitize protozoa and that some of these mechanisms do not function
within macrophages.
*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, University of Kentucky Chandler Medical Center, Lexington, KY 40536-0084. Phone: (606) 323-3873. Fax: (606)
257-8994. E-mail: yabukw{at}pop.uky.edu.
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