Abstract:
Pathogenic Escherichia coli, including enteropatho-
genic E. coli (EPEC), enterohaemorrhagic E. coli
(EHEC), enteroinvasive E. coli (EIEC) and enterotoxi-
genic E. coli (ETEC) are major causes of food and
water-borne disease. We have developed a genetically
tractable model of pathogenic E. coli virulence based
on our observation that these bacteria paralyse and
kill the nematode Caenorhabditis elegans. Paralysis
and killing of C. elegans by EPEC did not require
direct contact, suggesting that a secreted toxin medi-
ates the effect. Virulence against C. elegans required
tryptophan and bacterial tryptophanase, the enzyme
catalysing the production of indole and other mole-
cules from tryptophan. Thus, lack of tryptophan in
growth media or deletion of tryptophanase gene failed
to paralyse or kill C. elegans. While known tryptophan
metabolites failed to complement an EPEC tryptopha-
nase mutant when presented extracellularly, comple-
mentation was achieved with the enzyme itself
expressed either within the pathogen or within a coc-
ultured K12 strains. Thus, an unknown metabolite of
tryptophanase, derived from EPEC or from commen-
sal non-pathogenic strains, appears to directly or
indirectly regulate toxin production within EPEC.
EPEC strains containing mutations in the locus of
enterocyte effacement (LEE), a pathogenicity island
required for virulence in humans, also displayed attenuated capacity to paralyse and kill nematodes.
Furthermore, tryptophanase activity was required for
full activation of the LEE1 promoter, and for efficient
formation of actin-filled membranous protrusions
(attaching and effacing lesions) that form on the sur-
face of mammalian epithelial cells following attach-
ment and which depends on LEE genes. Finally,
several C. elegans genes, including hif-1 and egl-9,
rendered C. elegans less susceptible to EPEC when
mutated, suggesting their involvement in mediating
toxin effects. Other genes including sek-1, mek-1,
mev-1, pgp-1,3 and vhl-1, rendered C. elegans more
susceptible to EPEC effects when mutated, suggest-
ing their involvement in protecting the worms. More-
over we have found that C. elegans genes controlling
lifespan (daf-2, age-1 and daf-16), also mediate sus-
ceptibility to EPEC. Together, these data suggest that
this C. elegans/EPEC system will be valuable in elu-
cidating novel factors relevant to human disease that
regulate virulence in the pathogen or susceptibility to
infection in the host.
