Deciphering the
genetics of food poisoning
Source of
Article: http://www.scientistlive.com/European-Food-Scientist/Q&A/Deciphering_the_genetics_of_food_poisoning/22724/
Scientists in Portugal and France managed to follow the patterns of gene
expression in food-poisoning bacteria Listeria monocytogenes
(L. monocytogenes)
live during infection for the first time. The work about to be published in PLoS Pathogens shows how
the bacterial
genome
shifts to better adapt to infection by activating genes involved in virulence
and subversion of the host defences, as well as adaptation to the host
conditions. This is the first time that the molecular interactions between L. monocytogenes and its
host, as they occur during the different steps of infection, are followed in
real time paving the way, not only to the development of new therapies
against this potentially lethal bacterium, but also for the study of other
pathogen/host interactions.
Scientist Live spoke with
Didier Cabanes, one of the lead authors, about the study and its
implications.
What prompted you to study real-time gene expression of Listeria during
infection?
The gram-positive bacterium Listeria
monocytogenes is the causative agent of listeriosis, a severe
food-borne infection characterized by abortion, septicaemia, or
meningoencephalitis. L.
monocytogenes is an intracellular pathogen that has one of the
highest hospitalisation and mortality rates among food-borne infections and
has become one of the best-characterized bacterial systems for the molecular
analysis of invasion and intracellular parasitism. However, our knowledge of
its infectious process in
vivo is still fragmentary.Virulence is a trait that only
manifests in a susceptible host, involving a highly coordinated interaction
between bacterial factors and host components. Also we thought that the best
means to better understand the
Listeria-host interaction, would be the development of a global
approach to be used in a context where both elements of the host-pathogen
interplay are accessible.
How did you design the
experiment and why did you employ DNA arrays? What advantages did it offer?
Macroarray technology is a global approach that allows the determination of
the transcriptional status of the bacteria at the level of the whole genome,
and that can in turn allow the identification of new bacterial genes
critical for the infectious process, and lead to a better understanding of
the molecular events responsible for Listeria
infection. In order to determine the detailed expression kinetics of the
complete L. monocytogenes
genome in the course of the infection, and identified new Listeria virulence factors
whose expression is highly up regulated in
vivo, we compared expression profiles of L. monocytogenes grown
in standard culture medium in exponential phase vs. bacteria recovered from
mouse spleens 24, 48 and 72 hours after intravenous infection.
What did you learn
about how Listeria infection occurs?
Our results indicate that in the spleen of infected mice, ? 20% of the Listeria genome is
differentially expressed, essentially through gene activation, as compared to
growth in rich broth medium. During infection, Listeria is in an active multiplication
phase, as revealed by the high expression of genes involved in replication,
cell division and multiplication. In
vivo bacterial growth requires differential expression of genes
involved in adaptation of the bacterial metabolism and stress responses, in
particular to oxidative stress. Listeria
interaction with its host induces cell wall metabolism and surface expression
of virulence factors. During infection, L.
monocytogenes also activates subversion mechanisms of host
defenses, including resistance to cationic peptides, peptidoglycan
modifications and release of muramyl peptides. We showed that the in vivo differential
expression of the Listeria
genome is coordinated by a complex regulatory network, with a central role
for the PrfA-SigB interplay. In particular,
L. monocytogenes up regulates in
vivo the two major virulence regulators, PrfA and VirR, and their
downstream effectors.
You discovered a new
virulence factors. Can you describe them?
Mutagenesis of in vivo
induced genes allowed the identification of novel L. monocytogenes virulence factors,
including an LPXTG surface protein, and suggesting a role for S-layer
glycoproteins and for cadmium efflux system in Listeria virulence. However, characterization
of the exact role of these new virulence factors reserves further
investigations.
What is the next step
for your laboratory?
We currently try to characterize the function of the new virulence genes
identified in our study, but we also try to identified other virulence
factors by mutagenesis of target genes differentially expressed during mouse
spleen infection. Our work provides a powerful tool for the detection of
novel virulence determinants and a better understanding of the complex
strategies used by pathogens to promote infections.
We are also planning to perform the same in
vivo genome profile analysis on different infected organs
(intestine, liver, brain) and using different animal models in order to
identify organ- or host-specific virulence factors.
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