download report - Istituto Pasteur

P a r t i c i p a n t s :
Gianni Prosseda, researcher; Monica Pompili, post-doc
fellow; Marialuisa Barbagallo, Maria Letizia Di Martino,
PhD students.
C o l l a b o r a t i o n s :
Dipartimento di Biologia, Università di Roma Tre (Prof.
Mariassunta Casalino); Dipartimento di Biologia Molecolare,
Cellulare e Animale, Università di Camerino (Dr. Maurizio
Falconi); Istituto di Biologia e Patologia Molecolari, CNR, Roma
(Dr. Gioacchino Micheli).
Report of activity
Shigella is a facultative intracellular pathogen causing
a severe enteric syndrome in humans, mainly in
the developing countries. The pathogenicity mechanism
of Shigella is based on the capacity to reach and
invade colonic epithelial cells, multiply intracellularly
and spread to adjacent cells with consequent cell
death and destruction of the colonic mucosa. This
process is highly complex and requires the coordinated
expression of virulence factors, encoded not
only by chromosomal genes but also by the plasmid
genome. The transition of Shigella towards a pathogenic
phenotype is a good model for understanding
how the virulence phenotype is triggered in a commensal
organism. Major mechanisms possibly
accounting for this phenomenon are the acquisition
of additional genes encoding virulence determinants
and the inactivation or loss of pre-existing genes.
The acquisition of the virulence plasmid (pINV) has
been undoubtedly one of the most critical events in
the evolution of the pathogenic lifestyle of Shigella.
The pINV plasmid contains all the genes required
for invasion and for intra- and inter-cellular spread,
including their positive activators VirF and VirB. To
obtain a proper sensing of the host environment
and an adequate virulence response, the expression
of virulence determinants is integrated in layers of
Pathogenetic mechanisms of microbially associated diseases - AREA 2
Deciphering the complexity of the virulence networks in Shigella
and enteroinvasive Escherichia coli (EIEC): the interplay among
nucleoid proteins, promoter DNA structure and regulatory factors
Principal investigator: Bianca Colonna
Professor of General Microbiology
Dipartimento di Biologia Cellulare e dello Sviluppo
Tel: (+39) 06 49917580, 06 49917582; Fax: (+39) 06 49917594
bianca.colonna@uniroma.it
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iterative regulative networks, ensuring a coordinated
and on-time synthesis of all determinants necessary
to induce invasivity. Like in other life-threatening
human pathogens, also in Shigella the entry from
the outer environment into the warmer host milieu
is one of the crucial events triggering the expression
of virulence factors. The plasmid regulatory
cascade of Shigella represents an interesting system
to understand how pathogenic bacteria have evolved
to prevent wasteful expression of virulence factors
while ensuring strong and appropriate activation
when this is required. The arrival of additional regulators
in the novel pathogen through the acquisition
of the pINV might have altered the transcriptional
program of the ancestral E. coli cell. By transcriptomic
analysis we have investigated on which
E. coli genes are up- or down- regulated in response
to an increasing level of the VirF protein, which is
the first positive activator of the Shigella invasivity
cascade. The results indicate that in E. coli several
genes are up-regulated including those involved in
biosynthesis of bacterioferritin, tryptophan and
sperimidine. Surprisingly several E. coli VirF upregulated
genes are partially or completed deleted in
all four Shigella species, thus suggesting that overexpression
of these genes might have a deleterious
effect on the intracellular life of pathogenic Shigella.
Analysis of the effect on the invasivity process of
Shigella strains transformed with plasmids containing
the E. coli VirF up-regulated genes will strongly
contribute to the understanding of the molecular
processes which determine the optimal expression
of the virulence phenotype.
The acquisition of new positive activators encoded
by the pINV is counterbalanced by the loss of some
ancestral regulators. In the novel habitat the new
pathogen reaches optimal fitness by modifications of
its genome which often redesign important metabolic
properties. Besides the loss of traits important for
the survival in the environment, but redundant for
the life inside the host, the new pathogen may also