5 The role of quorum-sensing in the virulence of Pseudomonas ...

as hypothetical proteins (He et al. 2004). It is important to note that none of the
ORFs have been previously investigated using in vivo respiratory infection models.
The work presented in this thesis is the first reported analysis of the contribution of
these ORFs to virulence within an acute respiratory model.
There are two, two-component regulatory systems (TCS) within PAPI-1 that have
been highlighted through previous studies as important for general virulence of the
strain (He et al. 2004, Lee et al. 2006). Two component regulatory systems act to
change the gene expression within a cell to favour changes in the external
environment. They consist of two parts; one is the sensor kinase, which senses the
changes in the environment by being bound to the cellular membrane and two; the
response regulator, which mediates the intracellular response to the changes in
environment. These external changes manifest in response to changes in
temperature, pH, presence of antibiotics and changes in nutrient availability.
The TCS involving RcsC and RcsB described in the literature is involved in the
adaptation of the cell surface by changing its structure and integrity (Huang,
Ferrieres & Clarke 2006). This TCS has been identified in a number of
Enterobacteriae species, in which the system has the ability to activate capsule
synthesis, suppress flagella biosynthesis and mobility as well as encouraging
biofilm synthesis (Huang, Ferrieres & Clarke 2006). Literature suggests it is
involved in a phenotypic change from mobility and attachment to cells during
infection, to that of creating a stable biofilm for a more permanent environment.
Interestingly, in Yersinia species, RcsB has been shown to be a positive regulator of
Type III secretion system (TTSS) secretion (Huang, Ferrieres & Clarke 2006). In E.
coli there are three components involved in this system, RcsC, the sensor kinase,
which activates RcsD, an intermediate, which in turn activates RcsB, the response
regulator. As of yet, no RcsD homologue has been discovered in P. aeruginosa
PA14. Importantly, the Yersinia genome does not encode a functional RcsD, which
suggests that the TCS has taken on a different role (Huang, Ferrieres & Clarke
2006). This is also the case in PA14 as mutagenesis of either RcsB or RcsC results
in attenuation in a murine burns-sepsis model of infection (He et al. 2004), if they
were redundant parts left over, mutagenesis should have no effect. In the
circumstances that the rcs TCS system could still be involved in some of its usual
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