PhD Thesis MJProl .pdf - digital-csic Digital CSIC - Consejo Superior ...

New strategies for the control of bacterial infections in marine fish larval rearing 17
aquaculture (Moriarty, 1998; Rengpipat et al., 2003) might partly be due to inactivation
of QS molecules, apart from the production of antagonistic substances (Dong et al.,
2000, 2004; Molina et al., 2003). A direct immunomodulatory activity was produced in
several mammalian systems by a QS molecule from Pseudomonas aeruginosa (Hooi et
al., 2004; Ritchie et al., 2005; Thomas et al., 2006).
The importance of QS in situ gene expression regulation is becoming more evident
for pathogenic bacteria but hardly for putative probiotic bacteria. Some problems could
arise, such as the development of resistances (Zhu et al., 1998) or lack of specificity, as
not all bacteria found to contain QS systems are pathogens. Therefore, there is a need to
know more about the importance of QS molecules on their in vivo metabolism and their
interaction with the non-pathogenic microbial community and with the host.
1.2.2. Acting on host-microbe interactions
• Immunostimulants
Fish are intimately in contact with a complex and dynamic microbial world, and a
large fraction of these microbes cling to and colonize epithelial surfaces. In rare
circumstances microbes cause disease, either directly, by damaging or traversing
epithelial layers, or indirectly, by inducing tissue-damaging inflammatory responses. If
microbial pathogens invade the host, innate and adaptive defence mechanisms are
activated for preventing further spread of the infection (Gómez and Balcázar, 2008).
The fish immune system possesses two integral components:
i) The innate, natural or non-specific defence formed by series of cellular and
humoral components.
ii) The adaptive, acquired or specific immune system characterized by the
humoral immune response through the production of antibodies and by the
cellular immune response mediated by T-lymphocytes.
However, it is believed that larvae do not have the ability to develop adaptive
immunity during the early stages of development. In this regard, the innate immune
system is probably the major defence against microbes (Vadstein, 1997). These