Determinantes genéticos de la virulencia en los procesos infecciosos en acuicultura

Abstract

Microbial virulence factors are mechanisms by which pathogenic micro-organisms can multiply inside the host and can cause an injury in theircells and tissues. These mechanisms are very complex and diverse and canbe different for each microorganism, although some of them are widespreadamong different pathogens. Among these factors could be cited the mecha-nisms to obtain nutrients (especially iron) from the host, mechanisms thatallow the pathogen to overcome the host defenses, or the production of cito-toxic substances. These virulence factors are common to many vertebrates including fish, and even invertebrates. These mechanisms are often encodedby gene clusters that are coordinately expressed and that are regulated at thetranscriptional level by specific regulatory pathways. Among them we cancite the transcriptional regulator Fur, which acts as a global regulator of mul-tiple iron regulated genes, maintaining the cell iron homeostasis and regula-ting many other genes including those codifying for virulence factors. Otherimportant regulatory system is the quorum sensing, which turns on certaingenes when a particular bacterial cell density is reached. Several virulencefactors in fish pathogens are encoded by genes harbored by plasmids thatbecome then virulence plasmids. This is the case of Vibrio anguillarum,Aeromonas salmonicida or Photobacterium damselae. Very often someof the virulence factors are encoded by genomic islands named pathogenicityislands, although these are not yet very well known among fish pathogens.Infective processes depend on interactions between the pathogen andthe host, interactions that are poorly studied in fish pathogens. To identifybacterial genes related to the infective process, several molecular techniqueshave been developed during the last years. By means of these techniques thegenetic determinants of virulence can be isolated and characterized. Amongthem we can cite the techniques to isolate bacterial genes expressed duringthe infection, such as IVET (in vivo expression technology) to select the pro-moters that are expressed in vivo or the signature-tagged transposon muta-genesis (STM) that identifies genes specifically involved in virulence. Othertechniques that allow the identification of genes related to virulence are thein vivo transposon mutagenesis or the suppression subtractive hybrizationthat permits to compare two whole bacterial genomes.