Evidence of the Presence of a Functional Dot/Icm Type IV-B Secretion System in the Fish Bacterial Pathogen <em>Piscirickettsia salmonis</em>

<div><p><em>Piscirickettsia salmonis</em> is a fish bacterial pathogen that has severely challenged the sustainability of the Chilean salmon industry since its appearance in 1989. As this Gram-negative bacterium has been poorly characterized, relevant aspects of its life cycle, virulence and pathogenesis must be identified in order to properly design prophylactic procedures. This report provides evidence of the functional presence in <em>P. salmonis</em> of four genes homologous to those described for Dot/Icm Type IV Secretion Systems. The Dot/Icm System, the major virulence mechanism of phylogenetically related pathogens <em>Legionella pneumophila</em> and <em>Coxiella burnetii</em>, is responsible for their intracellular survival and multiplication, conditions that may also apply to <em>P. salmonis</em>. Our results demonstrate that the four <em>P. salmonis dot/icm</em> homologues (<em>dotB, dotA, icmK</em> and <em>icmE</em>) are expressed both during <em>in vitro</em> tissue culture cells infection and growing in cell-free media, suggestive of their putative constitutive expression. Additionally, as it happens in other referential bacterial systems, temporal acidification of cell-free media results in over expression of all four <em>P. salmonis</em> genes, a well-known strategy by which SSTIV-containing bacteria inhibit phagosome-lysosome fusion to survive. These findings are very important to understand the virulence mechanisms of <em>P. salmonis</em> in order to design new prophylactic alternatives to control the disease.</p>