The Phagosome-Lysosome Fusion Is the Target of a Purified Extract (PQSE) in Reducing Infection of Fish Macrophages by the Bacterial Pathogen

, the etiological agent of Piscirickettsiosis, is a Gram-negative and facultative intracellular pathogen that has affected the Chilean salmon industry since 1989. The bacterium is highly aggressive and can survive and replicate within fish macrophages using the Dot/Icm secretion system to evade the host's immune response and spread systemically. To date, no efficient control measures have been developed for this disease; therefore, the producers use large amounts of antibiotics to control this pathogen. In this frame, this work has focused on evaluating the use of saponins from as a new alternative to control the Piscirickettsiosis. It has been previously reported that purified extract of (PQSE) displays both antimicrobial activity against pathogenic bacteria and viruses and adjuvant properties. Our results show that PQSE does not present antimicrobial activity against , although it reduces infection in an in vitro model, promoting the phagosome-lysosome fusion. Additionally, we demonstrate that PQSE modulates the expression of and in infected cells, promoting the immune response against the pathogen and reducing the expression of pathogen virulence genes. These results together strongly argue for specific anti-invasion and anti-intracellular replication effects induced by the PQSE in macrophages.