Abstract

Piscirickettsia salmonis, the etiological agent of Piscirickettsiosis, is a Gram-negative and faculta-tive intracellular pathogen that has affected the Chilean salmon industry since 1989. The bacte-rium 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 Quillaja saponaria as a new alternative to control the Pis-cirickettsiosis. It has been previously reported that purified extract of Q. saponaria (PQSE) dis-plays both antimicrobial activity against pathogenic bacteria and viruses and adjuvant proper-ties. Our results show that PQSE does not present antimicrobial activity against P. salmonis, alt-hough it reduces P. salmonis infection in an in vitro model, promoting the phagosome–lysosome fusion. Additionally, we demonstrate that PQSE modulates the expression of IL-12 and IL-10 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.