Abstract

Piscirickettsia salmonis, 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 Quillaja saponaria as a new alternative to control the Piscirickettsiosis. It has been previously reported that purified extract of Q. saponaria (PQSE) displays both antimicrobial activity against pathogenic bacteria and viruses and adjuvant properties. Our results show that PQSE does not present antimicrobial activity against P. salmonis, although 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.

Highlights

  • We found that purified Quillaja saponaria extracts (PQSEs) do not have a direct antimicrobial effect on P. salmonis (MIC > 30 mg/mL) but, if they reduce the invasion and intracellular replication of P. salmonis in macrophages (SHK-1 fish cell lines), downregulate bacterial genes encoding for virulence factors such as dotB ATPase from the T4-BSS and the chaperone protein chaperone HSP60 (chaPs), upregulate genes encoding for proinflammatory interleukin 12 (IL-12) and anti-inflammatory Interleukin 10 (IL-10) cytokines, favoring IL-12 over IL-10, characteristic of an activated-destructive macrophage, and favor the phagosome

  • PQSE, the cell viability was reduced to around 60%

  • The findings show that the incubation of macrophages with PQSE reduced the gene exexpression of dotB and the P. salmonis chaPs protein, generate a modupression of dotB and the P. salmonis chaPs protein, generate a modulation of the balance of proinflammatory (IL-12) and anti-inflammatory (IL-10) interleukins that favors IL-12 expression over IL-10 expression, and increases significantly the phagosome

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Summary

Introduction

Piscirickettsiosis (SRS), the main bacterial infectious disease and the main cause of antimicrobial use in the Chilean salmon industry, does not currently have an adequate control measure that can be implemented industry-wide [1,2]. P. salmonis, the etiological agent of SRS, has been identified in salmon net-pens in Norway, Canada, Ireland, and Scotland, but with a reduced virulence compared with the Chilean strains [3]. Public and private efforts have been made in Chile to counteract Piscirickettsiosis (SRS), the lack of efficient strategies has led to the use of significant amounts of antimicrobials, for which there is a global safety concern [4,5,6,7,8,9,10,11]

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