Quillaja saponaria (Molina) Extracts Inhibits In Vitro Piscirickettsia salmonis Infections

SIMPLE SUMMARY: Bacterial diseases causes massive mortalities in aquaculture and antibiotic use remains the main measure to keep these under control. Pisciricketssia salmonis, an intracellular bacterium only present in Chile, produces high mortalities in farmed salmon and is currently the main reason for using antimicrobials compared to other salmon-producing countries such as Norway. Environmental and antimicrobial resistance concerns have been raised by the local and global public and society, although no scientific evidence has demonstrated such an impact. Thus, there is a constant search for new alternatives that can complement or reduce the use of antimicrobial in intensive salmon farming. Phytochemicals such as saponins from Quillaja saponaria extracts have been proven to prevent and control diseases in other animal production systems. This study explored the safety and efficacy of quillaja extract in in vitro infections with P. salmonis. The results of this study showed a good in vitro safety and efficacy to infections. The efficacy proved to be dependent on the quantity of saponins and toxicity dependent on purification. The results showed that quillaja extracts could be potentially used as a new sustainable and eco-friendly alternative to control P. salmonis infection, contributing to decreased fish mortality, antibiotic use and antimicrobial resistance in intensive aquaculture worldwide. ABSTRACT: P. salmonis infections are the cause of major bacterial disease in salmonids in Chile, and the reason for using more antibiotics compared to other salmon-producing countries. Vaccination and antibiotics have not been efficient and new approaches are needed. The safety of Quillaja saponaria extracts was measured by cytotoxicity using flow cytometry of cytopathic and death of fish cell cultures and efficacy was assessed using in vitro infection models with pathogenic P. salmonis. Cytotoxicity was low and control of in vitro infections was achieved with all products, with protection of over 90%. Minimum inhibitory concentrations were much higher than those in the infection using cell cultures. These results suggest a dual mechanism of action where less purified extracts with a combination of saponin and non-saponin components simultaneously decrease P. salmonis infection while protecting cell lines, rather than exerting a direct antimicrobial effect. Quillaja saponins controlled in vitro infections with P. salmonis and could be considered good candidates for a new, safe and sustainable method of controlling fish bacterial infectious diseases.