The illness with the biggest economic impact on Chile’s salmon farming sector is Salmonid Rickettsial Septicemia (SRS) caused by the bacterium Piscirickettsia salmonis, which results in significant mortality rates during the last phase of their productive cycle at sea (Rozas & Enriquez, 2014). Antimicrobials, most frequently florfenicol, remain the major treatment and control option for this infection because currently and commercially available vaccinations have not shown the expected efficacy levels (SERNAPESCA, 2016) . Botanicals are frequently employed in zootechnical feed additives for terrestrial animals, and they are gaining popularity as an alternative to using antibiotics for their antimicrobic and immunostimulants properties (Beltran & Esteban, 2022 ; Kuralkar & Kuralkar, 2021; Rossi et al., 2020). To the best of our knowledge, there are not many research on aquaculture, and nothing is known about how they affect SRS control. The aim of this study was to evaluate the feasibility in using the active principles of P rototype α, a thymol-based blend of botanicals, as a mean to support the control of the seasonal outbreaks of SRS.
Materials and methods
In order to reach the aim of the study, Minimum Inhibitory Concentration (MIC) assays were performed. The antimicrobial activity of the compound was evaluated using the microdilution method in CASO broth (Vera et al., 2012) , in which the active principles of the blend were dissolved ranging from 10 to 5000 ppm. Wells with no addition of products and with ethanol equivalent to the highest concentration were used as control and vehicle respectively while wells without bacterial inoculum served as negative controls. The trial was performed in quintuplicate. The Chilean P. salmonis isolate PS005 belonging to the EM genogroup was inoculated at a final concentration of 107 CFU/mL, and the plates were incubated for 96 h at 18 °C. After incubation, bacterial growth was evaluated by absorbance measurement at 620 nm and the MIC was defined as the lowest concentration of the tested compounds that totally inhibited the growth of the bacterium. The data were analyzed with two-way ANOVA followed by the Dunnet’s post hoc test, and differences were considered significant at p ≤ 0.0001.
The in-vitro effect of Prototype α’s active principles on P. salmonis were evaluated in a MIC assay providing bacterial growth inhibition to a statistically significant degree starting as early as 10 ppm until proving a complete inhibition of the pathogen from 200ppm upward. Results are provided below in
Aquaculture is o ne of the fastest expanding sectors, contributing worldwide to food provision (FAO, 2022). Due to the limitations and concerns on anti-m icrobial resistance coming from consumers and institutions alike (FAO, 2023) , it is one of the challenges of feed additive to support the aquaculture production by regulating harmful bacteria, promote growth and stimulate the immune system. These in-vitro results, coupled with many studies proving the health boost and growth enhancement of botanicals additive, show a promising new opportunity for these compounds and their use in support to SRS control.
Beltrán J. M. G. & Esteban M. A. (2022). Nature-identical compounds as feed additives in aquaculture, Fish & Shellfish Immunology, Volume 123, 2022, Pages 409-416, ISSN 1050-4648, https://doi.org/10.1016/j.fsi.2022.03.010.
FAO (2022) . The State of World Fisheries and Aquaculture 2022. Towards Blue Transformation. Rome, FAO. https://doi.org/10.4060/cc0461en.
FAO (2023) Antimicrobial Resistance : What is it? Food and Agriculture Organization of the United Nations. Accessed in February 2023, from https://www.fao.org/antimicrobial-resistance/background/what-is-it/en/
Kuralkar, P. & Kuralkar S.V. (2021). Role of herbal products in animal production – An updated review, Journal of Ethnopharmacology, Volume 278, 114246, ISSN 0378-8741, https://doi.org/10.1016/j.jep.2021.114246.
SERNAPESCA (2016). Informe sobre uso de antimicrobianos por la salmonicultura nacional: 2015. Subdirección de Acuicultura, Departamento de Salud Animal,
Rossi, B., Toschi, A., Piva, A., & Grilli, E. (2020). Single components of botanicals and nature-identical compounds as a non-antibiotic strategy to ameliorate health status and improve performance in poultry and pigs. Nutrition Research Reviews, 33(2), 218-234. doi:10.1017/S0954422420000013
Rozas M. & Enriquez R. (2014). Piscirickettsiosis and Piscirickettsia salmonis in fish: a review. J. Fish. Dis. 2014; 37: 163–188. doi:10.1111/jfd.12211
Vera, T., Isla, A., Cuevas, A., & Figueroa, J. (2012). Un nuevo medio de cultivo líquido para el patógeno Piscirickettsia salmonis. Archivos de medicina veterinaria, 44(3), 273-277.