Citation: Yanez-Lemus, F.; Moraga, R.; Smith, C.T.; Aguayo, P.; Sánchez-Alonzo, K.; García-Cancino, A.; Valenzuela, A.; Campos, V.L. Selenium Nanoparticle-Enriched and Potential Probiotic, Lactiplantibacillus plantarum S14 Strain, a Diet Supplement Beneficial for Rainbow Trout. Biology 2022, 11, 1523. https://doi.org/10.3390/ biology11101523 Academic Editor: Nuria De Pedro Received: 24 August 2022 Accepted: 8 October 2022 Published: 18 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biology Article Selenium Nanoparticle-Enriched and Potential Probiotic, Lactiplantibacillus plantarum S14 Strain, a Diet Supplement Beneficial for Rainbow Trout Francisco Yanez-Lemus 1,2 , Rubén Moraga 3 , Carlos T. Smith 1 , Paulina Aguayo 1,4,5 , Kimberly Sánchez-Alonzo 6,7 , Apolinaria García-Cancino 6 , Ariel Valenzuela 8 and Víctor L. Campos 1, * 1 Environmental Microbiology Laboratory, Department of Microbiology, Faculty of Biological Sciences, Universidad de Concepcion, Concepcion 4070386, Chile 2 Escuela de Medicina Veterinaria, Facultad de Recursos Naturales y Medicina Veterinaria, Universidad Santo Tomás, Santiago 8370003, Chile 3 Microbiology Laboratory, Faculty of Renewable Natural Resources, Arturo Prat University, Iquique 1100000, Chile 4 Faculty of Environmental Sciences, EULA-Chile, Universidad de Concepcion, Concepcion 4070386, Chile 5 Institute of Natural Resources, Faculty of Veterinary Medicine and Agronomy, Universidad de Las Américas, Sede Concepcion, Chacabuco 539, Concepcion 3349001, Chile 6 Laboratory of Bacterial Pathogenicity, Department of Microbiology, Faculty of Biological Sciences, Universidad de Concepcion, Concepcion 4070386, Chile 7 School of Medical Technology, Faculty of Medicine and Science, Universidad San Sebastian, Concepcion 4080871, Chile 8 Laboratory of Pisciculture and Aquatic Pathology, Department of Oceanography, Faculty of Natural and Oceanographic Sciences, Universidad de Concepcion, Concepcion 4070386, Chile * Correspondence: vcampos@udec.cl; Tel.: +56-41-2204144 Simple Summary: Potential probiotic bacteria for aquacultured species should be naturally occurring and non-pathogenic in the native habitat of the host, easy to culture, and able to grow in the intestine of the host. Se nanoparticles (Se 0 Nps) can be effectively used as a growth promoter, antioxidant, and immunostimulant agent in aquacultured species. Dietary supplementation with probiotics and Se 0 Nps contributes to the balance of the intestinal microbiota and probiotics have been proposed as an alternative to chemotherapeutants and antibiotics to prevent disease outbreaks, to mitigate the negative effects of stress and to strengthen the antioxidant capacity and the immune system of fish. Our results reported the isolation of a probiotic strain obtained from healthy rainbow trout. The strain was identified as Lactiplantibacillus plantarum species. This strain showed characteristics typically present in probiotics and, concurrently, the capacity to biosynthesize Se 0 Nps. The supplementation of the rainbow trout fish diet with LABS14-Se 0 Nps showed a positive effect on innate immune response parameters, oxidative status, well-being, and a better growth performance than the supplementation of the diet with the bacterium LABS14 alone. Therefore, we propose LABS14-Se 0 Nps as a promising alternative for the nutritional supplementation for rainbow trout or even other salmonids. Abstract: Lactic acid bacteria (LAB), obtained from rainbow trout (Oncorhynchus mykiss) intestine, were cultured in MRS medium and probiotic candidates. Concurrently, producers of elemental selenium nanoparticles (Se 0 Nps) were selected. Probiotic candidates were subjected to morphological characterization and the following tests: antibacterial activity, antibiotic susceptibility, hemolytic activity, catalase, hydrophobicity, viability at low pH, and tolerance to bile salts. Two LAB strains (S4 and S14) satisfied the characteristics of potential probiotics, but only strain S14 reduced selenite to biosynthesize Se 0 Nps. S14 strain was identified, by 16S rDNA analysis, as Lactiplantibacillus plantarum. Electron microscopy showed Se 0 Nps on the surface of S14 cells. Rainbow trout diet was supplemented (10 8 CFU g −1 feed) with Se 0 Nps-enriched L. plantarum S14 (LABS14-Se 0 Nps) or L. plantarum S14 alone (LABS14) for 30 days. At days 0, 15, and 30, samples (blood, liver, and dorsal muscle) were obtained from both groups, plus controls lacking diet supplementation. Fish receiving LABS14-Se 0 Nps for 30 days improved respiratory burst and plasmatic lysozyme, (innate immune response) and glutathione peroxidase (GPX) (oxidative status) activities and productive parameters Biology 2022, 11, 1523. https://doi.org/10.3390/biology11101523 https://www.mdpi.com/journal/biology