Effect of Dietary Supplementation of <i>Lactobacillus Sakei</i> Subsp <i>. Sakei</i> on the Growth Performance, Immune Responses, and Antioxidant Defense of Rainbow Trout ( <i>Oncorhynchus Mykiss</i> ) Exposed to the Herbicide Butachlor ^*

Abstract The aim of this work was to determine the benefits of dietary supplementation with the probiotic Lactobacillus sakei subsp. sakei (LS) on the growth, intestinal microbiota, digestive enzymes, immunological responses, humoral antioxidant markers and liver enzymes of rainbow trout ( Oncorhynchus mykiss ) exposed to the herbicide butachlor (BU). Six experimental groups including C1: non-supplemented and non-exposed fish, LS1: 1×10 6 (cfu/g) L. sakei , LS2: 1×10 8 (cfu/g) L. sakei , BU: 12.5% LC 50 butachlor, LS1 + BU: 1×10 6 (cfu/g) L. sakei + 12.5% LC 50 butachlor, LS2 + BU: 1×10 8 (cfu/g) L. sakei + 12.5% LC 50 butachlor were designed. After the 30-day experimental period, a significant increase was observed in the survival rate (SR), final weight (FW), and weight gain (WG) of the fish fed with LS (P&lt;0.05). The total intestine bacteria (TBC) were elevated in the groups supplemented with probiotic, while a significant decrease was observed in the fish exposed to butachlor (P&lt;0.05). Fish fed with probiotic showed a higher value of protease activity, while a significant decrease was observed in protease activity in the butachlor group (P&lt;0.05). The fish of probiotic group showed a significant increase in immunity parameters components including lysozyme and total Ig. Furthermore, an improvement was observed in hormonal antioxidant markers [superoxide dismutase (SOD) and catalase (CAT)] (and liver enzymes in the probiotic groups) (P&lt;0.05). Overall, results suggested a protective role for LS against the harmful effects of butachlor through restoring gut health, the digestive enzyme activities, immune and antioxidant responses in the rainbow trout.