The Indigenous Probiotic Lactococcus lactis PH3-05 Enhances the Growth, Digestive Physiology, and Gut Microbiota of the Tropical Gar (Atractosteus tropicus) Larvae

Probiotics in aquaculture hold promise for enhancing fish health and growth. Due to their increased specificity and affinity for their host, indigenous probiotics may offer isolated and potentially amplified benefits. This study investigated the effects of <i>Lactococcus lactis</i> PH3-05, previously isolated from adults of tropical gar (<i>Atractosteus tropicus</i>), on the growth, survival, digestive enzyme activity, intestinal morphology, expression of barrier and immune genes, and intestinal microbiota composition in the larvae of tropical gar. Larvae were fed with live <i>L. lactis</i> PH3-05 concentrations of 10⁴, 10⁶, and 10⁸ CFU/g for 15 days alongside a control diet without probiotics. Higher concentrations of <i>L. lactis</i> PH3-05 (10⁶ and 10⁸ CFU/g) positively influenced larval growth, increasing hepatocyte area and enterocyte height. The 10⁶ CFU/g dose significantly enhanced survival (46%) and digestive enzyme activity. Notably, the 10⁸ CFU/g dose stimulated increased expression of <i>muc-2</i> and <i>il-10</i> genes, suggesting enhanced mucosal barrier function and anti-inflammatory response. Although <i>L. lactis</i> PH3-05 did not significantly change the diversity, structure, or Phylum level composition of intestinal microbiota, which was constituted by Proteobacteria, Bacteroidota, Chloroflexi, and Firmicutes, an increase in <i>Lactobacillus</i> abundance was observed in fish fed with 10⁶ CFU/g, suggesting enhanced probiotic colonization. These results demonstrate that administering <i>L. lactis</i> PH3-05 at 10⁶ CFU/g promotes growth, survival, and digestive health in <i>A. tropicus</i> larvae, establishing it as a promising indigenous probiotic candidate for aquaculture applications.