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 PH3-05, previously isolated from adults of tropical gar ( ), 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 PH3-05 concentrations of 10 , 10 , and 10 CFU/g for 15 days alongside a control diet without probiotics. Higher concentrations of PH3-05 (10 and 10 CFU/g) positively influenced larval growth, increasing hepatocyte area and enterocyte height. The 10 CFU/g dose significantly enhanced survival (46%) and digestive enzyme activity. Notably, the 10 CFU/g dose stimulated increased expression of and genes, suggesting enhanced mucosal barrier function and anti-inflammatory response. Although 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 abundance was observed in fish fed with 10 CFU/g, suggesting enhanced probiotic colonization. These results demonstrate that administering PH3-05 at 10 CFU/g promotes growth, survival, and digestive health in larvae, establishing it as a promising indigenous probiotic candidate for aquaculture applications.