Effect of Methyl Gallate on Physiological and Gut Microbiota-Associated Responses Following a Single Intramuscular Administration of Tylosin in Weaned Piglets

Tylosin (TYL), a macrolide antibiotic, is commonly used to treat respiratory infections in weaned piglets. Despite its utility, TYL administration is associated with adverse side effects, including immune dysregulation and gut microbiota imbalance. This study evaluated whether methyl gallate (MG), a natural polyphenolic compound with antimicrobial and immunomodulatory properties, could mitigate these side effects of TYL. Forty weaned piglets were randomly assigned to a saline control group, a TYL (10 mg/kg) group, an MG (10 mg/kg) group, and a TYL + MG (10 mg/kg each) group. Changes in body weight, hematological parameters, plasma cytokines, culturable fecal bacterial populations, and 16S rRNA-based gut microbiota composition were evaluated in all weaned piglets. There were no significant differences (p > 0.05) in ADG, ADFI, or FE among the treatment groups. Hematological analysis revealed significant changes in neutrophil and eosinophil counts, showing significant fluctuations in the MG-treated group compared with the control. TYL administration significantly decreased TNF-α and IL-1β levels while increasing IL-10 and IL-1ra concentrations, indicating immunomodulatory effects. In contrast, MG significantly increased adaptive immune-associated cytokines, including IL-2, IL-4, IL-6, and IL-12. Culture-based fecal microbiological analysis revealed that TYL reduced Lactobacillus abundance and lowered the culture-based Lactobacillus/Enterobacteriaceae ratio, suggesting acute dysbiosis. Co-administration of MG partially attenuated these microbiota-associated alterations and promoted the recovery of bacterial populations. Moreover, 16S rRNA sequencing demonstrated that similar relative abundances of Proteobacteria were observed in the TYL and TYL + MG groups. Therefore, no clear reduction in relative abundance of Proteobacteria was demonstrated under the present experimental conditions. These findings support the possibility that MG may partially attenuate selected TYL-associated immunological and gut microbiota-related changes under the conditions of the present exploratory study. A graphical summary of the study design and the principal findings is presented in the Graphical Abstract.