Antibacterial effects of epigallocatechin gallate against calf diarrhea-causing Escherichia coli: in vitro and in vivo investigations

IntroductionCalf diarrhea caused by Escherichia coli poses severe economic burdens to the cattle industry, and antibiotic overuse has triggered drug resistance and residue risks. This study screened 10 plant extracts for antibacterial activity against a clinically isolated calf diarrhea-causing E. coli strain in vitro, and investigated the protective effects of epigallocatechin gallate (EGCG)—the most effective extract—on E. coli-infected mice.MethodsAntimicrobial susceptibility was assessed by disk diffusion, minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) assays. In vivo, mice were intraperitoneally challenged with E. coli and treated with 50 mg/kg or 100 mg/kg EGCG. Bacterial loads in liver and small intestine were quantified, intestinal pathology was evaluated by HE staining, and expression of tight junction proteins (ZO-1, Occludin, Claudin-1) was analyzed by immunohistochemistry and Western blotting.ResultsEGCG exhibited the largest inhibition zone (19.7 ± 0.3 mm) against the pathogen, with an MIC of 3.125 mg/mL and MBC of 50 mg/mL. In vivo, both 50 mg/kg and 100 mg/kg EGCG significantly reduced bacterial loads in the liver and small intestine of infected mice, alleviated small intestinal pathological damage, and the 100 mg/kg EGCG group showed markedly upregulated expression of ZO-1, Occludin, and Claudin-1 in the small intestine (p < 0.05).DiscussionOur findings confirm that EGCG exerts potent in vitro antibacterial effects against calf diarrhea-causing E. coli and protects against E. coli-induced small intestinal injury in mice, highlighting its potential as a natural antibiotic alternative for controlling colibacillary diarrhea in calves.