In vitro selective inhibitory activity of ethnoveterinary plant extracts, antibiotic growth promoters, antibiotics, phytochemicals and their synthetic analogues against diarrhoea-associated and probiotic-bacteria of livestock.

Infectious diarrhoea is a serious threat to farm animals, especially newborns. This study aimed to investigate in vitro selective growth-inhibitory effects of 15 Indian ethnoveterinary plant ethanolic extracts, 5 antibiotic growth promoters, 5 antibiotics, 15 phytochemicals and their synthetic analogues against 12 diarrhoea-associated and 12 probiotic bacteria of veterinary concern. Minimum inhibitory concentrations (MIC≥ 80) of crude extracts and chemicals were determined by the broth microdilution method and log value of the ratio of selective indices (SI) were calculated. Among the tested agents, 8-hydroxyquinoline and chloroxine (SI = 1.3 and 0.7) showed selective antibacterial effects comparable to those of ciprofloxacin, doxycycline, and tetracycline (SI = 1.1, 0.8 and 0.9), respectively. The extracts of Embelia ribes fruit and Punica granatum epicarp (SI = 0.3 and 0.2, respectively) produced weak selectivity, inhibiting the growth of diarrhoea-causing strains at lower concentrations than beneficial microbiota. An ultra-high-performance liquid chromatography equipped with a mass spectrometer and a high-resolution accurate mass (UHPLC/MS-HRAM) analysis of E. ribes extract, that showed a certain degree of selectivity, highest amongst tested species, quantified abundant embelin (256.7-327.6 µg/mg of dry extract), which could be responsible for its antibacterial activity. To our knowledge, this is the first study investigating the growth-inhibitory effects of E. ribes and embelin toward gastrointestinal bacteria of veterinary significance. In conclusion, our study identifies potential growth-inhibitory activities of 8-hydroxyquinoline and E. ribes extract, supporting further investigation of their potential relevance for veterinary antidiarrhoeal research. However, further studies are necessary to explore their applications in farm animals.