Antimicrobial Activities of Grape Seed, Green Tea, and Rosemary Phenolic Extracts Against Liver Abscess Causing Bacterial Pathogens in Cattle.

Liver abscesses, which occur in finishing cattle, are of significant economic concern to the feedlot industry. The causative agents include both subspecies ( and . ), (), and serotype Lubbock (S. Lubbock). Tylosin, a macrolide antibiotic, is supplemented in the feed to reduce liver abscesses. However, due to the concern with emergence of antimicrobial resistance, the antimicrobial activities of the plant-based phenolic compounds could be an antibiotic alternative to control liver abscesses. We investigated the inhibitory activities of phenolic compounds extracted from grape seed, green tea, and rosemary on liver-abscess-causing bacterial pathogens. Total phenolic content was determined spectrophotometrically. Anaerobic Brain-Heart Infusion broth (for ) and Muller-Hinton broth (for and ) with phenolic extracts at 0, 0.1, 1, and 2 mg/mL were prepared. Growth was measured at 0, 12, 24 and 48 h by determining bacterial concentrations. A micro-broth dilution method was used to quantify the inhibition. Grape seed and green tea phenolics inhibited growth of both subspecies, and . Green tea at 1 mg/mL concentration was more effective in inhibiting the growth of when compared to grape seed and rosemary. Green tea at 2 mg/mL was more effective than at 1 mg/mL against . The inhibitory effect was dose-dependent, which was consistent across all strains within the same bacterial species. The phenolic extracts were inhibitory against with minimum inhibitory concentration ranging from 6.25 to 12.5 µg/mL. Among the phenolic extracts tested, green tea showed the most potent activity, suggesting its strong potential as a natural alternative to conventional antibiotics. Plant-based phenolic compounds supplemented in the feed may have the potential to control liver abscesses.