Oregano essential oils, castor vegetable oil, and Baccharis hydroethanolic extract on growth inhibition of rumen gram-positive and gram-negative bactéria

Natural additives are a promising tool in modulating rumen fermentation due to the recent concerns regarding microbial resistance to antibiotics. We evaluate the antimicrobial capacity of different levels (10, 20, 50 and 100 mg/L) of monensin, oregano essential oil (Origanum vulgare L), castor oil (Ricinus communis) and Baccharis dracunculifolia (BD) hydroethanolic extract against gram-negative (Prevotella albensis, Prevotella bryantii and Treponema saccharophilum) and gram-positive (Ruminococcus albus, Ruminococcus flavefaciens and Streptococcus bovis) bacteria. Optical density (600 nm) was measured at 8, 12 and 24 h. Monensin inhibited gram-positive bacterial growth (P < 0.05). However, higher dosages were also effective against gram-negative bacteria, possibly due to a toxicological effect. Oregano essential oil inhibited (P < 0.05) both gram-negative and gram-positive bacteria. Castor oil had marginal-to-no effect on gram-negative bacteria, but inhibited the growth of cellulolytic bacteria (Ruminococcus albus) at 12 h. Concentrations of Baccharis dracunculifolia used in this study (up to 100 mg/L) had no inhibitory effect on rumen bacteria. Natural additives are a promising tool to modulate rumen fermentation, which highlights the importance of further studies to evaluate the potential of new products.