The objective of this study was to evaluate the effect of an antibiotic resistant bacterium (Lactococcus lactis; L. lactis) on nutrient apparent digestibility, fermentation parameters, ruminal pH and ammonia nitrogen, productivity and ruminal bacterial abundance in multiparous lactating Holstein cows during ruminal acidosis challenge. Four rumen-fistulated Holstein dairy cows were assigned to the following treatments as: 1) basal diet without any additive (CON) and 2) basal diet inoculated with 1×1011 cfu d-1 transgenic L. lactis (BACT) which was infused into the rumen for two consecutive periods each consisting of seven days and separated by 10 days recovery. During the study, ruminal acidosis was induced by direct introduction of ground corn grain and whey powder into the rumen (3 kg DM per head per d) for 4 days. Milk fat content was increased in cows inoculated with BACT (P<0.05). Cows treated with BACT had higher apparent digestibility of dry matter, crude protein, neutral detergent fiber, acid detergent fiber and ether extract. Both ruminal pH and ammonia nitrogen concentration did not show significant responses to the experimental treatments, while their pattern related to the sampling time, for 8 h after BACT inoculation, was significant (P<0.05). Moreover, bacterial-treated group exhibited an increase in total ruminal volatile fatty acid production and molar concentration of acetate (P<0.05). L. lactis inoculation increased (P<0.05) the abundance of lactic acid utilizing (M. elsdenii) and cellulolytic (R. flavefaciens) bacteria. Our results demonstrated that ruminal inoculation with the antibiotic resistant bacterium L. lactis might improve rumen fermentation pattern, as seen in acetate concentration, and change bacterial population in benefit of hydrogen consuming bacteria during ruminal acidosis.

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