Reduction in Rumen Tetracycline-Insensitive Bacteria during a Grain Challenge Using the Isoflavone Biochanin A.

The isoflavone biochanin A was previously shown to promote weight gain in growing steers by selectively inhibiting rumen bacteria-like growth-promoting feed antibiotics. The hypothesis that biochanin A inhibited the action of drug efflux pumps was tested by enumerating tetracycline-insensitive bacteria from steers in a subacute rumen acidosis (SARA) challenge. Steers ( n = 3/group) treatment groups were forage only, SARA control, SARA with monensin (0.2 g d -1 ), and SARA with biochanin A (6.0 g d -1 ). As the steers were stepped up from the forage-only basal diet to 70% cracked corn, the number of rumen bacteria enumerated on two tetracycline-containing media types (nutrient glucose agar and tetracycline, and bile esculin azide and tetracycline) increased ( p < 0.05) from as little as 1.7(10 5 ) to as great as 6.7(10 6 ) cfu mL -1 on the nutrient glucose agar in the SARA and monensin control groups. The biochanin A group maintained the same number of tetracycline-insensitive bacteria as the forage-only controls ( p > 0.05). The effects were similar to the more selective media type, but the differences were smaller. These results support the hypothesis that biochanin A inhibits the activity of drug efflux pumps in vivo.