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

SIMPLE SUMMARY: Biochanin A is an isoflavone, a small molecule produced by red clover (Trifolium pratense) and other legumes. Biochanin A promoted weight gain in growing steers in a manner similar to feed antibiotics. In laboratory experiments, biochanin A prevented bacteria from pumping out antibiotics and other inhibitory compounds, which increased their sensitivity to those inhibitors. It seemed logical that biochanin A might reduce the number of bacteria that were insensitive to an inhibitor, for example, the antibiotic tetracycline. The number of tetracycline-insensitive bacteria in the bovine rumen increases when the animals are fed high-starch diets. We fed steers (Bos taurus) diets with increasing concentrations of cracked corn (Zea mays) and enumerated the tetracycline-insensitive bacteria. Experimental groups received (1) biochanin A, (2) a well-studied feed antimicrobial, (3) no intervention, or (4) they were left on a forage-only diet. Biochanin A mitigated the increase in tetracycline-insensitive bacteria during the high-starch periods. These results support the idea that biochanin A can make some bacteria more sensitive to antibiotics and other inhibitors in animals, just as it does in the laboratory. More research is needed to determine the roles of forage legumes as alternatives to growth-promoting antibiotics and in combating the spread of antibiotic-resistant bacteria. ABSTRACT: 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.