Reduction in rumen tetracycline-insensitive bacteria during a grain challenge using the isoflavone biochanin A

Authors: Flythe, Michael; Harlow, Brittany; Kagan, Isabelle

Submitted to: Veterinary Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2023
Publication Date: 4/4/2023
Citation: Flythe, M.D., Davis, B.E., Kagan, I. 2023. Reduction in rumen tetracycline-insensitive bacteria during a grain challenge using the isoflavone biochanin A. Veterinary Sciences. 10(4). Article 273. https://doi.org/10.3390/vetsci10040273.
DOI: https://doi.org/10.3390/vetsci10040273

Interpretive 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 antibi-otics 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, e.g., the antibiotic tetracycline. The number of tetracycline-insensitive bacteria in the bo-vine rumen increase 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 tetracy-cline-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 miti-gated 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.

Technical Abstract: The isoflavone, biochanin A was shown to promote weight gain in growing steers by selectively inhibiting rumen bacteria and in a manner similar to growth promoting feed antibiotics. The hy-pothesis 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)., 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 tetracy-cline-containing media types (nutrient glucose agar + tetracycline, bile esculin azide + tetracy-cline) increased (P < 0.05) from as little as 1.7(105) to as great as 6.7(106) 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 with 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.