The effect of biochanin A on the antibiotic susceptibilities of Escherichia coli grown under different respiration and metabolic conditions on agar media

Antimicrobials have been used for a number of purposes in beef cattle production. However, increased prevalence of pathogens, including zoonotic bacterial species, such as Escherichia coli, is a concern. The flavonoid, biochanin A (BCA), was identified as a natural feed component to potentially limit the spread of antibiotic-resistant microorganisms by potentiating the activity of other antimicrobials. The current study investigated the synergistic effects of 120 ppm BCA with five classes of antibiotics on two strains of E. coli: 25922 and 51659 (O157:H7) on two agar culture media by using commercially available ETEST® strips and the FDA/CLSI breakpoint interpretation criteria. The effect of 120 ppm BCA on minimum inhibitory concentration (MIC) across the antibiotic classes was relatively consistent where it had largely no effect, an occasionally negative effect, or a neutral effect. BCA addition yielded negligible changes to ciprofloxacin (susceptible) for either strain under the redox and metabolic conditions tested. The culture conditions seemed more influential over MIC than BCA. The absence of oxygen generally conferred clinical resistance or a higher MIC, and at times media composition altered the MICs as well. For example, neither macrolide inhibited either strain (> 256 µg mL− 1) under anaerobic conditions but generally had measurable MICs aerobically. Additionally, the MICs for streptomycin were consistently higher across each strain when cultured on tryptic soy agar rather than Mueller Hinton broth. Despite previous studies in which biochanin A decreased MICs in other bacterial species, neither strain of E. coli was consistently impacted by the presence of 120 ppm BCA in the culture medium. The data indicate that BCA is likely not a reliable resensitizing/synergistic candidate for resistant E. coli. Regardless, the data demonstrates the impact culture conditions have on MIC across antibiotic class. These differences in susceptibility may be necessary to consider when treating E. coli infections under different conditions.