Increased production of biofilms by Escherichia coli in the presence of enrofloxacin.

The literature has demonstrated that subinhibitory concentrations of some antimicrobials are able to induce biofilm formation by certain bacterial species. Biofilms present in the mammary glands of cattle contribute to antimicrobial resistance, resulting in the appearance of persistent mastitis and consequent great losses to the dairy sector worldwide. The present study aimed to investigate the induction of biofilm formation by enrofloxacin in Escherichia coli isolates obtained from bovine mastitis. Twenty-seven isolates were reactivated in brain heart infusion (BHI) broth supplemented with different subinhibitory concentrations of enrofloxacin. Biofilm formation in microtiter plates was measured and confirmed by scanning electron microscopy (SEM). Isolates submitted to the concentration 0.0125 mg/mL of enrofloxacin showed greater biofilm formation compared to the control (p<0.001). Biofilm formation results obtained for the other concentrations did not differ from those obtained for the control (p>0.05). Using SEM it was possible to visualize the typical architecture of biofilms. These results represent the first report of inducing the production of biofilms in the presence of enrofloxacin, a quinolone antibiotic used to treat clinical mastitis.