Inhibitory effect of fluoroquinolones against biofilm formation by Staphylococcus aureus isolated from milk of dairy cows with mastitis

Bovine mastitis may cause intractable, chronic infection in cases where the causative pathogen forms a biofilm; however, data are lacking on Staphylococcus aureus (S. aureus) biofilm in Japanese dairy herds, and the relative efficacy of antimicrobial agents to treat it. Accordingly, we aimed to investigate the biofilm-forming potential of S. aureus isolates obtained from the milk of Holstein dairy cows with clinical mastitis, and to elucidate the effects of five clinically used antibiotic drugs against S. aureus biofilms. We found that 14/24 S. aureus isolates formed biofilm. All five antibiotic drugs demonstrated an inhibitory effect on S. aureus at the planktonic stage (based on minimum inhibitory concentrations); however, only the three fluoroquinolone antibiotics (orbifloxacin, enrofloxacin, and marbofloxacin) and cefazolin inhibited biofilm formation (based on minimum biofilm inhibitory concentrations). However, none of the five antibiotics proved effective against formed biofilms (based on minimum biofilm-eradicating concentrations). Accordingly, we suggest that systemic fluoroquinolone antibiotic or intra-mammary cefazolin may prevent biofilm formation in mastitic cows infected with S. aureus, if administered at a sufficiently high concentration and a sufficiently early stage of the disease (before biofilms are actually formed), but that antibiotics may have little efficacy once the biofilm is established.