A comparison of antimicrobial susceptibility patterns for Staphylococcus aureus in organic and conventional dairy herds.

Selective pressure from antimicrobial use, mutations, or acquisition of foreign resistance determinants mediate antimicrobial resistance. If antimicrobial use is the major selective pressure encouraging the development of resistance, then reduced use should result in decreased resistance. We compared antimicrobial susceptibility patterns of Staphylococcus aureus isolates obtained from milk samples from 22 organic (nonantibiotic using) dairy herds to isolates from 16 conventional dairy herds. Susceptibility testing was performed by disk diffusion, and zone diameters were recorded in millimeters for 144 isolates from organic farms and 117 isolates from conventional farms and were also classified as susceptible or not-susceptible (intermediate and resistant categories combined). Strength of association between high or low use and proportion susceptible was evaluated by Chi-square analysis and differences in mean zone diameter for isolates from organic farms versus isolates from conventional farms were compared by analysis of variance. Analysis was done for each antimicrobial and deemed significant at p < or = 0.05. Differences in antimicrobial susceptibility were observed between S. aureus isolates from organic and conventional herds for seven of the nine antibiotics studied. Herds that were certified organic had S. aureus isolates that were more susceptible to antimicrobials. Overall, S. aureus isolates from both organic and conventional herds showed good susceptibility to most commonly used bovine mastitis antimicrobials; however, isolates from organic herds were significantly more susceptible. Longitudinal studies of herds undergoing the transition to organic farming would help elucidate the dynamics of antimicrobial resistance and the potential return of antimicrobial susceptibility.