Antibiotic susceptibility patterns of mastitis pathogens isolated from Bloemfontein dairy herds

The antibiotic susceptibility patterns of bacteria associated with subclinical mastitis in Bloemfontein fresh milk dairy herds were determined. A total of 141 bacterial strains tested, consisted of Staphylococcus aureus (93 strains), coagulase negative staphylococci (17), streptococci (12), Corynebacterium bovis (8), Pseudomonas aeruginosa (7) and enterobacteria (4). Antibiotic susceptibility was determined qualitatively using the Kirby-Bauer disc diffusion method and quantitatively by determining the minimal inhibitory concentrations using the agar dilution method. The utilization of commercial intramammary antibiotic preparations on the dairy farms is also discussed. The Gram-positive bacteria were generally not exceptionally resistant to the antibiotics tested. S. aureus susceptibility figures for penicillin G were 66% and for methicillin (or cloxacillin) 100%. The coagulase negative staphylocci in contrast were relatively more resistant than the coagulase positive staphylococci. The enterobacteria and particularly the P. aeruginosa strains, were extremely resistant to all antibiotics tested. In the latter case even carbenicillin and gentamicin susceptibility figures were low. A general mastitis control programme is discussed.