Sensitivity of bovine and equine streptococci to beta-lactam antibiotics (benzylpenicillin, ampicillin, oxacillin, cefotaxim) in the agar diffusion test and E-test

368 streptococcal strains from udder secretions of cattle (Sc. agalactiae, Sc. dysgalactiae, Sc. uberis) and 191 streptococcal isolates from horse spe cimens (Sc.. equi ssp. zooepidemicus, Sc. equi ssp. equi) originating from different agricultural regions in Germany (Lower Saxony, in particular the region of Weser-Ems, Bavaria, Altmark) were investigated for their sensitiv ity to 4 beta-lactam antibiotics (benzylpenicillin, ampicillin, oxacillin, cefotaxime). Two different test methods were applied: the agar diffusion te st for determination of the diameter of the zone of inhibition and the E-te st for determination of the minimal inhibitory concentration (MIC). Accordi ng to the evaluation code of DIN 58 940, 98 % to 100 % of the isolates from the cow udder as well as of the Streptococcus strains from horses were sen sitive to the four antibiotics tested. Only Sc. uberis was less sensitive t o benzylpenicillin (79.7 %) and Oxacillin (83.2 %). The strains from differ ent agricultural regions did not differ from each other concerning their se nsitivity to beta-lactams. The results of the two methods of sensitivity te sting were in satisfactory agreement: Compared to the MIC reference values, misclassifications occurred in the agar diffusion test only at an error ra te of between 1.62 % (for ampicillin) and 5.21 % (for benzylpenicillin).