Comparative erythromycin and tylosin susceptibility testing of streptococci from bovine mastitis

Tylosin, a 16-membered macrolide, is - besides other indications - used for the treatment of bovine mastitis. So far, there is only limited information available on the tylosin susceptibility of streptococci isolated from mastitis. The aim of the present study was to comparatively investigate 303 streptococci from bovine mastitis, including 101 Streptococcus agalactiae, 100 Streptococcus dysgalactiae and 102 Streptococcus uberis, for their tylosin and erythromycin susceptibility by broth microdilution and agar disk diffusion. Both tests followed the recommendations of the Clinical and Laboratory Standards Institute (CLSI). For erythromycin, the results were interpreted using the CLSI-approved clinical breakpoints. Moreover, erythromycin-resistant isolates were tested for the presence of macrolide resistance genes and for inducible macrolide resistance. In general, both testing methods showed a good correlation for the three streptococcal species, although for the erythromycin susceptibility testing 11 S. uberis isolates fell into the very major error category. All but one of the erythromycin-resistant isolates harbored at least one macrolide resistance gene, with the erm(B) gene being most common. Moreover, single isolates of S. agalactiae and S. dysgalactiae proved to be inducibly macrolide-resistant. Since inducible macrolide resistance can easily switch to constitutive resistance, tylosin should not be used for the treatment of infections caused by inducibly resistant streptococci.