Tylosin susceptibility of staphylococci from bovine mastitis
Although the 16-membered macrolide tylosin is commonly used for the treatment of bovine mastitis, little information is currently available about the susceptibility of mastitis pathogens to tylosin. In the present study, 112 Staphylococcus aureus and 110 coagulase-negative Staphylococcus (CoNS) spp. isolates from cases of bovine mastitis were tested by broth microdilution and agar disk diffusion with 30 μg tylosin disks. Susceptibility to erythromycin was tested by broth microdilution and disk diffusion using 15 μg disks. Both test populations showed bimodal distributions of minimal inhibitory concentrations (MICs) and zone diameters with eleven S. aureus and eight CoNS isolates showing tylosin MICs of ≥256 μg/ml and no zones of growth inhibition around the tylosin 30 μg disks. All 19 isolates with tylosin MICs of ≥256 μg/ml were also resistant to erythromycin. For six additional erythromycin-resistant isolates, tylosin MICs of 1–8 μg/ml were observed. One S. aureus and two CoNS isolates showed inducible macrolide resistance. PCR analysis of the 25 erythromycin-resistant staphylococcal isolates identified the resistance genes erm(A), erm(B), erm(C), erm(T), mph(C) and msr(A) alone or in different combinations. An excellent correlation between the results of the different tylosin susceptibility tests (broth microdilution versus disk diffusion) was seen for S. aureus and CoNS isolates. Since tylosin does not induce the expression of the aforementioned erm genes, isolates with an inducible resistance phenotype may – if only tylosin is tested – be falsely classified as tylosin-susceptible. Thus, erythromycin should be tested in parallel and tylosin should only be used for the treatment of infections caused by erythromycin-susceptible staphylococci.