Phenotypic and genotypic characterization of methicillin-, tetracycline- and erythromycin-resistant strains of Staphylococcus intermedius of canine origin

Introduction: Staphylococcus intermedius (S. intermedius) is a coagulase positive zoonotic staphylococcus found in several domestic animals. It is a common commensal of oral, nasal and skin flora in healthy dogs, where it can also cause invasive diseases. Studies on genotypic characterization of antibiotic resistance have shown that the distribution of the antibiotic resistance genes seems to be vary among staphylococci of different animal origin. Materials and Methods: The specimens were collected from diseased dogs which attended the Microbiology Laboratory of the Department of Veterinary Medicine and Animal Production of the University of Naples Federico II. The swabs from different origins were inoculated onto Mannitol Salt Agar (MSA) and identified by automated VITEK 2 (bioMérieux®). The isolated strains were tested for their susceptibility to 14 antibiotics by using Kirby-Bauer disk diffusion test. Multiplex PCR were performed to determine genetic profiles of antibiotic resistance. Results: We isolated 25 strains of S. intermedius. Phenotypically penicillin resistance was present in 18 bacterial strains; however, only 4 of these isolated strains carried the mecA gene. The resistance rates for tetracycline and erythromycin were 60 and 56 per cent, respectively. Tetracycline resistance genes tet(K) and tet(M) were found positive in 15 isolates which were phenotypically tetracycline-resistant. Erythromycin resistance gene erm(B) was found positive in 14 isolates which were phenotypically erythromycin-resistant. In particular, nine of the tetracycline-resistant strains carried only tet(K) gene, while six carried both tet(K) and tet(M) genes. All the erythromycin- resistant isolates had erm(B) gene. Discussion and Conclusions: We noticed 56 per cent multidrug resistant S. intermedius strains. Moreover, our results show that the phenotypic and genotypic characterization of methicillin resistance are not correlated and so further studies are needed, whereas the phenotypic and genotypic characterization of tetracycline and erythromycin resistance are well correlated. The distribution and the prevalence of the antibiotic resistance genes among Staphylococcus spp. suggest that effective measures should be taken to control antibiotic use in pet animals.