Microbiological characterisation of Staphylococcus pseudintermedius isolates from dogs

Abstract number: P1380

Faires M., Gard S., Aucoin D., Weese J.S.

Objectives: In dogs, Staphylococcus intermedius has traditionally been regarded as the predominant pathogenic Staphylococcus species and a leading cause of skin and soft tissue infections. However, it has been recently reported that most isolates identified conventionally as S. intermedius are truly the related species S. pseudintermedius. The objectives of this study were to determine the prevalence of S. pseudintermedius among isolates from infections from dogs that have been classified, phenotypically, as S. intermedius and determine the prevalence of selected virulence factors and methicillin-resistance of S. pseudintermedius isolates.

Methods: Isolates from various infections in dogs that were phenotypically identified as S. intermedius were collected. Isolates were molecularly identified by sequence analysis of the sodA gene. For all isolates identified as S. pseudintermedius, genes for exfoliative toxins A (ETA) and B (ETB), S. intermedius exfoliative toxin (SIET), toxic shock syndrome toxin – 1 (TSST-1), Panton-Valentine leukocidin (PVL) toxin, and methicillin-resistance (mecA) were investigated. Each mecA positive isolate was evaluated for susceptibility to oxacillin (1 mg) and cefoxitin (30 mg) using the disk diffusion method and the presence of the penicillin-binding protein 2a (PBP2a) using a latex agglutination test (LAT).

Results: 102 isolates phenotypically identified as S. intermedius, were analyzed. 88/102 (86.3%) were molecularly identified as S. pseudintermedius. None were identified as S. intermedius. The SIET gene was detected in 60.2% (53/88) of S. pseudintermedius isolates. Genes for ETA, ETB, TSST-1, and PVL were not detected. The mecA gene was identified in 15.9% (14/88) isolates. 11/14 (78.6%) methicillin-resistant strains were phenotypically resistant to oxacillin and produced PBP2a. However, none were identified as resistant to cefoxitin.

Conclusion: The re-classification of a large proportion of S. intermedius isolates as S. pseudintermedius provides additional support to the hypothesis that S. pseudintermedius is the predominant pathogenic Staphylococcus species in dogs. The SIET gene was common and its role in disease requires further study. The low rate of cefoxitin-resistance but high rate of oxacillin-resistance in methicillin-resistant strains is opposite to that reported for S. aureus and must be considered when developing testing regimens for methicillin-resistant S. pseudintermedius.