Characterization of Staphylococcal Cassette Chromosome Elements from Methicillin-Resistant Infections in Australian Animals.

We examined the oxacillin resistance phenotype and genomic structure of staphylococcal cassette chromosome (SCC) elements from 77 veterinary methicillin-resistant (MRSP) isolates. Isolates were characterized by oxacillin broth microdilution, whole-genome sequencing, and bioformatics analysis. Five previously described SCC elements, and a sixth novel element, were identified: SCC III (also known as II-III), ΨSCC, and SCC (a SCC VII variant), all previously described in MRSP, and SCC IVg and SCC V, previously described in both methicillin-resistant (MRSA) and MRSP. The sixth element was novel and found among nine geographically clustered isolates. This novel pseudostaphylococcal cassette chromosome (ΨSCC) contained a class A gene complex but lacked genes. It also harbored heavy metal (cadmium) resistance determinants. The median oxacillin MIC values among ΨSCC, SCC III, and SCC V isolates were significantly higher than those determined for the SCC VII variant isolates and ΨSCC and SCC IVg isolates. ΨSCC was found exclusively in sequence type 497 (ST497), an MRSP clone that is locally successful in Victoria, Australia. Future studies are necessary to determine if this clone has disseminated further afield and if ΨSCC has moved into other MRSP lineages or staphylococcal species. is a significant veterinary pathogen and occasional cause of infections in humans. β-Lactams are an important group of antimicrobials used to treat staphylococcal infections in humans and animals. However, when staphylococci become methicillin resistant via the acquisition of a mobile genetic element called staphylococcal cassette chromosome (SCC), they become resistant to all β-lactams. This study detected a novel SCC element among a cluster of methicillin-resistant isolates from animals in Australia. It also detected SCC elements in that had high similarity to those identified in methicillin-resistant , demonstrating how human and animal pathogens can share the same resistance determinants.