Novel Organization of the Staphylococcal Cassette Chromosome mec Composite Island in Clinical Staphylococcus haemolyticus and Staphylococcus hominis Subspecies hominis Isolates from Dogs.

Staphylococcus haemolyticus and Staphylococcus hominis subsp. hominis are common coagulase-negative staphylococcus opportunistic pathogens. In Thailand, the clinical strains S. haemolyticus 1864 and 48 and S. hominis subsp. hominis 384 and 371 have been recovered from sick dogs. These strains were methicillin resistant with the nontypeable staphylococcal cassette chromosome mec (NT-SCC mec ). The SCC mec element distribution in the clinical isolates from dogs was analyzed using whole-genome sequencing, which revealed the presence of different SCC mec composite islands (CIs) and gene structure. The SCC mec -CIs of ψSCC mec 1864 (13 kb) and ψSCC 1864 (11 kb) with a class C1 mec complex but no ccr gene were discovered in S. haemolyticus 1864. The CIs of ψSCC mec 48 with a C1 mec complex (28 kb), SCC 48 with ccrA4B4 (23 kb), and ψSCC 48 (2.6 kb) were discovered in S. haemolyticus 48. In SCC 48 , insertion sequence IS 256 contained an aminoglycoside-resistant gene [ aph(2″)-Ia ]. Two copies of IS 431 containing the tetracycline-resistant gene tet (K) were found downstream of ψSCC 48 . In S. hominis subsp. hominis , the SCC mec -CI in strain 384 had two separate sections: ψSCC mec 384 (20 kb) and SCC ars (23 kb). ψSCC mec 384 lacked the ccr gene complex but carried the class A mec complex. Trimethoprim-resistant dihydrofolate reductase ( dfrC ) was discovered on ψSCC mec 384 between two copies of IS 257 . In strain 371, SCC mec VIII (4A) (37 kb) lacking a direct repeat at the chromosomal end was identified. This study found SCC mec elements in clinical isolates from dogs that were structurally complex and varied in their genetic content, with novel organization. IMPORTANCE In Thailand, the staphylococcal cassette chromosome mec (SCC mec ) element, which causes methicillin resistance through acquisition of the mec gene, has been studied in clinical coagulase-negative Staphylococcus isolates from various companion animals, and Staphylococcus haemolyticus and Staphylococcus hominis subsp. hominis were found to have the most nontypeable (NT)-SCC mec elements. These species are more prone to causing illness and more resistant to a variety of antimicrobials than other coagulase-negative staphylococci. However, full characterization of NT-SCC mec in clinical S. haemolyticus and S. hominis subsp. hominis isolates from such animals has been limited. Our findings support the use of full nucleotide sequencing rather than PCR designed for Staphylococcus aureus in further research of novel SCC mec elements. Moreover, several antimicrobial resistance and heavy metal resistance genes were identified on the SCC mec elements; these are important as they could limit the therapeutic options available in veterinary medicine.