Multidrug-Resistant Staphylococcus haemolyticus ST42 Carrying ΨSCCmec57395-like SCCmec and Resistant Islands with Type I aj1–LP–fusB Structure Emerges in Taiwan Hospitals

<b>Background/Objectives:</b><i>Staphylococcus haemolyticus</i> is a common commensal bacterium that has emerged as an important nosocomial pathogen. Its multi-antibiotics resistance presents substantial therapeutic challenges in healthcare settings worldwide. Despite its growing clinical relevance, most investigations into antimicrobial resistance determinants have been focused on <i>Staphylococcus aureus</i> or <i>Staphylococcus epidermidis</i>, leaving <i>S. haemolyticus</i> comparatively understudied. This study aimed to elucidate the genetic basis of multi-drug resistance by characterizing mobile genetic elements associated with predominant <i>S. haemolyticus</i> clones circulating in Taiwan. <b>Methods:</b> From 2010 to 2017, 140 clinical targeted isolates of <i>S. haemolyticus</i> were obtained from individual patients. Two representative strains, SH53 (ST3) and SH51 (ST42), were sequenced using the PacBio^TM platform. The structural organization of SCC<i>mec</i> cassettes and phage-associated resistance islands in the remaining 138 isolates was analyzed by polymerase chain reaction (PCR) using specifically designed primers. <b>Results:</b> Of the 140 isolates, 92 (65.7%) were ST42 and 48 (34.3%) were ST3. PCR analysis showed that over two-thirds harbored heavy metal resistance genes. <i>cadD</i>, <i>cadX</i>, <i>arsC</i>, <i>arsB</i>, and <i>arsR</i> occurred in 90.2% of ST42 isolates, with <i>copA</i> in 71.7%. In ST3, these five genes were present in 89.6%, and <i>copA</i> in 64.6%. Fusidic acid (FA) resistance was more frequent in ST42 (46.7%) than ST3 (22.9%) (<i>p</i> = 0.015). Only one ST42 isolate carried <i>fusC</i>. The remaining 52 FA-resistant isolates contained a type I <i>aj1</i>-leader peptide (LP)-<i>fusB</i> structure downstream of <i>smpB</i>, except for a single ST42 isolate with the type IV structure. <b>Conclusions:</b> MDR ST42 <i>S. haemolyticus</i> carrying SCC<i>mec</i> cassettes with heavy metal resistance genes and phage-related islands carrying type I <i>aj1</i>-leader peptide (LP)-<i>fusB</i> structures may represent emerging opportunistic pathogens in Taiwan. Continued longitudinal surveillance is warranted to track the evolution of resistance-associated mobile elements under selective antimicrobial pressure.