Identification of fusB-Mediated Fusidic Acid Resistance Islands in Staphylococcus epidermidis Isolates

To understand the high prevalence of fusB genes in fusidic acid-resistant Staphylococcus epidermidis, analysis of resistance elements in 34 isolates was performed. First, sequence analysis of the aj1-LP-fusB region indicated that at least three types were present. Type I contained full-length aj1, type II contained a partial aj1 truncated from nucleotide position 93 to 421, and type III contained a more truncated aj1 that retained only the last 37 bp. Isolates with type I or type II aj1 displayed slightly higher levels of resistance to fusidic acid (MICs, 8 to 32 mu g/ml) than did those with type III aj1 (MICs, 4 to 16 mu g/ml). Subsequent sequencing of the flanking regions of fusB from four selected isolates carrying different types of aj1-LP-fusB regions revealed that the fusB genes were all located on phage-related resistance islands (RIs), referred to as SeRIfusB-2793, SeRIfusB-704, SeRIfusB-5907, and SeRIfusB-7778, respectively. Among them, three islands (SeRIfusB-2793, SeRIfusB-704, and SeRIfusB-5907) were located downstream of groEL (corresponding to the 44-min position based on Staphylococcus aureus whole genomic sequences), and one (SeRIfusB-7778) was located downstream of rpsR (corresponding to the 8-min position). All of the RIs were inserted into integrase-recognized att sites. Among 34 isolates, the insertion sites of fusB RIs were mostly (28/34, 82%) located downstream of groEL and two were located downstream of rpsR, but four remained unidentified. The pulsotype distribution indicated that fusB-containing S. epidermidis isolates were heterogeneous. In conclusion, the fusB resistance determinant in S. epidermidis was highly associated with phage-related RIs. This is the first report of fusB RI in S. epidermidis.