Mupirocin is effective for the prevention and treatment of MRSA infections. However, the resistance (including MuL) is able to lead to MRSA treatment and eradication failure [22, 23]. The prevalence rate of MUP resistance is various in MRSA clinical isolates worldwide, from 0.5–10.1% for MuH and 2.4–8.6% for MuL in America, from 0–75% for MuH and 0%% to 46.7% for MuL in Asia, and from 0.8–98% for MuH and 0–31.2% for MuL in Europe . In our present study, the isolation rates of MuH and MuL were low, namely 4.1% (50/1206) and 1.0% (12/1206), respectively. Recent large studies displayed that the prevalence of MuH is mediated by plasmid-borne mupA gene , this is the same as our results. Although mupB, also a plasmid-borne gene, is correlated with MuH , this mechanism is rarely examined in staphylococci, including the isolates investigated in the present study. The point mutations in the ileS gene, resulting in amino changes in MUP-binding site (also named Rossman fold), are the main mechanisms determining MuL . V588F and V631F are well identified the most frequent mutations in IleS responsible for MuL . In this study, only two MuL isolates (PT300 and wu9) contained the V588F mutation, and no MuL isolates harbored the V631F mutation. Notably, all MuL isolates harbored N213D mutation, which located in a hotspot amino acid sequence between 200 to 350 described by Lee et al . The N213D mutation had been previously reported, and are considered to have no impact on the sensitivity of MUP . Although the mupA gene located on chromosome is also associated with MuL , we did not detect the gene in our MuL isolates. In addition, no other mutations in IleS were found. Lee et al.  reported that a mutation of S634F could confer phenotype of susceptibility or MuL in diverse isolates. In view of this phenomenon, the contribution of N213D mutation to MuL should be evaluated further.
Fusidic acid is a steroidal antimicrobial agent, and suppresses the production of bacterial proteins by stopping the dissociation of elongation factor G (EF-G) from ribosome [6, 26]. In clinic, the main applications of topical FA are the treatment of SSTIs and decolonization of S. aureus including MRSA, which is similar to those for MUP . The prevalence of FA resistance reported by recent large studies varies in MRSA isolates from USA (0%-0.3%), Australian (4.1%-5.1%), Denmark (17.8%), Greece (57.0%) and other European countries (9.9%) . In China, the resistance levels in MRSA are also different in different areas, for example 3.0%-5.3% in MRSA from Beijing, Shanghai, Shenyang and Shenzhen cities [10, 27], and 27.1% in MRSA from Wenzhou city . Compared with the aforementioned data from China, our results showed a very low resistance rate (1.0%, 12/1026).
In S. aureus, the mutations in fusA (encoding EF-G) or fusE (coding for ribosome protein L6, RplF) lead to a decreased affinity of FA for the EF-G ribosome complex [3, 28]. There are over thirty point mutations in FusA sequence to be described, however, only a few were experimentally verified playing a role in FA resistance [3, 29, 30]. The mutations V90I, H457Y, H457Q and L461K observed in this study have been previously identified causing FAH in S. aureus [29, 31], and the L461K is the most prevalent mechanism among clinical FAL S. aureus strains . In our results, the L461K also existed in most (80%, 4/6) FAL isolates. One substitution with E8K located in domain I (amino acids 1 to 280) of EF-G was identified for the first time, and occurred with the mutations of V90I and L461K in a FAH isolate. Whether the novel mutation is associated with FA resistance is not clear and needs further clarification.
Protection of EF-G by FusB family molecules is another mechanism conferring the resistance (low-level) of FA . FusB family proteins (including FusB, FusC and FusD) are able to restore the translation of protein by binding to EF-G when FA exists . Previous studies showed that fusB was the most prevalent in Netherlands and mainland China [9, 10, 32], and fusC main existed in isolates from Taiwan, Australia, USA and European collections [16, 33, 34]. In our isolates with FA resistance, the fusB existed in all FAL isolates (6/6), and the fusC was most prevalent in FAH isolates (66.7%, 4/6). The fusD gene was identified in Staphylococcus saprophyticus, and relates to the ‘‘intrinsic resistance of FA’’ among this species . To date, this determinant is rarely detected in S. aureus strains.
Retapamulin, a semisynthetic drug, represses the synthesis of bacterial proteins by interacting with domain V of 50S ribosomal subunit . This drug has a potency to act as an alternative to MUP to eradicate the S. aureus colonization, except used for the treatment of SSTIs of S. aureus [8, 35]. For RET resistance, very little published data are available among clinical S. aureus strains worldwide. The resistance rates of 664 UK S. aureus (74% of them were MRSA), 155 USA MRSA, 403 USA MRSA and 400 USA S. aureus from several different studies were 0.15%, 2.6%, 0.25%, and 9.5%, respectively [8, 35–37]. In this study, the prevalence of RET resistance was very low (0.24%, 3/1226). In UK and USA, the RET resistance among S. aureus or MRSA with MUP resistance was < 1–2.6% [8, 37]. In the present study, only one MRSA isolate was observed to be simultaneous resistance to RET and MUP. In our 3 RET resistance isolates, no resistance mechanisms studied were examined. The genetic basis for resistance to RET in these isolates remains unclear, and other potential mechanisms may need to be further explored.
ST239 and ST5 are two predominant sequence types in China. However, in this study our strains were mainly belonged to ST764 (31.6%), which was more than the total percentage of ST239 and ST5 (16/76, 21.1%). ST764 MRSA, first reported in Japan, is a single-locus of ST5 nosocomial MRSA clone with or without the arginine catabolic mobile element (ACME, a feature of CA-MRSA) [38, 39]. In recent years, several studies have reported the S. aureus clone with ST764 in China [11, 14]. Notably, multiple MUP resistance MRSA clones with different genetic patterns, such as PFGE B-ST764, PFGE B-ST1821, PFGE B-ST239, PFGE B-ST5 and PFGE B-ST630, mainly occurred in the same hospital (Shanghai General Hospital) (Fig. 1), which indicates that the dissemination of different clones is responsible for the resistance of MUP in this hospital. However, for FA resistance isolates, the genetic background (PFGE-ST) exhibited more heterogeneity. This may be due to the situation that these isolates were from different hospitals (Fig. 1).