As show in Table 1 and Fig. 2, the 263 MRSA isolates was assigned into 20 sequence types, 49 spa types, and 12 SCCmec types. The evolutionary and genetic diversity of all MRSA isolates was analyzed by MLST and spa typing. Twenty distinct STs were identified within the 263 MRSA isolates, of which ST5 was identified as the most prevalent ST type, accounting for 49.8% (131/263), followed by ST59 (44/263, 16.7%), ST965 (21/263, 8.0%), ST398 (13/263, 4.9%), ST1 (7, 2.7%) ST188 (6, 2.3%), ST88 (5, 1.9%), ST630 (5, 1.9%), ST72 (5, 1.9%), ST239 (3, 1.1%) and ST22 (3, 1.1%). Each of the remaining STs accounted for only one or two isolates, and the STs for 10 isolates were not identified. As show in Fig. 1, ST5 was the most prevalent ST type in Lishui, Hangzhou and Jiaxin, and ST59 in Ningbo. These MRSA isolates were classified into 49 spa types, and 5 isolates had unknown spa types. The majority of MRSA isolates belonged to one major spa type, t311 (111/263, 42.2%), followed by t062 (19, 7.2%), t437 (19, 7.2%), t172 (14, 5.3%), t034 (12, 4.6%), t002 (10, 3.8%), t4549 (6, 2.3%), t189 (5, 1.9%), t2460 (4, 1.5%), t127 (4, 1.5%), t437 (4, 1.5%) and t163 (4, 1.5%). The rest of the spa types represented by fewer than three isolates. By SCCmec typing, five types (types II, III, IV, V and XII) were found among 263 MRSA isolates, 2 isolates were classified as nontypable for SCCmec typing. The most common was type II (135/263, 51.3%), followed by type IV (90, 34.2%), type V (32, 12.2%), type III (3, 1.1%) and type XII (1, 0.4%).

Phylogenetic tree of all 263 MRSA. Locations are distinguished by different colors with lines. ST types, Spa types, and SCCmec types are color-coded in the inner rings

Among the 263 MRSA isolates, ST5-SCCmec II-t311 isolates accounted for 41.8% (110/263), being the predominant clone, followed by ST59-SCCmec IVa-t437 (21/263, 8.0%) and ST398-V-t034 (11/263, 4.2%), as determined by spa typing, MLST, and SCCmec typing (Fig. 2).

All MRSA isolates were resistant to penicillin and oxacillin, and no MRSA isolates were resistant to vancomycin, daptomycin and linezolid. The antimicrobial susceptibility test results for the other ten antibiotics are shown in Table 2, and the antimicrobial susceptibility test results of MRSA ST5 and ST59 are shown in Table S2. Most of the MRSA isolates exhibited multiple antibiotic-resistance profiles. Specifically, more than 50% of isolates were resistant to four of the ten remaining antibiotics, namely erythromycin (81.4%), ciprofloxacin (57.0%), levofloxacin (63.9%), moxifloxacin (61.2%). The resistance rates to other antibiotics tested were 46.8% to clindamycin, 1.5% to trimethoprim-sulfamethoxazole, 36.99% to tetracycline, 1.4% to rifampicin 0.8% to amikacin and 3.4% to tigecycline.

Compared with non-ST5-II-t311 isolates, ST5-II-t311 isolates were more resistant to erythromycin, tetracycline, levofloxacin ciprofloxacin and moxifloxacin, but more susceptible to clindamycin (P <  0.05). In addition, ST5-II-t311 isolates had higher rates of multidrug resistance than the non-ST5-II-t311 isolates (Table 2).

Besides, compared with ST5 isolates, ST59 isolates were more susceptible to erythromycin, tetracycline, levofloxacin, ciprofloxacin and moxifloxacin, but more resistant to clindamycin and all ST59 isolates were susceptible to trimethoprim-sulfamethoxazole and rifampicin (Table S2).

The distribution of 30 representative virulence genes varied among the 263 BSI MRSA isolates according to MLST types (Table 3). In our study, the toxin genes etb and see were not discovered among all MRSA isolates, and all isolates simultaneously harbored at least four virulence genes (icaA, hlb, hlg and eta). 59.3% (156/263) isolates harbored ≥19 tested virulence genes, among which one isolate harbored 23 genes, 98 isolates harbored 22 genes, 21 isolates harbored 21 genes, 11 isolates harbored 20 genes, 18 isolates harbored 19 genes.

The adhesion genes were existed in most MRSA isolates, among which 100% carried the icaA genes, 92.8% carried clfA genes, 97.3% harbored sdrC genes, and 89.4% carried sdrE genes. Fifteen classical enterotoxin genes (sea, seb, sec, sed, see, seg, seh, sei, sej, sel, sem, sen, seo, sep and seq) were detected within 263 MRSA isolates (Table 3). The positivity rates for sed (0.4%), seh (3.0%), sej (0.4%) and sep (3.0%) among all MRSA isolates were low, most of the remaining enterotoxin genes have a positive rate of more than 50% except for seb (17.9%) and seq (20.2%). The toxin-encoding genes detected were hly/hla (100%), lukD/E (71.5%), hlg/hlb (100%), tsst1 (52.1%) and fnbA (98.9%), and fifteen isolates (5.7%) carried lukS/F-pv genes.

As show in Table 3, the distribution of enterotoxin genes is related to ST type. The genes seq and seb were not detected in ST398 but in ST5, ST65 and ST965, and the genes sed were not detected in ST398, ST65 and ST965 but in ST5. The sep genes were more frequently detected in ST5, ST65 and ST965 than in ST72 (P <  0.05). The sej genes were only detected in ST5 but not in ST59, ST965 and ST398. The carrier rate of most virulence genes in ST59 was slightly lower than that in ST5, but it was not statistically significant (p > 0.05). In comparison to the non-ST5-II-t311 isolates, ST5-II-t311 isolates showed no not statistically significant in virulence genes detected (p > 0.05) (Table 4).