BackgroundA significant trend towards increased fusidic acid (FA) resistance among Staphylococcus aureus with increased duration of use is of concern. The aim of the present study is to investigate the dissemination of fusidic acid resistance among Staphylococcus aureus clinical isolates.MethodsThe susceptibility of S. aureus isolates to antimicrobial agents was determined by disc-diffusion method. The minimal inhibitory concertrations(MICs) of fusidic acid and vacomycin for fusidic acid-resisitant isolates were determined by ager dillution method. FA resistance determinants were determined by PCR and DNA sequencing. SCCmec typing, spa typing and multi-locus sequence typing were used for the determination of molecular characteristics for S. aureus isolates.ResultsA total of 392 non-duplicate S. aureus isolates including 181 methicillin-resistant S. aureus (MRSA) isolates, which were isolated from the clinical specimens of patients at a Chinese tertiary hospital from January, 2012 to September, 2013, were collected for investigating FA resistance. Among 392 S. aureus clinical isolates tested, 56 (14.3 %) with FA MIC values ranging from 2 μg/ml to ≥128 μg/ml were resistant to FA. The proportions of FA resistance among MRSA and MSSA isolates were 27.1 % (49/181) and 3.3 % (7/211). There was a trend of rapidly increased FA resistance among S. aureus and MRSA isolates from 5.2 % and 8.9 % in 2012 to 24.9 % and 45.1 % in 2013. Acquired FA resistance gene, fusB, was present in 73.2 % (41/56) of FA-resistant S. aureus isolates. fusC and fusA mutation were not found in any of tested isolates. A total of 9 sequence types (STs) and 12 spa types were identified among the 56 FA-resistant S. aureus isolates. ST5 accounting for 66.1 % (37/56) was the most prevalent ST. The majority (92.9 %, 52/56) of the isolates tested belonged to clonal complex 5(CC5). t2460 was the most prevalent spa type, accounting for 67.9 % (38/56) . ST5-MRSA- II-t2460 was predominant clone, accounting for 75.5 % (37/49) of FA-resistant MRSA isolates and 66.1 % (37/56) of FA-resistant S. aureus isolates. Five of 7 FA-resistant MSSA isolates belonged to ST630-MSSA.ConclusionIncreased FA resistance among S. aureus isolates was found in China. fusB was predominant FA resistance determinant. The spread of CC5 clone, especially novel ST5-MRSA- II-t2460 clone with high-level resistance to FA, was responsible for the increase of FA resistance.

A better understanding of the antimicrobial susceptibility, carriage of virulence determinants and molecular characteristics of Staphylococcus aureus isolates associated with skin and soft tissue infections (SSTIs) may provide further insights related to clinical outcomes with these infections. From January 2012 to September 2013, a total of 128 non-duplicate S. aureus isolates were recovered from patients with SSTIs. All 128 S. aureus SSTI isolates carried at least five virulence genes tested. Virulence genes detected among at least 70% of all tested isolates included hld (100%), hla (95.3%), icaA (96.9%), clf (99.2%), sdrC (79.7%), sdrD (70.3%), and sdrE (72.7%). The prevalence of MRSA isolates with 10 virulence genes tested (54.4%, 31/56) was significantly higher than that among MSSA isolates (35.2%, 25/71) (p<0.05). The positive rates of seb, sen, sem, sdrE and pvl among MRSA isolates were significantly higher than among MSSA isolates (p<0.05). ST7 and ST630 accounting for 10.9% were found to be the predominant STs. The most prevalent spa type was t091 (8.6%). MRSA-ST59-SCCmec IV was the most common clone (12.3%) among MRSA isolates whereas among MSSA isolates the dominant clone was MSSA-ST7 (15.5%). Six main clonal complexes (CCs) were found, including CC5 (52.3%), CC7 (11.7%), CC59 (8.6%), CC88 (6.3%), CC398 (4.7%), and CC121 (3.1%). A higher carriage of seb and sec was found among CC59 isolates. In comparison to CC5 and CC7 isolates, those with the highest carriage rates (>80.0%) of sdrC and sdrD, CC59 isolates had lower prevalence of these two virulence genes. All CC59 isolates were susceptible to gentamicin and trimethoprim/sulfamethoxazole, while CC5 and CC7 isolates had resistance rates to these two antimicrobials of 25.4% and 20.9%, and 40.0% and 40.0%, respectively. The resistance rates for tetracycline, clindamycin, and erythromycin among CC5 isolates were lower than among CC7 and CC59 isolates. In conclusion, the molecular typing of S. aureus SSTI isolates in the present study showed considerable heterogeneity. ST7 and ST630 became prevailing clones. Different S. aureus clones causing SSTIs were associated with specific antimicrobial resistance and virulence gene profiles.

The increased vancomycin minimum inhibitory concentration values (MICs) for methicillin-resistant Staphylococcus aureus (MRSA) isolates are associated with treatment failure and mortality of MRSA infections. In the present study, 553 non-duplicate MRSA isolates from various specimens of patients with infections at a Chinese tertiary hospital from January 2003 to December 2014, were selected randomly for investigating the shift of vancomycin MICs determined by E-test method. The percentages of the MRSA isolates with vancomycin MICs of ≥2.0, 1.5, 1.0, and ≤0.75 mg/L were 16.3% (90/553), 38.5% (213/553), 35.6% (197/553), and 9.9% (55/553), respectively. The highest geometric mean MIC (GM MIC) value (1.648 mg/L) and the lowest GM MIC (0.960 mg/L) were found in the first year (2003) and the last year (2014) over the study period, with significant difference (p < 0.05). The GM MICs over the study period fluctuated by year, with the elevated values in 2005, 2011, and 2013 and the decreased values in other years relative to the respective former year. The vancomycin GM MIC (1.307 mg/L) for MRSA isolates from sputum was the highest relative to that for the MRSA isolates from other specimens. By contrast, the vancomycin GM MIC value (1.156 mg/L) for MRSA isolates from pus was the lowest, with similar value to that for the isolates from blood. The vancomycin GM MICs in period I (2003–2005), period II (2006–2008), period III (2009–2011), and period IV (2012–2014) were 1.501, 1.345, 1.177, and 1.139 mg/L, respectively, with the continuous decreased trend. Compared with period I, the vancomycin GM MIC for MRSA isolates in period IV was significantly lower (p < 0.01), with a 1.318- fold decrease. The percentages of the isolates with vancomycin MIC ≥2 mg/L in four periods were 25, 15.6, 15.2, and 12%, respectively, with a continuous decrease. While the percentages of the isolates with vancomycin MIC ≤0.75 mg/L in four periods increased from 1.7% in period I to 19.3% in period IV. Taken together, a decreased trend in vancomycin MICs for MRSA isolates from a Chinese tertiary teaching hospital has been found. This pnenomenon was mainly associated with a decrease in the proportion of the MRSA isolates with vancomycin MIC ≥2 mg/L and an increase in the proportion of the MRSA isolates with vancomycin MIC ≤0.75 mg/L.

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Epidemiological characteristics and molecular features of carbapenem-resistant Enterobacter (CR- Ent ) species remain unclear in China. In this study, we performed a genomic study on 92 isolates from Enterobacter -caused infections from a multicenter study in China. Whole genome sequencing (WGS) was used to determine the genome sequence of 92 non-duplicated CR- Ent strains collected from multiple tertiary health centres. The precise species of Enterobacter strains were identified by average nucleotide identity (ANI) and in silico DNA–DNA hybridization (isDDH). Molecular features of high-risk CR- Ent sequence type (ST) lineages and carbapenemase-encoding plasmids were determined. The result revealed that the most common human-source CR- Ent species in China was E. xiangfangensis (66/92, 71.93%), and the proportion of carbapenemase-producing Enterobacter (CP- Ent ) in CR- Ent was high (72/92, 78.26%) in comparison to other global regions. Furthermore, ST171 and ST116 E. xiangfangensis were the major lineages of CP- Ent strains, and ST171 E. xiangfangensis was more likely to cause infections in older patients. Genomic analysis also highlighted the likelihood of intra-hospital/inter-hospital clonal transmission of ST171 and ST116 E. xiangfangensis . In addition, the bla NDM -harbouring IncX3-type plasmid was identified as the prevalent carbapenemase-encoding plasmid carried by CR- Ent strains, and was experimentally confirmed to be able to self-transfer with high frequency. This study detailed the genomic and clinical characteristics of CR- Ent in China in the form of multicenter for the first time. The high risk of carbapenemase-producing ST171 and ST116 E. xiangfangensis , and the bla NDM -harbouring IncX3-type plasmid were detected and emphasized.

IntroductionThe raise of multi-drug resistant bacteria involving carbapenem, colistin, or tigecycline resistance constitutes a threat to public health, which partly results from the transmission of corresponding mobile resistance genes, such as blaKPC and blaNDM for carbapenem, mcr for colistin, and tmexCD-toprJ gene cluster for tigecycline. Herein, we described the emergence of an Aeromonas veronii strain HD6454 co-harboring blaKPC–2, mcr-3.17, and tmexC3.2-tmexD3.3-toprJ1b gene cluster from hospital sewage.MethodsWhole genome sequencing (WGS) was used to determine the genome sequence of HD6454, and the detailed genomic analysis of genetic elements or regions carrying key antimicrobial resistance genes (ARGs) from HD6454 were performed. Cloning experiment was conducted to confirm the function of key ARGs in mediating antimicrobial resistance. Conjugation experiment was conducted to determine the mobility of the plasmid.ResultsThe results showed that this strain belonged to a novel sequence type (ST) variant ST1016, and carried 18 important ARGs. Among them, the blaKPC–2 was carried by non-self-transmissible IncP-6 plasmid, while tmexC3.2-tmexD3.3-toprJ1b gene cluster and mcr-3.17 were carried by integrative and mobilizable element (IME) or IME-related region in chromosome. The mcr-3.17, mcr-3.6, and mcr-3-like3 genes were further inferred to originate from IMEs of Aeromonas species. Additionally, for the first time, the mcr-3.17 was confirmed to confer low-level resistance to colistin under inducible expression, while tmexC3.2-tmexD3.3-toprJ1b gene cluster was confirmed to confer low-level resistance to tigecycline.DiscussionThis is the first report of a strain co-harboring blaKPC–2, mcr-3.17, and tmexC3.2-tmexD3.3-toprJ1b gene cluster. Although the resistance and/or mobility of these ARGs are limited in this strain, the emergence of this multiple important ARGs-carrying strain deserves further attention.