Current status of oxacillin-susceptible mecA-positive Staphylococcus aureus infection in Shanghai, China: A multicenter study

Emerging strategies and therapeutic innovations for combating drug resistance in Staphylococcus aureus strains: A comprehensive review

In recent years, antibiotic therapy has encountered significant challenges due to the rapid emergence of multidrug resistance among bacteria responsible for life-threatening illnesses, creating uncertainty about the future management of infectious diseases. The escalation of antimicrobial resistance in the post-COVID era compared to the pre-COVID era has raised global concern. The prevalence of nosocomial-related infections, especially outbreaks of drug-resistant strains of Staphylococcus aureus, have been reported worldwide, with India being a notable hotspot for such occurrences. Various virulence factors and mutations characterize nosocomial infections involving S. aureus. The lack of proper alternative treatments leading to increased drug resistance emphasizes the need to investigate and examine recent research to combat future pandemics. In the current genomics era, the application of advanced technologies such as next-generation sequencing (NGS), machine learning (ML), and quantum computing (QC) for genomic analysis and resistance prediction has significantly increased the pace of diagnosing drug-resistant pathogens and insights into genetic intricacies. Despite prompt diagnosis, the elimination of drug-resistant infections remains unattainable in the absence of effective alternative therapies. Researchers are exploring various alternative therapeutic approaches, including phage therapy, antimicrobial peptides, photodynamic therapy, vaccines, host-directed therapies, and more. The proposed review mainly focuses on the resistance journey of S. aureus over the past decade, detailing its resistance mechanisms, prevalence in the subcontinent, innovations in rapid diagnosis of the drug-resistant strains, including the applicants of NGS and ML application along with QC, it helps to design alternative novel therapeutics approaches against S. aureus infection.

Comparative Analysis of Two Commercial Automated Systems with Agar Dilution for Oxacillin Susceptibility and Their Association with Genotypes of Invasive Staphylococcus aureus Isolates (2011-2021)

Background

Determining oxacillin susceptibility using reference methods and automated systems is crucial for treating invasive infections caused by Staphylococcus aureus. This study compares the oxacillin susceptibility results from the two automated systems with agar dilution and correlates them with genotypes of invasive S. aureus.

Methods

Non-duplicate S. aureus invasive isolates were collected over an 11-year period. The oxacillin susceptibility was determined with Phoenix 100 (Jan 2011 to Aug 2018) or Vitek 2 (Sep 2018 to Dec 2021), and susceptibility for oxacillin and cefoxitin was determined with agar dilution. Methicillin-resistant S. aureus (MRSA) was confirmed with mecA existence, and the genotype was determined using SCCmec. The association between genotype and antibiotic susceptibility using two automated systems and agar dilution was evaluated.

Results

A total of 842 invasive S. aureus, including 443 mecA+ MRSA and 399 mecA- MSSA, were collected. The susceptibility rates of oxacillin determined by two automated systems and agar dilution were 68.8% (76.8% for Phoenix 100 and 57.6% for Vitek 2) and 54.0%, respectively. When compared with the oxacillin susceptibility using agar dilution, the categorical agreement for Phoenix 100 and Vitek 2 were 0.46% and 0.88%, respectively (p < 0.001). One hundred and forty-three isolates were misinterpreted as oxacillin-susceptible S. aureus (OSSA) using automated systems while comparing with agar dilution, among which molecularly community-associated MRSA (CA-MRSA) outnumbered healthcare-associated MRSA (HA-MRSA) (99 vs 34, p < 0.001). There were 70 mecA+ OSSA (OS-MRSA) using agar dilution, among which 42 harbored SCCmec types were predominantly categorized as CA-MRSA (38, p < 0.001).

Conclusion

The categorical agreement of Vitek 2 in determining oxacillin susceptibility and predicting mecA existence is comparable with agar dilution, whereas Phoenix 100 is not. Most of those ORSA determined by agar dilution but misinterpreted as OSSA by automated systems and OS-MRSA are categorized as CA-MRSA.

Phenotypic and genomic characteristics of oxacillin-susceptible mecA-positive Staphylococcus aureus, rapid selection of high-level resistance to beta-lactams

The aim of this study is to describe the phenotypic and genetic properties of oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA) isolates and their beta-lactam resistant derivatives obtained after selection with oxacillin. A collection of hospital- (HA-) and community-acquired (CA-) MRSA was screened for oxacillin susceptibility. Antibiotic susceptibility testing, population analysis profile (PAP), mecA expression analysis, and whole genome sequencing (WGS) were performed for 60 mecA-positive OS-MRSA isolates. Twelve high-level beta-lactam resistant derivatives selected during PAP were also subjected to WGS. OS-MRSA were more prevalent among CA-MRSA (49/205, 24%) than among HA-MRSA (11/575, 2%). OS-MRSA isolates belonged to twelve sequence types (ST), with a predominance of ST22-t223-SCCmec IVc and ST59-t1950-SCCmec V lineages. OS-MRSA were characterized by mecA promoter mutations at - 33 (C→T) or - 7 (G→T/A) along with PBP2a substitutions (S225R or E246G). The basal and oxacillin-induced levels of mecA expression in OS-MRSA isolates were significantly lower than those in control ST8-HA-MRSA isolates. Most of the OS-MRSA isolates were heteroresistant to oxacillin. High-level beta-lactam resistant OS-MRSA derivatives selected with oxacillin carried mutations in mecA auxiliary factors: relA (metabolism of purines), tyrS, cysS (metabolism of tRNAs), aroK, cysE (metabolism of amino acids and glycolysis). Cefoxitin-based tests demonstrated high specificity for OS-MRSA detection. The highest positive predictive values (PPV > 0.95) were observed for broth microdilution, the VITEK® 2 automatic system, and chromogenic media. Susceptibility testing of CA-MRSA requires special attention due to the high prevalence of difficult-to-detect OS-MRSA among them. Mis-prescription of beta-lactams for the treatment of OS-MRSA may lead to selection of high-level resistance and treatment failures.

Progress in the Prevalence, Classification and Drug Resistance Mechanisms of Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus is a common human pathogen with a variety of virulence factors, which can cause multiple infectious diseases. In recent decades, due to the constant evolution and the abuse of antibiotics, Staphylococcus aureus was becoming more resistant, the infection rate of MRSA remained high, and clinical treatment of MRSA became more difficult. The genetic diversity of MRSA was mainly represented by the continuous emergence of epidemic strains, resulting in the constant changes of epidemic clones. Different classes of MRSA resulted in different epidemics and resistance characteristics, which could affect the clinical symptoms and treatments. MRSA had also spread from traditional hospitals to community and livestock environments, and the new clones established a relationship between animals and humans, promoting further evolution of MRSA. Since the resistance mechanism of MRSA is very complex, it is important to clarify these resistance mechanisms at the molecular level for the treatment of infectious diseases. We firstly described the diversity of SCCmec elements, and discussed the types of SCCmec, its drug resistance mechanisms and expression regulations. Then, we described how the vanA operon makes Staphylococcus aureus resistant to vancomycin and its expression regulation. Finally, a brief introduction was given to the drug resistance mechanisms of biofilms and efflux pump systems. Analyzing the resistance mechanism of MRSA can help study new anti-infective drugs and alleviate the evolution of MRSA. At the end of the review, we summarized the treatment strategies for MRSA infection, including antibiotics, anti-biofilm agents and efflux pump inhibitors. To sum up, here we reviewed the epidemic characteristics of Staphylococcus aureus, summarized its classifications, drug resistance mechanisms of MRSA (SCCmec element, vanA operon, biofilm and active efflux pump system) and novel therapy strategies, so as to provide a theoretical basis for the treatment of MRSA infection.

Cas12a/Guide RNA-Based Platforms for Rapidly and Accurately Identifying Staphylococcus aureus and Methicillin-Resistant S. aureus

In order to ensure the prevention and control of methicillin-resistant Staphylococcus aureus (MRSA) infection, rapid and accurate detection of pathogens and their resistance phenotypes is a must. Therefore, this study aimed to develop a fast and precise nucleic acid detection platform for identifying S. aureus and MRSA. We initially constructed a CRISPR-Cas12a detection system by designing single guide RNAs (sgRNAs) specifically targeting the thermonuclease (nuc) and mecA genes. To increase the sensitivity of the CRISPR-Cas12a system, we incorporated PCR, loop-mediated isothermal amplification (LAMP), and recombinase polymerase amplification (RPA). Subsequently, we compared the sensitivity and specificity of the three amplification methods paired with the CRISPR-Cas12a system. Finally, the clinical performance of the methods was tested by analyzing the fluorescence readout of 111 clinical isolates. In order to visualize the results, lateral-flow test strip technology, which enables point-of-care testing, was also utilized. After comparing the sensitivity and specificity of three different methods, we determined that the nuc-LAMP-Cas12a and mecA-LAMP-Cas12a methods were the optimal detection methods. The nuc-LAMP-Cas12a platform showed a limit of detection (LOD) of 10 aM (~6 copies μL-1), while the mecA-LAMP-Cas12a platform demonstrated a LOD of 1 aM (~1 copy μL-1). The LOD of both platforms reached 4 × 103 fg/μL of genomic DNA. Critical evaluation of their efficiencies on 111 clinical bacterial isolates showed that they were 100% specific and 100% sensitive with both the fluorescence readout and the lateral-flow readout. Total detection time for the present assay was approximately 80 min (based on fluorescence readout) or 85 min (based on strip readout). These results indicated that the nuc-LAMP-Cas12a and mecA-LAMP-Cas12a platforms are promising tools for the rapid and accurate identification of S. aureus and MRSA. IMPORTANCE The spread of methicillin-resistant Staphylococcus aureus (MRSA) poses a major threat to global health. Isothermal amplification combined with the trans-cleavage activity of Cas12a has been exploited to generate diagnostic platforms for pathogen detection. Here, we describe the design and clinical evaluation of two highly sensitive and specific platforms, nuc-LAMP-Cas12a and mecA-LAMP-Cas12a, for the detection of S. aureus and MRSA in 111 clinical bacterial isolates. With a limit of detection (LOD) of 4 × 103 fg/μL of genomic DNA and a turnaround time of 80 to 85 min, the present assay was 100% specific and 100% sensitive using either fluorescence or the lateral-flow readout. The present assay promises clinical application for rapid and accurate identification of S. aureus and MRSA in limited-resource settings or at the point of care. Beyond S. aureus and MRSA, similar CRISPR diagnostic platforms will find widespread use in the detection of various infectious diseases, malignancies, pharmacogenetics, food contamination, and gene mutations.

National surveillance of antimicrobial susceptibilities to ceftaroline, dalbavancin, telavancin, tedizolid, eravacycline, omadacycline, and other comparator antibiotics, and genetic characteristics of bacteremic Staphylococcus aureus isolates in adults: Results from the Surveillance of Multicenter Antimicrobial Resistance in Taiwan (SMART) program in 2020

Methicillin-resistant Staphylococcus aureus (MRSA) causes invasive infections and is associated with community-acquired infections (CAIs) and hospital-associated infections (HAIs). In 2020, 315 S. aureus isolates, including 145 methicillin-susceptible S. aureus (MSSA) and 170 MRSA, mainly associated with bacteremia and mostly CAIs, were collected from 16 hospitals in different regions of Taiwan. Minimum inhibitory concentrations (MICs) were determined using the Sensititre™ complete automated AST system. Staphylococcal cassette chromosome mec (SCCmec) types were analysed using multiplex polymerase chain reaction. The median age of patients infected with MRSA was significantly higher than that of patients infected with MSSA (72.5 years vs. 67.0 years, P=0.027). MIC₅₀/MIC₉₀ values of eravacycline and omadacycline were 0.06/0.12, and 0.25/0.5, respectively. Of the MRSA isolates, 4.1% presented susceptible dose-dependence to ceftaroline, most of which (85.7%) were HAI- and Panton-Valentine leukocidin (PVL)-negative. Among the MRSA isolates, 7.1% were not susceptible to telavancin and tedizolid (mainly type IV, PVL-negative, and CAI), 0.6% were not susceptible to daptomycin (type III, PVL-negative, and HAI), and 1.8% were not susceptible to quinupristin/dalfopristin (three isolates were type III, IV, and V_T, respectively, and all were PVL-negative), but all were susceptible to dalbavancin. In conclusion, patients with bacteremia caused by MRSA were older than those with bacteremia caused by MSSA, SCCmec type IV was more predominant in CAI than in HAI, and MRSA isolates not susceptible to novel anti-MRSA antimicrobials belonged to types II, III, or IV. Further studies that include comprehensive demographics and more detailed descriptions of other antimicrobial-resistant genes are urgently needed.

Comparison between Some Phenotypic and Genotypic Methods for Assessment of Antimicrobial Resistance Trend of Bovine Mastitis Staphylococcus aureus Isolates from Bulgaria

Simple Summary

The aim of the study was to assess the resistance of bovine mastitis Staphylococcus aureus isolates to antimicrobials by comparison of laboratory methods and statistical analysis. For this, 546 mastitis milk samples at 14 farms from 9 districts in Bulgaria were examined. A total of 92 Staphylococcus aureus strains were isolated and identified. The results showed high resistance to sulfadimethoxine (87%), followed by resistance to penicillin (33.7%), erythromycin (13%), streptomycin (8.7%), tetracycline (6.5%) and gentamicin (1.1%). The comparison of the methods demonstrated more than 90% agreement for 9 tested antimicrobial drugs, hence proving reliability of results from monitoring of resistance. Considering the detected discrepancies for some of isolates, an integral evaluation through standard and modern molecular methods for Staphylococcus aureus is recommended. The results from this study add to the microbiology laboratory experience and strategies for mastitis prevention and control programmes.

Abstract

The aim of this study was to assess the resistance of bovine mastitis S. aureus isolates from farms in Bulgaria to different classes of chemotherapeutic drugs by comparison of some phenotypic and genotypic methods by means of Cohen’s kappa statistics. The study comprised 546 milk samples from subclinical and clinical mastitis at 14 farms from 9 districts in the country. A total of 92 Staphylococcus aureus strains were isolated from tested samples and identified by nuc PCR. The results demonstrated high levels of resistance to sulfadimethoxine (87%), followed by resistance to penicillin (33.7%), erythromycin (13%), streptomycin (8.7%), tetracycline (6.5%) and gentamicin (1.1%). The comparison of both phenotypic tests with respect to 9 antimicrobials revealed strong agreement with kappa coefficient 0.836. An almost complete agreement was evidenced between phenotypic resistance to penicillin and blaZ gene presence, to methicillin with mecA gene, to tetracycline with tet genes, but the agreement between erythromycin resistance and erm genes presence was moderate. This study was the first to demonstrate discrepancy between the behaviour to cefoxitin in the disk diffusion test and oxacillin in the MIC test for an isolate shown to carry the mecA gene in the subsequent genetic analysis. Considering the detected discrepancies for some of isolates, an integral evaluation through phenotypic and molecular methods for monitoring of antimicrobial resistance of Staphylococcus aureus is recommended.

Recent Developments in Phenotypic and Molecular Diagnostic Methods for Antimicrobial Resistance Detection in Staphylococcus aureus: A Narrative Review

Staphylococcus aureus is an opportunistic pathogen responsible for a wide range of infections in humans, such as skin and soft tissue infections, pneumonia, food poisoning or sepsis. Historically, S. aureus was able to rapidly adapt to anti-staphylococcal antibiotics and become resistant to several classes of antibiotics. Today, methicillin-resistant S. aureus (MRSA) is a multidrug-resistant pathogen and is one of the most common bacteria responsible for hospital-acquired infections and outbreaks, in community settings as well. The rapid and accurate diagnosis of antimicrobial resistance in S. aureus is crucial to the early initiation of directed antibiotic therapy and to improve clinical outcomes for patients. In this narrative review, I provide an overview of recent phenotypic and molecular diagnostic methods for antimicrobial resistance detection in S. aureus, with a particular focus on MRSA detection. I consider methods for resistance detection in both clinical samples and isolated S. aureus cultures, along with a brief discussion of the advantages and the challenges of implementing such methods in routine diagnostics.

Bacteriophages and phage-delivered CRISPR-Cas system as antibacterial therapy

Multidrug-resistant (MDR) bacterial infections in humans are increasing worldwide. The global spread of antimicrobial resistance poses a considerable threat to human health. Phage therapy is a promising approach to combat MDR bacteria. An increasing number of reports have been published on phage therapy and the successful application of antibacterials derived using this method. Additionally, the CRISPR-Cas system has been used to develop antimicrobials with bactericidal effects in vivo. The CRISPR-Cas system can be delivered into target bacteria in various ways, with phage-based vectors being reported as an effective method. In this review, we briefly summarise the results of randomised control trials on bacteriophage therapy. Moreover, we integrated mechanisms of the CRISPR-Cas system antimicrobials in a schematic diagram and consolidated the research on phage-delivered CRISPR-Cas system antimicrobials.

Active Surveillance, Drug Resistance, and Genotypic Profiling of Staphylococcus aureus Among School-Age Children in China

Methicillin-susceptible (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) nasal colonization predisposes individuals for endogenous infections and is a major threat to children. Recently, oxacillin/cefoxitin-susceptible mecA-positive S. aureus (OS-MRSA) has been reported worldwide. Herein, a prospective, cross-sectional study was conducted across five schools, representing three educational stages, in Guangzhou, China. Nasal swabs from 2,375 students were cultured for S. aureus and all isolates were subjected to antibiotic susceptibility testing phenotypically and confirmed by femB and mecA genetic detection; all the isolates were classified as MSSA, MRSA, or OS-MRSA. All strains were also analyzed by multi-locus sequence typing. Among the 2,375 swabs, S. aureus was detected in 744 children (31.3%, 95% CI: 25.9–36.7%), of whom 72 had MRSA (3.0%, 95% CI: 0.6–5.4%) and 4 had OS-MRSA (0.2%, 95% CI: 0.1–0.3%), of which an oxacillin- and cefoxitin-susceptible MRSA strain was identified. The prevalence of S. aureus and MRSA was higher in younger children. The highest percentage of drug resistance of the S. aureus isolates (n = 744) was to penicillin (85.5%), followed by erythromycin (43.3%) and clidamycin (41.0%). The most prevalent sequence types (STs) were ST30, ST45, and ST188 in MSSA, accounting for 38.7% of the total isolates, whereas ST45, ST59, and ST338 accounted for 74.6% of the MRSA isolates and ST338 accounted for 50.0% of the OS-MRSA isolates. The MRSA and OS-MRSA isolates (n = 76) were grouped into three clades and one singleton, with clonal complex (CC) 45 as the most predominant linkage. The top nine multi-locus sequence typing-based CCs (CC30, CC45, CC5, CC1, CC15, CC944, CC398, CC59, CC7) represented 86.7% of all S. aureus isolates. All CC30 isolates were resistant to erythromycin and clidamycin, and almost all these isolates were also resistant to penicillin (99.2%). The CC45 and CC59 isolates exhibited high resistance rates to oxacillin at 31.5 and 59.0%, respectively. This study provides updated data valuable for designing effective control strategies to mitigate the burden of disease and to improve the adequacy of empirical antimicrobial treatments for potentially harmful infections.

Ceftriaxone and methicillin-susceptible staphylococcus aureus: a perspective from pharmacokinetics/pharmacodynamics studies

INTRODUCTION

: Usage of ceftriaxone-based therapy to treat Methicillin-Susceptible Staphylococcus aureus (MSSA) infections is a controversial issue, from in vitro to clinical studies.

AREA COVERED

: We conducted a literature review using PubMed of articles with ceftriaxone pharmacokinetics parameters and built a probability of target attainment (PTA) based on PK values from stable conditions (non-critically-ill patients) with goals of fT>55%, fT>75%, and fT>100%. Ceftriaxone's minimal inhibitory concentration from 31 MSSA strains (0.25-64mg/L) was used to build the cumulative fraction response (CFR). The isolates were clinically relevant from blood, bronchoalveolar lavage, and soft tissue biopsy.

EXPERT OPINION

The results from controversies about using ceftriaxone for MSSA infections have been commonly addressed in the literature. However, variables such as (i) pharmacokinetic profile, (ii) pharmacodynamic target, (iii) site of infection, and (iv) MIC distributions may influence divergences. From this pharmacokinetics-pharmacodynamics perspective, ceftriaxone may be a reasonable option for MSSA infections when the MIC50 and MIC90 were 4mg/L and 8mg/L. CFR analysis demonstrated that ceftriaxone 1g q24h could be used if bacteriostasis is the aim (fT>55%), while 1g q12h should be used for bactericidal effects (fT>75% or fT>100%). Since there is a lack of data from clinical trials, the findings should be interpreted cautiously.

First Report of Oxacillin Susceptible mecA-Positive Staphylococcus aureus in a Children's Hospital in Kunming, China

Purpose

The present study investigated the prevalence characteristics of oxacillin susceptible mecA-positive Staphylococcus aureus (OS-MRSA) in a children's hospital in Kunming from January 2019 to December 2020.

Methods

A total of 499 S. aureus strains were included in the study and tested for oxacillin susceptibility using the VITEK 2 Compact automated antimicrobial susceptibility test system. All oxacillin-susceptible strains were detected mecA and mecC by polymerase chain reaction (PCR). E-test was used to compare the minimum inhibitory concentration (MIC) values of methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and OS-MRSA for oxacillin, cefoxitin, penicillin, vancomycin, erythromycin, and clindamycin. Molecular typing of OS-MRSA was performed by MLST and SCCmec typing. Toxin genes were detected by PCR.

Results

Forty-five OS-MRSA strains were detected, for an overall rate of 9.02% (45/499). The MICs of MSSA, OS-MRSA, and MRSA against oxacillin were concentrated at 0.38, 0.38, and 12 μg/mL, respectively; the cefoxitin MICs of MSSA and MRSA were concentrated at 2 and 32 μg/mL respectively; and MICs of OS-MRSA were concentrated at 2 and 8 μg/mL; penicillin, vancomycin and erythromycin MICs against MSSA, OS-MRSA, and MRSA showed same centralized points and were 32, 1, and 256 μg/mL, respectively; the MICs of clindamycin against MSSA were 0.5 μg/mL, while that against OS-MRSA and MRSA were concentrated at 256 μg/mL. Molecular typing of OS-MRSA was dominated by ST59-SCCmec IV. The carrier rates of hemolysin genes (hl-a, hl-d) and fibrinogen-binding clumping factor genes (clfA, clfB) were 100% in OS-MRSA, followed by 40% (18/45) for enterotoxin genes (sea, seb).

Conclusion

OS-MRSA has a high detection rate in children, and main molecular typing is ST59-SCCmecIV in Kunming. The identification ability of automated antibacterial drug sensitivity test detection systems for OS-MRSA is very limited. A combination of phenotypic analysis and molecular detection should be used to improve OS-MRSA identification.

Clonal Diversity, Low-Level and Heterogeneous Oxacillin Resistance of Oxacillin Sensitive MRSA

Purpose

This study investigates the phenotypic and genotypic resistance features of OS-MRSA clinical isolates and their distinctive sensitivities to oxacillin.

Methods

1200 clinical isolates of Staphylococcus aureus were enrolled in this study. Automated antibiotics susceptibility tests on VITEK-2 and BD Phoenix-100^TM, cefoxitin disc diffusion method, oxacillin broth microdilution method, mecA, and mecC gene detection were performed to identify OS-MRSA. MLST, PFGE, SCCmec, and spa typing methods were employed to determine genotypes of OS-MRSA isolates. Heterogeneous resistance of OS-MRSA isolates was detected using the population analysis profiling method, and PBP2a latex agglutination assay was used to detect the expression of PBP2a protein for 14 OS-MRSA isolates and their highly resistant subpopulations.

Results

A total of 14 OS-MRSA isolates (1.17%, 14/1200) were identified, and all of the isolates were confirmed to be positive with the mecA gene and negative with the mecC gene. All of the 14 OS-MRSA isolates were identified as MSSA by VITEK-2, BD Phonenix-100, and oxacillin broth microdilution methods, while 21.43% (3/14) isolates were determined to be MRSA by the cefoxitin disk diffusion method. Genotypes of the 14 OS-MRSA isolates were diverse, and no dominant clones were identified. The prevalence of pvl gene among 14 OS-MRSA isolates was high up to 64.29% (9/14). All of the isolates showed heterogeneous resistance to oxacillin, while frequencies of the oxacillin-resistant subpopulations ranged from 10^-9 to 10^-5 and differed significantly among different isolates.

Conclusion

The overall prevalence of OS-MRSA was relatively lower, but lower oxacillin MICs, low testing sensitivity of routine antibiotics susceptibility testing methods and weak PBP2a protein expression were observed in this study. 14 OS-MRSA showed diverse genotypes and universal heterogeneous resistance, and inaccurate laboratory identification and improper antimicrobial usage may promote the induction of highly resistant subpopulations and lead to treatment failure.