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A consistently decreasing prevalence of MRSA infections in China has been reported, however, the underlying mechanism of molecular processes responsible for this decline in MRSA infections has been poorly understood. We conducted an epidemiologic investigation to determine the dynamic changes of Staphylococcus aureus infections. A total of 3695 S. aureus isolates was recovered from 2008 to 2017, and subsequently characterized by infection types, resistance profile, and clone types. The frequency of respiratory infection decreased over the study period from 76% to 52%. The proportion of MRSA remarkably decreased (from 83.5% to 54.2%, 2008-2017) (p < .0001). The prevalence of predominant healthcare-associated MRSA (HA-MRSA) clones, ST239-t030 and ST239-t037, significantly decreased (from 20.3% to 1% and 18.4% to 0.5%, 2008-2017, respectively); both of them were replaced by the continually growing ST5-t2460 clone (from 0% to 17.3%, 2008-2017). Epidemic community-acquired MRSA (CA-MRSA) ST59 and ST398 clones also increased (from 1.0% to 5.8% and 1.8% to 10.5%, 2008-2017, respectively). These results demonstrated a significant decrease in the previously dominant HA-MRSA ST239 clones, leading to a marked decrease in the prevalence of MRSA over the past decade, and shed new light on the complex competition of S. aureus clones predominating within the health-care environment.

Clostridium difficile infection (CDI) is increasingly recognized globally as a cause of significant morbidity and mortality. This study aimed to provide insight into the various dynamics of C. difficile transmission and infection in the hospital. We monitored the toxin and resistance profiles as well as evolutionary relationships of C. difficile strains to determine the epidemiology over time in a teaching hospital in Shanghai, China between May 2014 and August 2015. The CDI incidence of inpatients and outpatients were 67.7 cases and 0.3 cases per 100,000 patient-days, with a nosocomial patient-environment-patient transmission in May and June 2015. C. difficile genotype ST81, a clone with tcdA-negative and tcdB-positive, was not only the most common strain (30.8%, 28/91) but also had much higher resistance rates to clindamycin and moxifloxacin compared with non-ST81 genotypes. Hospitalized patients infected with ST81 genotypes were over 65 years of age and had more comorbidities, however patients infected with ST81 presented with less clinical symptoms than non-ST81 infected patients. This study provides initial epidemiological evidence that C. difficile ST81 is a successful epidemic genotype that deserves continuous surveillance in China.

During 2005–2014, community-associated methicillin-resistant Staphylococcus aureus infections increased in Shanghai, China. Most infections were caused by sequence type 59 S. aureus that lacked Panton-Valentine leukocidin. This finding challenges the notion that Panton-Valentine leukocidin is necessary for epidemiologic success of community-associated methicillin-resistant S. aureus.

Background Severe infections with highly virulent community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) are a global problem. However, the molecular events defining the evolution of CA-MRSA are still poorly understood. MRSA of sequence type (ST) 398 is known to frequently infect livestock, while ST398 isolates infecting humans are commonly methicillin-susceptible or represent MRSA originating from livestock-associated (LA)-MRSA. Methods We used whole genome sequencing of newly detected CA-MRSA ST398 isolates, in comparison to geographically matched LA-MRSA and methicillin-sensitive ST398, to determine their evolutionary history. Furthermore, we used phenotypic analyses including animal infection models to gain insight into the evolution of virulence in these CA-MRSA isolates. Finally, we determined methicillin resistance and expression of the methicillin resistance-conferring gene mecA and its penicillin-binding protein product, PBP2a, in a large series of CA-MRSA strains of divergent STs. Results We report several cases of severe and fatal infections due to ST398 CA-MRSA. The responsible isolates showed the typical genetic characteristics reported for human-adapted methicillin-sensitive ST398. Whole genome sequencing demonstrated that they evolved from human-adapted, methicillin-susceptible clones on several different occasions. Importantly, the isolates had not undergone consistent genetic alterations or changes in virulence as compared to their methicillin-susceptible predecessors. Finally, we observed dramatically and consistently lower methicillin resistance and expression of the resistance gene mecA, as compared to hospital-associated MRSA strains, in a diverse selection of CA-MRSA strains. Conclusions Our study presents evidence for the development of highly virulent human-adapted ST398 CA-MRSA isolates from methicillin-susceptible predecessors. Notably, our investigation indicates that, in contrast to widespread notions, the development of CA-MRSA is not necessarily associated with the acquisition of specific virulence genes or other virulence-increasing changes. Rather, our findings emphasize the importance of the CA-MRSA-characteristic staphylococcal cassette chromosome mec types, which provide only low-level methicillin resistance, for that process. Our findings are of particular importance for the diagnosis of CA-MRSA, inasmuch as they indicate that the presence of specific virulence genes cannot generally be used for that purpose. Electronic supplementary material The online version of this article (doi:10.1186/s13073-018-0514-9) contains supplementary material, which is available to authorized users.

Background: The increasing prevalence of carbapenem-resistant Klebsiella pneumoniae (CRKP) poses an immediate threat to treatment worldwide. This retrospective study assessed the molecular epidemiology and determined the risk factors for and outcomes of CRKP infections in a general teaching hospital in Shanghai, China. Methods: From January 2013 to July 2015, 100 consecutive unique CRKP isolates isolated from hospitalized patients were collected. Isolates were screened for antibiotic resistance genes by polymerase chain reaction and molecular typing was performed by pulsed-field gel electrophoresis (PFGE). Patients infected with CRKP comprised the case group and were compared to the control group of patients infected with carbapenem-susceptible Klebsiella pneumoniae. Therapeutic effects were compared in the CRKP infection group. Results: Among the 100 CRKP isolates, the percentages of multidrug-resistant, extensively drug-resistant (XDR), and pandrug-resistant were 50.0, 50.0, and 0%, respectively. All the CRKP isolates produced KPC-2 and could be divided into 18 PFGE clusters (A-O) and 70 subtypes. No dominant intra-hospital PFGE type was detected using a cutoff of 80% similarity. The ratio of CRKP infection to colonization was 51 to 49. Risk factors correlated with CRKP infection included pulmonary disease (p = 0.038), ICU stay (p = 0.002), invasive ventilation (p = 0.009), blood transfusion (p = 0.028), parenteral nutrition (p = 0.004), sputum suction (p = 0.006), medical history of previous hospitalization (p = 0.022), exposure to antibiotics 90 days before infection (p = 0.030), and antibiotic exposure during hospital stay including carbapenems (p = 0.013), enzyme inhibitors (p = 0.021), nitroimidazoles (p = 0.029), and glycopeptides (p = 0.000). Multivariable analysis showed that sputum suction (odds ratio 3.090, 95% confidence intervals 1.004-9.518, p = 0.049) was an independent risk factor for CRKP infections. Patients infected with CRKP with longer carbapenems treatment course (p = 0.002) showed better outcome. Conclusion: This study showed the severity of CRKP infection in eastern China. Sputum suction was an independent risk factor for CRKP infection. Prolonged duration of treatment with carbapenems benefited the patients infected with CRKP.

The ESAT-6 secretion system (ESS) has been reported to contribute to the virulence and pathogenicity of several Staphylococcus aureus strains such as USA300 and Newman. However, the role of the ESS in community-associated S. aureus (CA-SA) lineage ST398 in China is not well understood. By comparing the ess locus of ST398 with the published S. aureus sequence in the NCBI database, we found one gene in the ess locus encoding a novel WXG superfamily protein that is highly conserved only in ST398. LC-MS/MS and Western blot analysis revealed that this protein is a novel secreted protein controlled by the ST398 ESS, and we named the protein EsxX. Although EsxX was not under the control of the accessory gene regulator like many other virulence factors and had no influence on several phenotypes of ST398, such as growth, hemolysis, and biofilm formation, it showed important impacts on immune evasion and virulence in ST398. An esxX deletion mutant led to significantly reduced resistance to neutrophil killing and decreased virulence in murine skin and blood infection models, indicating its essential contribution to the evasion of innate host defense and virulence to support the pathogenesis of ST398 infections. The function of this novel secreted protein EsxX might help us better understand the role of the ESS in the virulence and epidemic success of the CA-SA lineage ST398.

Staphylococcus aureus is highly pathogenic and can cause diseases in both humans and domestic animals. In animal species, including ruminants, S. aureus may cause severe or sub-clinical mastitis. This study aimed to investigate the molecular profile, antimicrobial resistance, and genotype/phenotype correlation of 212 S. aureus isolates recovered from cases of bovine mastitis from 2014 to 2015 in the Shanghai and Zhejiang areas of China. Nineteen sequence types (STs) were determined by multi-locus sequence typing, while the dominant ST was ST97, followed by ST520, ST188, ST398, ST7, and ST9. Within 14 methicillin-resistant S. aureus (MRSA) isolates and 198 methicillin-susceptible S. aureus (MSSA) isolates, ST97 was the predominant MSSA clone and ST9-MRSA-SCCmecXII-spa t899 was the most common MRSA clone. The MRSA strains showed much higher rates of resistance to multiple antibiotics than did MSSA strains. Compared with other MSSA strains, MSSA ST398 was more resistant to clindamycin, erythromycin, and ciprofloxacin. No isolates were resistant to vancomycin, teicoplanin, or linezolid. The molecular profiles of the virulence genes varied in different strains. ST520 strains carried seg-sei-sem-sen-seo genes, and ST9 and ST97 harbored sdrD-sdrE genes. Virulence phenotype analysis showed diversity in different clones. Biofilm formation ability was significantly enhanced in ST188 and ST7, and red blood cell lysis capacity was relatively strong in all S. aureus strains of animal origin except ST7. Our results indicate that MSSA was the predominant S. aureus strain causing bovine mastitis in eastern regions of China. However, the presence of multidrug resistant and toxigenic MRSA clone ST9 suggests that comprehensive surveillance of S. aureus infection should be implemented in the management of animal husbandry products.

The highly successful epidemic of healthcare-associated methicillin-resistant Staphylococcus aureus (HA-MRSA) ST239 is a growing concern worldwide, due to its progressive adaptation to the highly selective environment of the healthcare system. HA-MRSA ST239 display the reduced virulence and successfully colonize in hospital settings, while the emergent community-associated MRSA (CA-MRSA) maintain full virulence and cause infections in the community environment. Our aim was to investigate what enables S. aureus ST239 to be highly adaptive under hospital circumstances and gradually progress to a series of widespread invasive infections. We found that spa expression of HA-MRSA ST239 is much higher than that of CA-SA ST398. And we discovered that the highly production of staphylococcal protein A (SpA), having no concern with spa gene structure, enhances nasal colonization and cell adhesion in ST239. S. aureus ST239 defends against the adaptive immune response by resisting phagocytosis and inducing apoptosis of B cells through expression of surface-anchored and released protein A, facilitating its dissemination within the circulatory system to other organs. Protein A also plays another key role in subverting the host immune response through its ability to induce early shedding of TNF-α receptor 1 (TNFR1) from phagocytic cells. The increased levels of soluble TNFR1 present during experimental S. aureus ST239 infection may neutralize circulating TNF-α and impair the host inflammatory response. Protein A is also a virulence factor, as tested in our bacteremia model in mice, contributing to the durative tissue damage of abscess formation sites in ST239 infection. These functions of protein A eventually benefit to widespread infections of S. aureus ST239. We draw the conclusion that Staphylococcal Protein A may be a crucial determinant in the duration of colonization and immune evasion of ST239 infections, contributing to persistent spread in the hospital settings. These results suggest that antibodies against protein A may provide insights into the development of novel treatments against S. aureus, especially HA-MRSA.

Understanding virulence is vital for the development of novel therapeutics to target infections with community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), which cause an ongoing epidemic in the United States and are on a global rise. However, what defines virulence particularly of global CA-MRSA lineages is poorly understood. Threatening a vast population, the predominant Asian CA-MRSA lineage ST59 is of major epidemiological importance. However, there have been no molecular analyses using defined virulence gene deletion mutants in that lineage as of yet. Here, we compared virulence in skin, lung and blood infection models of ST59 CA-MRSA isolates with geographically matched hospital-associated MRSA isolates. We selected a representative ST59 CA-MRSA isolate based on toxin expression and virulence characteristics and produced isogenic gene deletion mutants of important CA-MRSA virulence determinants (α-toxin, PSM α, Agr) in that isolate for in-vitro and in-vivo analyses. Our results demonstrate strongly enhanced virulence of ST59 CA-MRSA over hospital-associated lineages, supporting the notion that enhanced virulence is characteristic for CA-MRSA. Furthermore, they show strong and significant contribution of Agr, α-toxin and PSMα to pathogenesis of ST59 CA-MRSA skin, lung and blood infection, emphasizing the value of drug development efforts targeted toward those virulence determinants.

Novel Staphylococcus aureus clones continue to emerge that cause infections in otherwise healthy people. One example is the sequence type (ST) 398 lineage, which we show here is increasing in importance as a significant cause of community-associated (CA) human infections in China. We have a profound lack of understanding about what determines the considerable virulence potential of such newly emerging clones. Information about the contribution to virulence of the more recently discovered ESAT-6 secretion system (ESS) has remained particularly scarce. The Chinese ST398 isolates exhibited significantly increased expression of ESS genes as compared to predominant hospital-associated clones, which we found is likely due to increased expression of the accessory gene regulator (Agr) system and control of ESS by Agr. Importantly, deletion of essB in ST398 resulted in significantly reduced resistance to neutrophil killing and decreased virulence in murine skin and blood infection models. Our results demonstrate a key function of ESS in promoting virulence and mechanisms of resistance to innate host defense in an important emerging CA-S. aureus lineage. They suggest that ESS has a so far underestimated role in promoting aggressive virulence and epidemiological success of S. aureus.