Background

Gram-positive bacteria, such as Staphylococcus aureus, Enterococcus faecalis, and Enterococcus faecium are dangerous and challenging agents of infection due to their increasing resistance to antibiotics [1]. Among these, S. aureus are the most frequently isolated, making up 29.1% of the isolated Gram-positive bacterial populations, and infections caused by S. aureus (both community-associated or nosocomial-associated) are reported all over the world [2, 3]. Enterococci (including E. faecalis and E. faecium) are the second frequently isolated Gram-positive bacteria at 19.5%. Past generations of enterococci were mainly associated to urinary-tract infection, but recently more and more enterococci are isolated from other infections [4], and an outbreak of vancomycin resistant enterococci (VRE) in a solid organ transplant unit was reported in 2018 [5].

The epidemiology of resistance in Gram-positive bacteria has undergone major changes in recent decades, with methicillin-resistant S. aureus (MRSA) and VRE now being of international concern [6]. Antimicrobial resistances of Gram-positive bacteria have been reported in many countries [7,8,9]; however, it was only in recent years that vanM-carrying E. faecalis strains were isolated from patients and the clinical environment in the Second Affiliated Hospital of Zhejiang University School of Medicine [27]. These may have a different resistance than strains with the vanM gene, seen in China [28] or in strains with vanA dominance, seen in Poland [29]. In our study, resistance rate of E. faecalis to linezolid increased from 1.6% in 2008 to 2.97% in 2016 [36] and can be attributed to patients with severe co-morbidities. Stratifying the data by patient age, it is observed that the proportion of MRSA and VRE was the highest in isolates from elderly patients older than 75 years, but was the lowest from children aged 2 years - 3 years and infants younger than 1 year. The high numbers of MRSA and VRE isolates from elderly patients may be due to these patients having more underlying diseases and a greater history of antibiotic use than the child and the infant group. As well, decreased nutrition and immune function, often seen in the elderly, may also be contributing elements. Other identified risk factors associated with MRSA and VRE include having a source of isolates. We found that puncture fluid (containing hydrothorax, ascites, articular cavity fluid, pericardial fluid and cerebrospinal fluid) has the lowest proportion (21.32%) of MRSA, though the underlying reason for this phenomenon needs to be further studied. In terms of treatment, we should be wary of methicillin-sensitive S. aureus when S. aureus is isolated from puncture fluid in Zhejiang province. With the noted exception of blood and urine, there is no difference in the specimen types in proportions of VRE isolates.

These findings will provide valuable information for infection control practices. Although many surveillance projects of antibiotic resistance have been carried out in China, they always cover only the tertiary hospitals. In our study, a wider range of scenarios were seen where resistant strains could occur, indicating the importance of performing regional antibiotic resistance surveillance. The current study had some limitations: we had a relatively short span of time (3 years) for the data collection, we had a limited number of IIB hospitals to contribute data to the study, and because ours was a retrospective analysis, we had an inability to obtain the original strains. In the future, we would like to expand this surveillance to cover more IIB and rural area clinics and also collect the original strains for intensive study.

Conclusion

The detection rates of MRSA stayed at moderate levels, and VRE stayed at low levels during the last three years, and local dissemination was found in MRSA and VRE isolates. The highest risk factors for MRSA and VRE infection were patient status in a IIIA hospital, age older than 75 years and hospitalization in the ICU ward. As a result of our findings, we suggest that sustained surveillance is necessary to prevent the spread or clonal dissemination of drug-resistant strains in Zhejiang China.