Prevalence of clinical meticillin-resistant Staphylococcus aureus (MRSA) with high-level mupirocin resistance in Shanghai and Wenzhou, China
Introduction
Meticillin-resistant Staphylococcus aureus (MRSA) continues to be a common human pathogen in nosocomial infections and is generally recognised as the most significant because of the burden of serious disease it causes and the worldwide dissemination of multidrug-resistant strains. Nasal and extranasal colonisation of MRSA has been linked to some invasive MRSA infections and plays a crucial role in the spread of these strains [1].
Mupirocin (pseudomonic acid A) is a topical antibiotic that has been extensively used for treating MRSA skin and soft-tissue infections, decreasing certain types of surgical site infections and eliminating nasal colonisation of MRSA among patients and medical staff. However, the massive use of this agent has led to the rapid emergence of resistance in MRSA worldwide [2], [3], [4], [5], [6].
Two mupirocin resistance phenotypes [low-level (MuL) and high-level (MuH) mupirocin-resistant] are defined in staphylococci. MuL is frequently related to site mutations in the chromosomal isoleucyl-tRNA synthetase gene (ileS-1), whereas MuH is generally due to acquisition of a plasmid hosting the mupA gene (ileS-2) encoding an additional novel isoleucyl-tRNA synthetase whose function is not inhibited by mupirocin [7]. Treatment with mupirocin is likely to be ineffective in the setting of MuH MRSA and there is some evidence to show that MuL may also lead to treatment failure [8]. Therefore, it is of significance to know the prevalence of mupirocin resistance in MRSA for treating infections of resistant strains and incorporating mupirocin into an infection control programme.
At present, the prevalence of mupirocin-resistant MRSA in hospitals in China is still unknown. We therefore undertook the current study to determine the extent of MuH and MuL in MRSA in two regions of China and investigated the molecular characteristic of resistant strains by staphylococcal chromosomal cassette mec (SCCmec), staphylococcal protein A (spa) and pulsed-field gel electrophoresis (PFGE) typing methods.
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Bacterial strains
A total of 803 MRSA isolates from various clinical specimens of individual inpatients were obtained from five university hospitals located in two geographical areas of China [four (Hospitals I–IV) in Shanghai (n = 411 isolates) and one (Hospital V) in Wenzhou (n = 392 isolates)] for screening for mupirocin resistance. The Wenzhou isolates were collected between January 2005 and May 2007 and the Shanghai strains were gathered from August 2005 to May 2008. Standard microbiological methods for
Prevalence of high-level mupirocin-resistant MRSA
Of the 803 MRSA isolates tested, 53 (6.6%) were confirmed to be MuH (MICs of all isolates ≥1024 μg/mL); MuL was not detected. Among the MuH isolates screened, 28 were identified in Shanghai (13 from Hospital I, 4 from Hospital II, 6 from Hospital III and 5 from Hospital IV) (6.8% of Shanghai isolates) and 25 in Wenzhou (Hospital V) (6.4% of Wenzhou isolates). Most of the MuH isolates were recovered from respiratory samples (86.8%). Intensive Care Units, surgical wards, burn wards and neurology
Discussion
The emergence of mupirocin resistance among MRSA isolates has been a well-defined phenomenon in many parts of the world. To our knowledge, the present study is the first to survey the prevalence of mupirocin resistance in staphylococci from hospitals in China. Our results demonstrated a prevalence of 6.6% for MuH among MRSA strains collected from Shanghai and Wenzhou regions of China; MuL was not found. The proportion of MuH in the two regions was similar (6.8% and 6.4%, respectively). This
Acknowledgment
The authors acknowledge the help of Li-zhong Han for his considerable effort in obtaining isolates and reference strains for SCCmec typing.
Funding: This study was supported by the fund of the Scientific Research Institute of Shanghai (grant number 05ZR14129).
Competing interests: None declared.
Ethical approval: Not required.
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