JAC Advance Access originally published online on March 31, 2006
Journal of Antimicrobial Chemotherapy 2006 57(6):1122-1127; doi:10.1093/jac/dkl114
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Risk factors for specific methicillin-resistant Staphylococcus aureus clones in a Korean hospital
1 Department of Microbiology, Kyungpook National University School of Medicine Daegu 700-422, Korea 2 Institute of Infectious Diseases, Kyungpook National University School of Medicine Daegu 700-422, Korea 3 Department of Internal Medicine, Kyungpook National University School of Medicine Daegu 700-422, Korea 4 Department of Emergency Medicine, Kyungpook National University School of Medicine Daegu 700-422, Korea
*Corresponding author. Tel: +82-53-420-4844; Fax: +82-53-427-5664; E-mail: leejc{at}knu.ac.kr
Received 2 November 2005; returned 12 December 2005; revised 3 March 2006; accepted 12 March 2006
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Abstract |
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Objectives: To analyse the risk factors for nosocomial methicillin-resistant Staphylococcus aureus (MRSA) infections caused by different clonal types.
Methods: A total of 134 non-duplicate nosocomial MRSA isolates were analysed for clonal types by molecular typing techniques. The medical records of 90 patients who had documented MRSA infection were evaluated retrospectively.
Results: Two predominant MRSA clones of sequence types (STs) ST239 (n = 75) and ST5 (n = 39) accounted for 85% of the isolates. Management of patients in the departments of orthopaedic surgery, neurosurgery and plastic surgery was identified as a risk factor for infection with MRSA of ST239, while the presence of intravascular catheters was a risk factor for infection with ST5. Pulmonary infection was significantly higher in the patients infected with ST239 strains than in the patients infected with ST5 strains (P < 0.05). The overall mean duration of antimicrobial therapy for the patients with ST239 infection was significantly more than that for the patients with ST5 infection (P < 0.05).
Conclusions: ST239 and ST5 were the predominant MRSA clones in the study hospital. Risk factors were significantly different between ST239 and ST5 strains. The results of this study will be of use in designing larger prospective epidemiological studies for MRSA infection based on clonal types.
Keywords: MRSA , nosocomial infections , clinical impact , sequence types
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Introduction |
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Over the past decade methicillin-resistant Staphylococcus aureus (MRSA) infections have increased in many countries and this notorious organism has become one of the leading causes of nosocomial infections.1–7 A high proportion of nosocomial MRSA infections are caused by a relatively few epidemic MRSA clones, and five major international clones, designated the Iberian, Brazilian, Hungarian, New York/Japan and paediatric clones, have been identified.8,9 The widespread occurrence of S. aureus strains might be facilitated by the acquisition of exogenous staphylococcal chromosomal cassette mec (SCCmec) DNA elements.10
The overall frequency of nosocomial infection caused by MRSA is currently estimated to be more than 70% in Korea, and MRSA infections contribute significantly to patient morbidity and mortality.11 In a recent study showing the prevalence of MRSA clones in Asian countries, the New York/Japan clone (ST5-MRSA-II) was found to be predominant in Korea and Japan, while the Hungarian (ST239-MRSA-III) and Brazilian clones (ST239-MRSA-IIIA) were predominant in many other Asian countries, including China, India, Indonesia, Singapore, Sri Lanka, Thailand, Vietnam and Hong Kong.12,13
Several studies have documented the risk factors and the mortality rates of MRSA infection in hospital settings. The risk factors for MRSA infection include longer duration of hospitalization, severe underlying diseases, intravascular catheter infections, soft tissue and wound infections, an intensive care unit stay and recent antimicrobial treatment.14–18 The mortality rate associated with MRSA bacteraemia is significantly higher than that for methicillin-susceptible S. aureus (MSSA) bacteraemia.19,20 However, differences in the risk factors, clinical courses, antimicrobial regimens and mortality rates for MRSA infection based on clonal types have not been elucidated yet. We analysed the clonal types of MRSA isolates from the tertiary-care teaching hospital in Daegu, Korea, during the period 2001 to 2003 and evaluated the risk factors, clinical courses and mortality rates for MRSA infection based on clonal types.
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Materials and methods |
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Bacterial strains
A total of 134 non-duplicate clinical MRSA isolates were collected from the tertiary-care teaching hospital in Daegu, Korea, during the period 2001 to 2003: 28 isolates from February to October 2001, 61 isolates from January to October 2002 and 45 isolates from February to December 2003. All MRSA isolates from clinical specimens were collected during this period. Patients with nosocomial infection, defined as an infection acquired 48 h after hospital admission, were included. Contaminants, colonizers of uninfected sites in routine cultures and MRSA isolates from the patients with polymicrobial infection were excluded from the study. No recognized MRSA outbreaks occurred in the hospital during the study period. S. aureus isolates were tested for phenotypic resistance to oxacillin by the salt agar dilution method according to the CLSI (formerly NCCLS).21 MRSA was genotypically confirmed using PCR for the nuc and mecA genes.3,22 MRSA isolates were obtained from 91 men (67.9%) and 43 women (32.1%). Patients were admitted in the following departments: internal medicine (n = 30, 22.4%), orthopaedic surgery (n = 25, 18.7%), general surgery (n = 20, 14.9%), plastic surgery (n = 16, 11.9%), neurosurgery (n = 13, 9.7%), paediatrics (n = 7, 5.2%) and other departments (n = 23, 17.2%). MRSA isolates were obtained from wounds (n = 53, 39.6%), blood (n = 41, 30.6%), pus (n = 11, 8.2%), urine (n = 10, 7.5%), the upper respiratory tract (n = 6, 4.5%) and other clinical sources (n = 13, 9.7%).
Clinical data collection
The medical records of 90 patients who had clinical and microbiological evidence of MRSA infections were evaluated. MRSA isolates were classified by an infectious disease physician as those causing infection or those causing colonization according to established diagnostic criteria for nosocomial infections.23 Colonization was defined when MRSA was isolated from a patient lacking clinical signs or symptoms of disease. The other 44 patients who were deemed to be colonized rather than infected with MRSA were not investigated further. The data that were collected retrospectively included patient demographics, the date on which each sample was obtained for the bacterial culture, the primary site of infection, the secondary site of infection, the time from the first positive culture to the initiation of antimicrobial therapy, the duration of hospitalization, the patient's underlying diseases, the presence of intravascular catheters or prostheses, recent history of antimicrobial therapy, the duration of antimicrobial therapy during MRSA infection, therapeutic procedures other than antimicrobial therapy and time from the positive culture to death. Patient outcomes were classified as ‘died of infection’, ‘died of causes other than infection’ and ‘recovery’ on the basis of clinical and microbiological data.
Molecular typing
Chromosomal DNA was extracted using the Wizard Genomic DNA Preparation Kit (Promega, Madison, WI, USA). Multilocus sequence typing (MLST) was performed using the method described by Enright et al.24 PCR fragments of seven genes, arcC, aroE, glpF, gmk, pta, tpi and yqiL, were directly sequenced in an ABI Prism 3100 analyzer (Applied Biosystems, Foster City, CA, USA). Allele number and sequence type (ST) were determined with the use of the database accessible via http://saureus.mlst.net/. Clonal complex (CC) was determined using the program eBURST v2 based on related STs (http://eburst.mlst.net/).25 Multiplex PCR was performed to identify SCCmec structural types.26
Statistical analyses
Univariate analyses were performed using the Student's t-test, 
2 test or Fisher's exact test. A multivariate analysis was performed using logistic regression analysis to determine the influence of variables on the probability of acquisition of MRSA. Statistical significance was assigned to two-sided P values of <0.05. Statistical analyses were performed using SPSS, version 12 for Windows (SPSS Inc, Chicago, IL, USA).
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Results |
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Characteristics of MRSA isolates
The 134 MRSA isolates were classified into five CCs, seven STs and three SCCmec types with variants (Table 1). The most prevalent MRSA clone was ST239, which accounted for 56% (n = 75). The second most frequent clone was ST5, which accounted for 29% (n = 39). The remaining five STs, ST72 (n = 9), ST344 (n = 6), ST1 (n = 3), ST221 (n = 1) and an undetermined ST (n = 1), accounted for 15% (n = 20). Six isolates belonging to ST344 were determined to belong to CC8 according to the eBURST program (http://www.mlst.net), although they were single locus variant of ST239. The isolation rates of MRSA belonging to ST239 and ST5 were different between the hospital wards. Isolates of ST239 were commonly isolated from orthopaedic surgery (n = 19, 25.3%), plastic surgery (n = 12, 16.0%) and neurosurgery (n = 10, 13.3%), while isolates of ST5 were rarely isolated from orthopaedic surgery (n = 2, 5.1%), plastic surgery (n = 3, 7.7%) and neurosurgery (n = 3, 7.7%). MRSA isolates of ST5 were more commonly found in internal medicine (n = 10, 25.6%) and general surgery (n = 8, 20.5%). Of 75 isolates of ST239, 52 (69.3%) were obtained from the patients with infections and 23 (30.7%) were obtained from the patients deemed to be colonized. Of 39 isolates of ST5, 24 (61.5%) were from infected patients and 15 (38.5%) were from colonized patients.
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Risk factors for MRSA infection
With regard to risk factors for infection with MRSA of ST239 and ST5, there were no significant differences with respect to age, sex, duration of hospitalization and underlying diseases (Tables 2 and 3). Isolates of ST239 were commonly isolated from orthopaedic surgery, neurosurgery and plastic surgery, and three surgical wards were categorized to compare isolation frequency of ST239 and ST5 clones according to isolation sites. The hospital departments in which patients were managed, including orthopaedic surgery, neurosurgery and plastic surgery, were the risk factor for ST239 infection in univariate analysis (P < 0.005) and were also statistically significant in multivariate analysis (P < 0.05). The presence of intravascular catheters was significantly higher in patients with ST5 infection than in those with ST239 infection (P < 0.05). Of the 12 patients with bloodstream infections due to ST5, 10 patients had intravascular catheters, suggesting that the presence of intravascular catheters is highly associated with bloodstream infection caused by this clone. The frequency of previous surgical or invasive procedures, the use of mechanical ventilation and the presence of prostheses in patients with ST239 infection were not significantly different from patients with ST5 infection. There was also no statistical difference in the recent antimicrobial therapy between the two groups. These results suggest that ST239 infection was highly associated with the hospital departments in which the patients were managed, while ST5 infection was highly associated with the presence of intravascular catheters.
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Clinical diagnoses and clinical courses of the patients infected with different MRSA clones
Bloodstream infection was the most common primary infection site both in patients with ST239 infections (35%) and in patients with ST5 infections (50%) (Table 4). Pulmonary infection caused by ST239 strains (27%) was significantly higher than that caused by ST5 strains (4%) (P < 0.05). The clinical courses, including development of septic shock, disseminated infection and recovery, were not statistically different between patients with ST239 infection and those with ST5 infection. Similarly, therapeutic procedures other than antimicrobial therapy, including removal of intravascular catheters, removal of prosthesis and surgical drainage procedures, were not significantly different between patients with ST239 infection and patients with ST5 infection. Antimicrobial agents used for the treatment of MRSA infections, such as vancomycin, teicoplanin, rifampicin, fluoroquinolones and trimethoprim/sulfamethoxazole, were not different between the two groups. However, the overall mean durations of antimicrobial therapy were significantly different between patients with ST239 infection (43.8 days) and patients with ST5 infection (29.2 days) (P < 0.05). The mortality rates at the 14th and 30th day after the first positive culture were not significantly different between patients with ST239 infection and patients with ST5 infection.
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Discussion |
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This study assessed the clinical impact of MRSA infection based on causative clonal types. In addition to characterization of nosocomial MRSA strains, we compared the risk factors, clinical diagnoses, treatment courses and clinical outcomes of the patients infected with two predominant MRSA clones, ST239 and ST5. The prevalence of MRSA infections was largely attributable to ST239 and ST5 strains, which accounted for 85% of the MRSA isolates. This result is in accordance with previous results showing the worldwide dominance of only a few epidemic MRSA clones in hospitals or countries.9,27
Our results demonstrate that the risk factors for ST239 and ST5 infections were different, although the study was limited to a single hospital. Several studies have described the risk factors for MRSA infection in the hospital setting,14–18 but the risk factors for MRSA infections based on clonal types have not been determined yet. Infection with MRSA of ST239 was highly associated with particular hospital departments, namely orthopaedic surgery, neurosurgery and plastic surgery, despite the fact that these three departments were physically discrete. This suggested that patients admitted to these three departments might have acquired their infections with ST239 on these wards. Of the risk factors associated with S. aureus infection, central and peripheral intravascular catheters were found to be an important route as an access site for primary infection.20,28 In the current study, the presence of intravascular catheters was significantly higher in patients with ST5 infection than in those with ST239 infection (P < 0.05). The current study demonstrated the specific risk factors for ST239 and ST5 clones, respectively, but it is difficult to say with any certainty that these risk factors can be applied to the same clones in other hospital settings. Accordingly, further prospective studies will be needed to elucidate the risk factors for MRSA infections based on clonal types.
The overall mean duration of antimicrobial therapy was significantly longer in patients with ST239 infection (43.8 days) than in those with ST5 infection (29.2 days), although the antimicrobial agents used did not greatly differ between the two groups. The mean duration of antimicrobial therapy with vancomycin, rifampicin, ciprofloxacin and trimethoprim/sulfamethoxazole was not significantly different between the two groups (data not shown), but the mean duration of teicoplanin therapy was 29.7 days in patients with ST239 infection (n = 20) and 18.0 days in patients with ST5 infection (n = 8). Therefore, teicoplanin was primarily responsible for the prolonged antimicrobial therapy in patients with ST239 infection. In the study hospital, teicoplanin is the preferred option for wound and musculoskeletal infections in orthopaedic surgery because of its high drug concentrations in bone tissues. However, the prolonged teicoplanin therapy can also be affected by physicians who are determining the length of antibiotic therapy, although MRSA infections are treated according to generally accepted treatment guidelines.
The mortality rates caused by MRSA infection on the 14th and 30th day were not significantly different between the patients with ST239 and ST5 infections. Ten patients infected with the predominant clones (CC239 and CC5) died on the 14th day, while all patients infected with the sporadic clones (CC72, CC8 and CC1) survived. However, the mortality rates on the 14th day caused by MRSA infection were not significantly different between the predominant clones and the sporadic clones. Even though MRSA bacteraemia is highly associated with increased mortality compared with MSSA bacteraemia,19,20 there is no evidence from the current study to suggest that the mortality rates of the patients infected with a specific MRSA clone are significantly different from those of other clones.
The current study analysed the risk factors for MRSA infection with the different clones of MRSA in a hospital setting, but the study was limited by being undertaken in a single hospital, the number of patients infected with MRSA strains and the retrospective nature of the study. The experience at a single medical centre, in a specific geographic location, limits the ability to make broad conclusions regarding individual MRSA clonal types related to infection. However, this study illustrates the need for further large prospective studies regarding the risk factors, clinical outcomes, an appropriate oral treatment regimen and effective preventive strategies based on MRSA clonal types. Further studies of MRSA clones in hospitals should also aim to identify risk factors for colonization as well as infection, as colonization and infection are inter-related.
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None to declare.
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Acknowledgements |
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We thank Jong Sook Jin for her assistance with the laboratory work. We would also like to thank Dr Herminia de Lencastre, from Laboratório de Genética Molecular, Instituto de Technologia Química e Biológica da Universiadade Nova de Lisboa, Portugal, for providing the MRSA strains COL, N315 and ANS46. This study was supported by a grant from Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea (03-PJ1-PG1-CH03-0002).
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References |
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