JAC Advance Access published online on August 27, 2008
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkn326
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Original research |
Are methicillin-resistant Staphylococcus aureus that produce Panton–Valentine leucocidin (PVL) found among residents of care homes?
1 Leeds Teaching Hospitals NHS Trust, Leeds LS1 3EX, UK 2 Faculty of Biological Sciences, University of Leeds, Leeds LS1 9NL, UK 3 Leeds Primary Care Trust, Stockdale House, Victoria Road, Leeds LS6 1PF, UK 4 Central Lancashire Primary Care Trust, Wigan Road, Ormskirk L39 2JW, UK 5 Health Protection Agency, Yorkshire and the Humber, Bridle Path, York Road, Leeds LS15 7TR, UK
* Correspondence address. Department of Microbiology, Leeds Teaching Hospitals NHS Trust, Leeds LS1 3EX, UK. Tel: +44-113-39-23929; Fax: +44-113-343-5649; E-mail: carolyne.smith{at}leedsth.nhs.uk
Received 5 June 2008; returned 3 July 2008; revised 16 July 2008; accepted 21 July 2008
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Abstract |
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Objectives: Panton–Valentine leucocidin (PVL)-positive Staphylococcus aureus are responsible for causing skin and soft tissue infections, with the potential to cause severe invasive disease. Recently, methicillin-resistant Staphylococcus aureus (MRSA) strains that produce PVL have emerged in the community. As residents of care homes are a key group at risk of MRSA colonization and infection, we have examined the epidemiology of MRSA in three large cohorts of residents in urban care homes to establish whether PVL-positive MRSA strains are present in this setting.
Methods: Nasal swabs (n = 3037) collected from consenting residents of 69 care homes in Leeds, UK, were screened for MRSA using chromogenic agar over three periods (June–August 2005, November–December 2006 and October–November 2007). PCR amplification was used to detect genes encoding PVL. Antibiogram profile and PFGE were also used to characterize MRSA isolates (n = 601).
Results: MRSA prevalence was 21%, 20% and 19% in each cohort, respectively. The majority of the isolates were related epidemiologically to the predominant local nosocomial epidemic MRSA strain, EMRSA-15 (78%). No isolate carried the genes encoding PVL. Twelve percent of the isolates (n = 74) had increased susceptibility to non-β-lactam agents and were distributed across 31 care homes.
Conclusions: MRSA strains that produced PVL were not found to be colonizing residents of care homes between 2005 and 2007. Continued surveillance is, however, necessary to understand the interaction between MRSA in care homes and hospitals, especially to reduce the chance that the former may amplify community-associated MRSA strains.
Key Words: community , epidemiology , MRSA
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Introduction |
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Methicillin-resistant Staphylococcus aureus (MRSA) remain a major threat to public health, not only in the healthcare setting but also in the community. Community-associated MRSA (CA-MRSA) strains that are currently the focus of concern have evolved worldwide and are genetically different from the MRSA strains typically associated with hospitals (HA-MRSA). These emerging CA-MRSA strains have a propensity for infecting previously healthy, younger individuals who often do not have traditional risk factors for HA-MRSA.1 CA-MRSA strains carry the mecA gene and are resistant to methicillin, yet they tend to be susceptible to classes of antibiotics other than β-lactams when compared with the multiple resistant HA-MRSA strains.2 Many clones of CA-MRSA, but not all, produce the Panton–Valentine leucocidin (PVL),2,3 a pore-forming cytotoxin that targets leucocytes. S. aureus strains that produce PVL are associated with skin and soft tissue infections and can also cause life-threatening invasive infections, such as necrotizing pneumonia and necrotizing fasciitis.1
Strains of CA-MRSA are becoming increasingly responsible for nosocomial infections,1 and there is a concern that more virulent CA-MRSA strains may become established in the healthcare environment and become epidemic in nature.4 The elderly population living in residential care homes is a key group at risk of colonization and infection with HA-MRSA strains.5 This population, living in the wider community but also requiring frequent hospitalization, may act as a portal for CA-MRSA to enter the nosocomial environment. We have therefore examined the epidemiology of MRSA in three large cohorts of residents in urban care homes. As a universal marker for detection of all CA-MRSA clones is not known, we screened MRSA isolates for the presence of the genes encoding PVL and for susceptibility to non-β-lactam antibiotics, characteristics that may indicate putative CA-MRSA strains.2 We determined the genetic background of MRSA isolates using PFGE.
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Materials and methods |
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Collection and culture of samples
Swabs were collected from the anterior nares of consenting residents from care homes registered with the Commission for Social Care Inspection in Leeds during three periods: 14 June–16 August 2005; 16 November–13 December 2006 and 1 October–12 November 2007, representing cohorts 1, 2 and 3, respectively. Only one swab was collected per resident per cohort. Each swab was inoculated onto a single MRSA Select agar plate (Bio-Rad, France) and incubated for 24 h at 37°C. Bright, fuchsia–pink colonies were considered presumptive MRSA. Presumptive MRSA colonies were confirmed to be S. aureus by DNAse agar testing and positive agglutination reaction using the PastorexTM Staph plus kit (Bio-Rad, France).
Susceptibility testing of MRSA
Methicillin resistance was confirmed by breakpoint antibiotic susceptibility testing using Iso-Sensitest agar supplemented with methicillin at concentrations of 4, 8 and 12 mg/L and Iso-Sensitest agar supplemented with cefoxitin at concentrations of 4 mg/L. Methicillin-susceptible S. aureus strain NCTC 6571 and MRSA strain NCTC 10442 were used to control confirmation and susceptibility testing. Antibiotic susceptibility testing for ciprofloxacin, erythromycin, fusidic acid, tetracycline and trimethoprim was conducted using the BSAC standardized disc susceptibility testing method, as was the detection of clindamycin dissociated resistance.
MRSA isolates with susceptibility to non-β-lactam agents were defined as follows: isolates susceptible to less than three antibiotic classes, excluding ciprofloxacin, that were indistinguishable, closely related or possibly related to EMRSA-15 control strain (NCTC 13142) or isolates susceptible to less than three antibiotic classes, including ciprofloxacin, that have a 
7 band difference in PFGE profile when compared with EMRSA-15. Evidence of transmission within the care home environment was defined as more than one isolate belonging to the same PFGE cluster occurring during the same collection period.
Detection of genes encoding PVL
Genomic DNA from MRSA isolates was extracted using the Wizard® Genomic DNA Purification kit according to the manufacturer's instructions (Promega UK Ltd, Southampton, UK). Detection of the genes encoding PVL (LukS-LukF) was achieved using primers and conditions published by Francois et al.6
Pulsed-field gel electrophoresis
Restriction endonuclease digestion of bacterial whole-cell DNA with SmaI followed by PFGE was used to assess epidemiological relatedness of the MRSA isolates using a standardized protocol.7 The resultant profiles were compared using BioNumerics software (Applied Maths BVBA, Belgium). Dendrograms were constructed by the unweighted pair group method with arithmetic mean clustering using Dice correlation coefficient. Isolates were considered to be indistinguishable, closely related, possibly related or different according to the guidelines recommended by Tenover et al.8
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Results and discussion |
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To our knowledge, this is the first prospective study to monitor the occurrence of MRSA positive for PVL in UK care homes. To obtain a representative collection of MRSA in care homes, we examined three large cohorts spanning three consecutive years. Six hundred and one MRSA isolates were identified from 3037 nasal swabs collected from residents during the periods stated. The mean MRSA colonization rate of residents was 20% (range 19% to 21%). Methods such as barrier nursing and/or routine decolonization of residents colonized with MRSA were not employed during the study periods.
In the care homes surveyed, the majority of residents who were colonized with MRSA (78%) were colonized with a strain that is closely related to the local HA-MRSA strain, EMRSA-15; 9% isolates were considered to be possibly related to EMRSA-15. Fifteen percent of the isolates had 7 band difference in PFGE profile compared with EMRSA-15 and were considered to be different strains (Table 1). Studies in the UK and Europe during the 1990s identified rates of MRSA prevalence in care homes between 0.14% and 17%, with strains showing similarities to those circulating in the local nosocomial environment.5 A higher rate of MRSA colonization 10 years later is not unexpected and is likely to mirror the increased rates of MRSA seen in hospitals.
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Previous studies have reported the presence of PVL-positive MRSA isolates in the nursing home setting where the MRSA isolates positive for PVL had susceptibility to non-β-lactam agents and a unique PFGE profile compared with local nosocomial MRSA strains.9,10 MRSA carrying genes encoding PVL were not found within our study population; however, 74 MRSA strains (12%) with susceptibilities to non-β-lactam agents were identified. MRSA isolates that were susceptible to non-β-lactam agents clustered into three major distinct clusters by PFGE (Figure 1). Cluster I comprises 11 isolates with a profile indistinguishable from (n = 5), closely related (n = 5) or possibly related (n = 1) to EMRSA-15. Clusters II (n = 12) and III (n = 41) are distinct from isolates in cluster I as they are different from EMRSA-15 by
7 bands. There are three other smaller clusters of isolates (n = 2–4) different from EMRSA-15 and a solitary isolate that was also different.
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MRSA isolates that were susceptible to non-β-lactam agents were distributed in multiple homes (n = 31) and did not cluster in any particular cohort (16%, 12% and 10% for cohorts 1, 2 and 3, respectively). While there was a decreasing prevalence in the occurrence of isolates that were susceptible to non-β-lactam agents, this trend was not statistically significant. There were 14 homes that had a single isolate and 17 homes that had two or more isolates that were susceptible to non-β-lactam agents. In eight homes (Home identification numbers 2, 17, 27, 28, 30, 38, 57 and 64; Figure 1), there was evidence of transmission of isolates that were susceptible to non-β-lactam agents, according to the definition stated in the Materials and methods section.
Data presented in our study indicate that 10% of the MRSA isolates (n = 59) were susceptible to non-β-lactam agents and had a different genetic background to the local HA-MRSA, as confirmed by PFGE; these may indicate putative CA-MRSA strains. It is important to note that the presence of the PVL toxin and/or antibiotic susceptibility profiles are not reliable markers for detecting CA-MRSA.2 Detection of CA-MRSA is made increasingly difficult when the predominant strain circulating in the hospitals, and thus the community, is an MRSA strain that carries the SCCmec IV element but is not otherwise multiple resistant, such as EMRSA-15. Isolates matching the PFGE profile of EMRSA-15 have been confused with CA-MRSA.3
Our results reinforce previous reports of high MRSA colonization rates in residents of long-term care facilities. We have not found evidence for PVL-positive MRSA strains in three large cohorts between 2005 and 2007; however, we have found modest rates of colonization with MRSA isolates that are susceptible to non-β-lactam agents, which may represent putative CA-MRSA strains. The data presented here provide an important baseline for future surveillance of MRSA in the care home setting. Continued surveillance is needed to understand the interaction between MRSA in care homes and hospitals to reduce the chance that the former may amplify CA-MRSA.
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Funding |
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This study was funded by a research grant from the Department of Health in England.
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None to declare.
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Acknowledgements |
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We wish to thank the residents and staff of participating care homes, MRSA Research Nurses, Leeds PCT, and Infection Control laboratory staff at Leeds Teaching Hospitals NHS Trust. This work was undertaken by members of the Leeds MRSA Care Home Study Group who received funding under the Department of Health's HCAI Initiative; the views expressed in this publication are those of the authors and not necessarily those of the Department of Health.
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References |
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