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What Is Community-Associated Methicillin-Resistant Staphylococcus aureus?

  1. Michael Z David1,
  2. Daniel Glikman1,
  3. Susan E Crawford1,
  4. Jie Peng1,
  5. Kimberly J King1,
  6. Mark A Hostetler2,
  7. Susan Boyle-Vavra1 and
  8. Robert S Daum1
  1. 1Infectious Diseases Section and
  2. 2Emergency Medicine Section, Department of Pediatrics, the University of Chicago, Illinois
  1. Reprints or correspondence: Dr. Michael Z. David, Section of Infectious Diseases and Department of Pediatrics, the University of Chicago, 5841 S. Maryland Ave., MC 6054, Chicago, IL 60637 (mdavid{at}medicine.bsd.uchicago.edu)
 
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Abstract

BackgroundA community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infection has been defined as an MRSA infection in a patient who lacks specific risk factors for healthcare exposure. We sought to determine whether the absence or presence of these risk factors still predicts the phenotypic or genotypic characteristics of MRSA strains

MethodsAll clinical MRSA isolates were prospectively collected at the University of Chicago Hospitals from July 2004 through June 2005. Patients were interviewed and/or their medical records were reviewed. Isolates underwent genotyping and susceptibility testing. Data on patients and isolates were stratified in accordance with 8 frequently cited criteria for the identification of CA-MRSA and compared for concordance

ResultsAmong 616 unique patients from whom MRSA isolates were recovered, 404 (65.6%) had risk factors for healthcare exposure. Of the 404 isolates recovered from these patients, 166 (41.1%) were clindamycin susceptible, 190 (47.0%) carried staphylococcal cassette chromosome mec (SCCmec) type IV, 145 (35.9%) carried the Panton-Valentine leukocidin genes (PVL+), and 162 (40.1%) were identified as sequence type (ST) 8 by multilocus sequence typing (MLST), all of which are characteristics commonly attributed to CA-MRSA strains

ConclusionsAssociation with the healthcare environment now has little predictive value for distinguishing patients with infection due to multidrug resistant MRSA isolates from those infected by CA-MRSA isolates, that is, isolates that are clindamycin-susceptible, PVL+, ST8, and/or contain SCCmec type IV. Defining CA-MRSA by the absence of risk factors for healthcare exposure greatly underestimates the burden of epidemic CA-MRSA disease

Beginning with its first report in 1961 [1], methicillin-resistant Staphylococcus aureus (MRSA) isolates have been associated with a variety of infectious syndromes in patients with exposure to the healthcare environment. A defined group of risk factors for exposure to MRSA was established, including hospitalization, surgery, receipt of hemodialysis, a stay in a long-term care facility, or undergoing surgery during the previous year, as well as the presence of an indwelling device or vascular catheter [2,3]. MRSA strains recovered from patients who frequent healthcare facilities, so-called healthcare-associated MRSA (HA-MRSA), were typically resistant to clindamycin and other non–β-lactam antimicrobials. This multidrug-resistant MRSA phenotype was due in part to the presence of relatively large DNA elements, the staphylococcal cassette chromosome mec (SCCmec), integrated into the bacterial genome. An SCCmec element carries the mecA gene, which mediates resistance to methicillin and, by inference, resistance to all other available β-lactam drugs and is the molecular hallmark of MRSA strains. The relatively large type II–III SCCmec elements associated with HA-MRSA strains also carry genes responsible for resistance to other antibiotic classes [4]

Beginning in the late 1990s, there appeared many reports of MRSA colonization and infection in the community that involved patients who had no recent contact with healthcare facilities [5–10]. It was initially suspected that these cases represented the spread of HA-MRSA strains into the community. Although HA-MRSA strains do sometimes circulate in the community, distinct community-associated MRSA (CA-MRSA) strains were identified that carry 1 of 2 novel, smaller SCCmec types, IV and V, and share genetic backgrounds with S. aureus strains not previously known to carry SCCmec elements [11–13]. These novel CA-MRSA strains differ from their HA-MRSA counterparts by usually being susceptible to clindamycin and other non–β-lactam antibiotics [10]. They also typically carry the 2 genes encoding Panton-Valentine leukocidin (PVL), a pore-forming toxin that was rarely found among clinical isolates of S. aureus [10, 14]. Certain S. aureus genotypes have been identified, distinct from HA-MRSA genotypes, that commonly cause CA-MRSA infections in the community. The predominant strain in the United States is designated USA300, which usually corresponds to sequence type 8 (ST8) as identified by multilocus sequence typing (MLST) [15, 16]. Several new clinical staphylococcal syndromes have been described that are associated with CA-MRSA strains [17–19], ranging from skin and soft tissue infections (SSTIs) that initially resemble the bite of an arachnid [5, 6, 10] to severe, life-threatening infections [18]

Many investigators have identified the isolates that cause CA-MRSA infection by using a definition based on the patient’s lack of healthcare risk factors, a definition advocated by the Centers for Disease Control and Prevention (CDC) in 2000 [2, 3]. This definition states that a case of MRSA infection is community-acquired when it is diagnosed in an outpatient or within 48 hours of hospitalization if the patient lacks the following traditional risk factors for MRSA infection: receipt of hemodialysis, surgery, residence in a long-term care facility, or hospitalization during the previous year; the presence of an indwelling catheter or a percutaneous device at the time culture samples were obtained; or previous isolation of MRSA

There are several problems with the use of an approach based on the lack of these healthcare risk factors. First, both CA-MRSA and HA-MRSA strains now circulate in the community. Second, so-called CA-MRSA strains are gradually becoming entrenched as nosocomial pathogens [20–25]. This complex, changing epidemiology raises doubts about the distinction between CA-MRSA and HA-MRSA strains based on healthcare exposure in both clinical practice and epidemiologic research. Third, new high-risk groups for MRSA infection and colonization in the community have been identified, including children [5, 6], athletes [26–28], incarcerated populations [29, 30], military recruits [31], Native Americans [7], Pacific Islanders [8], men who have sex with men [9], impoverished adults in the inner city [32], and adult emergency department patients [33]

It is useful to attempt to distinguish HA-MRSA isolates from CA-MRSA isolates and to distinguish patients with HA-MRSA infection from patients with CA-MRSA infection. Isolate characteristics help investigators to understand and monitor the rapidly changing epidemiology of MRSA. Patient characteristics may also be of help when a clinician is faced with a patient who has a potential MRSA infection. The distinction between HA-MRSA and CA-MRSA has been used to guide decisions about empirical therapy, because CA-MRSA isolates are more likely to be clindamycin susceptible

Accordingly, for all patients at an urban tertiary care medical center that had been experiencing epidemic CA-MRSA infections during the previous decade [5] who had MRSA isolates recovered during a 1-year period, we analyzed the clinical and demographic characteristics of the patients from whom the isolates were recovered, as well as the isolates’ genetic characteristics and antimicrobial resistance profiles. We also analyzed the temporal characteristics of recovery (e.g., date of patient admission and date of specimen procurement). We evaluated several criteria that have been associated with CA-MRSA strains or with the patients they infect in order to understand the relative importance of these various criteria in the arenas of clinical medicine, epidemiologic studies, and basic science

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Patients, Materials, and Methods

SettingThe University of Chicago Medical Center (UCMC) is a tertiary care medical center in Chicago, Illinois, with 577 beds for inpatients and 26,200 annual admissions. It includes an outpatient care facility that has 389,000 annual visits and an emergency department that has more than 71,000 annual visits. UCMC serves an inner-city population and draws tertiary referrals from the surrounding region. The study was approved by the Institutional review board of the Biological Sciences Division of the University of Chicago

Microbiological studiesThe UCMC Clinical Microbiology Laboratories prospectively identified all MRSA isolates collected between July 1, 2004 and June 30, 2005, from patients in all clinical settings as reported elsewhere [34]. We used an automated system (Vitek 2; bioMérieux) to determine the antimicrobial susceptibility profile of each isolate for oxacillin, erythromycin, clindamycin, ciprofloxacin, rifampin, gentamicin, linezolid, and vancomycin. All isolates identified by automated testing as susceptible to oxacillin underwent confirmatory disk diffusion testing for cefoxitin susceptibility. For isolates that were identified as resistant to erythromycin but susceptible to clindamycin, a D-test was performed to detect inducible clindamycin resistance, as was a disk diffusion test for susceptibility to trimethoprim-sulfamethoxazole. All assays were performed in accordance with Clinical and Laboratory Standards Institute guidelines [35]

Molecular studiesMLST was performed on all MRSA isolates, as described elsewhere [15]. Clonal complexes were assigned using the eBURST algorithm, as described elsewhere [36]. The presence of mecA was assessed, and the SCCmec type of each strain was determined by the molecular architecture of the ccr and mec complexes by use of criteria described elsewhere [37]. The presence of lukF-PV and lukS-PV encoding the PVL toxin was performed by polymerase chain reaction (PCR), as described elsewhere [16]

Patient informationAt least 5 attempts were made to contact each unique patient from whom an MRSA isolate was recovered to administer a standardized questionnaire that asked about any history of previous MRSA infection, surgery in the previous 6 months, hospitalization in the previous year, the presence of an indwelling catheter or any prosthetic device, and/or a history of hemodialysis. For patients who could not be contacted, the paper medical record was reviewed and abstracted by one of us (D.G.) using the same questionnaire. Additionally, for all patients, the electronic medical record, including hospital discharge summaries and radiographic, laboratory, pathology, operative, and outpatient clinic reports, was reviewed (by D.G. or S.E.C.) to determine the location of care in the medical center, date of admission, demographic characteristics, date of specimen procurement, the isolate’s antibiotic susceptibility profile, anatomic source of the culture sample, clinical indication for the culture, radiographic results and results of other tests, and any history of previous MRSA isolation at UCH since 1994. A clinical syndrome was assigned to each patient from whom MRSA was recovered on the basis of all available data. For isolates from inpatients, the number of days from admission to specimen procurement was calculated

Criteria for identifying CA-MRSAWe joined clinical information for each patient with molecular data on the isolate recovered from that patient (a combination hereafter referred to as a “patient-isolate set”). Data on patient-isolate sets were stratified on the basis of 8 commonly cited criteria for identifying CA-MRSA [2, 5–10, 23, 26–33]. These criteria were as follows: (1) the clindamycin susceptibility criterion, which identified all isolates not shown to be resistant to clindamycin either by single-agent testing or by D-test; (2) the non–multidrug resistance (non-MDR) criterion, which identified isolates resistant to <3 non–β-lactam antibiotics; (3) the SCCmec IV criterion, which identified all isolates that carried SCCmec type IV; (4) the SSTI criterion, which identified all isolates recovered from patients with SSTI (i.e., abscesses, burn infections, felons, paronychia, cellulitis, surgical wound infections, folliculitis, carbuncles, furuncles, myositis, and/or pyomyositis); (5) the PVL criterion (PVL+), which identified all isolates that carried the PVL genes; (6) the ST8 criterion, which identified all isolates that were ST8—and thus presumptively USA300, the most common background genotype now found in many US studies of CA-MRSA [15]; (7) the 48-hour criterion, which identified all isolates obtained from outpatients, emergency department patients not admitted to the hospital, and inpatients within 48 hours of admission; and (8) the “lack of healthcare risk factors” criterion, which identified the patients defined by criterion 7 who also lacked the following risk factors for exposure to HA-MRSA: hospitalization, receipt of hemodialysis, or residence in a long-term care facility during the previous year; surgery during the previous 6 months; the presence of an indwelling catheter or a percutaneous device at the time the culture sample was obtained; or previous isolation of MRSA. Our “lack of healthcare risk factors” criterion differed from the CDC definition for CA-MRSA [2,3] only in that we included surgery as a risk factor if it occurred during the 6 months prior to the time the isolate was recovered, rather than during the year prior to recovery of MRSA

Data analysis Bacteriologic and patient data were compiled in an electronic database using Access (Microsoft). Only the first isolate collected from each patient during the surveillance period was included in the database. Each criterion was applied individually to the entire collection of data on patient-isolate sets. Isolates that fulfilled a given criterion were deemed CA-MRSA, and the remainder were identified as HA-MRSA. Data were analyzed with Stata SE, version 9.2 (Stata). Comparisons between groups were performed by use of the χ2 test. The percentage of concordance was calculated to define the subset of isolates that would be identified as CA-MRSA, or the subset of patients who would be identified as having a CA-MRSA infection, by simultaneous application of any pair of the 8 criteria for CA-MRSA. Because risk-factor data related to healthcare exposure are often readily available to the clinician at the bedside, the “lack of healthcare risk factors” criterion for CA-MRSA was applied as a screening test for the presence of SCCmec IV and for clindamycin susceptibility. The positive predictive value (PPV), negative predictive value (NPV), sensitivity, and specificity were calculated for these tests. Both the pairwise concordance and the screening tests were applied to the entire sample and repeated for the following subgroups: children (<18 years), adults, inpatients, outpatients, emergency department patients, and children and adults, respectively, in the inpatient, outpatient, and emergency department settings. All hypotheses were evaluated by 2-tailed tests and results were considered significant if P<.05

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Results

There were 1225 MRSA isolates identified by the UCH Clinical Microbiology Laboratories during the study period, of which 560 were not analyzed further because they were recovered from patients who had already had MRSA isolates recovered during the study period. Thirty-five patients declined to participate in the study, 9 isolates were identified as methicillin-susceptible owing to the absence of the mecA gene, and 1 isolate was coagulase-negative Staphylococcus Two isolates came from patients who had no documentation of a S. aureus isolation in their medical record, and 2 isolates were not available for study. Ultimately, 616 patient-isolate sets were included in the analysis (table 1)

Figure 1
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Figure 1

Concordance between isolates identified as community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) and healthcare-associated MRSA (HA-MRSA) by the “lack of healthcare risk factors” criterion and the percentage of isolates identified as CA-MRSA and HA-MRSA by the 7 other study criteria. A Classification of the 212 isolates that were identified as CA-MRSA by the “lack of healthcare risk factors” criterion, in comparison with the other 7 study criteria. Of these 212 isolates, the majority were also classified as CA-MRSA by each of the 7 other criteria (i.e., 182 [85.8%] of the isolates were susceptible to clindamycin, 197 [92.9%] carried the staphylococcal cassette chromosome [SCC] mec type IV element, 197 [92.9%] were not multidrug resistant [MDR], 189 [89.2%] carried the Panton-Valentine leukocidin [PVL] toxin genes, and 180 [84.9%] had the sequence type 8 [ST8] genotype). In addition, 178 [84.0%] of the patients from whom the isolates were recovered had skin and soft tissue infection (SSTI) identified as their clinical syndrome. B Classification of the 404 isolates that were identified as HA-MRSA by the “lack of healthcare risk factors” criterion, in comparison with the other 7 study criteria. Of these 404 isolates, many also had traits characteristic of CA-MRSA (i.e., 166 [41.1%] were susceptible to clindamycin, 190 [47.0%] carried a type IV SCCmec element, 186 [46.0%] were not MDR, 145 [35.9%] carried the PVL toxin genes, and 162 [40.1%] had the ST8 genotype). In addition, 174 patients [43.1%] from whom the isolates were recovered had SSTI identified as their clinical syndrome

Table 1
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Table 1

Demographic and clinical characteristics of study patients from whom methicillin-resistant Staphylococcus aureus (MRSA) was isolated

Of the 616 patients, 320 (52.0%) were interviewed and paper medical records were reviewed for 296 (48.1%); electronic records were reviewed for all 616. Two predominant MLST types, ST8 and ST5, accounted for 342 (55.5%) and 211 (34.2%) of the isolates, respectively (table 2). Nearly two-thirds of the isolates carried SCCmec IV, including 11 (5.2%) of ST5 and 336 (98.5%) of ST8 isolates. A total of 9 isolates carried novel SCCmec elements, and none carried types I, III, or V. Erythromycin resistance was detected in the vast majority of isolates (564 [91.6%]). A D-test was performed on 347 isolates; 53 (15.3%) were positive for inducible clindamycin resistance. None of the isolates tested were resistant to vancomycin or linezolid (table 2)

Table 2
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Table 2

Genotypic and phenotypic characteristics of methicillin-resistant Staphylococcus aureus isolates

The percentage of isolates designated as CA-MRSA varied according to which criterion was applied (table 3). Surprisingly, the lowest percentage of isolates were identified as CA-MRSA by the “lack of healthcare risk factors” criterion, which classified 34.4% of isolates as CA-MRSA. The highest percentage of isolates was classified as CA-MRSA by the 48-hour criterion (78.1%). The percentage of isolates designated as CA-MRSA by each criterion was lower for adults than for children (P<.001) (table 3). For each criterion except the 48-hour criterion, the highest percentage of CA-MRSA cases was identified in the emergency department and the lowest in the inpatient setting (table 3). There were 91 isolates from patients who were known to have had an MRSA infection prior to the 1-year study period. Of these 91 isolates, 30 (33.0%) were susceptible to clindamycin, 38 (41.8%) carried SCCmec type IV, and 26 (28.6%) carried the genes for PVL; 22 (24.2%) met all 3 of these criteria for identifying CA-MRSA. Thus, a history of previous MRSA infection was often associated with the recovery of isolates that had features typical of CA-MRSA isolates

Table 3
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Table 3

Identification of community-associated methicillin-resistant Staphylococcus aureus in accordance with 8 criteria, for the entire sample and for isolates stratified by patient age group and location of patient care

The percentage of concordance was calculated to define the subset of patient-isolate sets that would be identified as CA-MRSA by simultaneous application of any 2 of the criteria used to identify CA-MRSA (table 4). A high percentage of patients were classified as having had a CA-MRSA isolate recovered by application of any 2 of the following criteria: SCCmec IV, non-MDR, PVL, ST8, and clindamycin susceptibility. In contrast, there was a relatively poor correlation between the SSTI criterion and each of the other 7 criteria. The concordance between the “lack of healthcare risk factors” criterion and each of the other 7 criteria was also relatively poor (table 4); this remained true even when patients were stratified by location of care, age group, or both

Table 4
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Table 4

Percentage concordance in identifying community-associated methicillin-resistant Staphylococcus aureus, for any 2 of the 8 study criteria applied simultaneously

The “lack of healthcare risk factors” criterion was applied to the patient-isolate set as a screening test for identifying CA-MRSA, here defined by the hypothetical gold standard of clindamycin susceptibility (table 5). The sensitivity of the “lack of healthcare risk factors” screening test was only 52.8%. The specificity was 89.2%, the PPV was 86.3%, and the NPV was 59.4%. These parameters showed some variation when the sample was stratified by patient age group and location of patient care. Sensitivity and the PPV were lower for patient-isolate sets that involved adult patients and higher for patient-isolate sets that involved pediatric patients. The sensitivity was highest among patient-isolate sets that involved patients treated in the emergency department and lowest among patient-isolate sets that involved patients treated in the inpatient setting. Not unexpectedly, the NPV was very low for patient-isolate sets that involved patients treated in the emergency department, where the prevalence of clindamycin-susceptible isolates was highest (84.4%). Compared with all other patients, emergency department patients were significantly more likely to be children (103 [80.5%] vs. 121 [24.8%]; P<.001) and to be covered by a state-administered health insurance program (103 [80.5%] vs. 326 [66.8%]; P=.003). When the SCCmec IV criterion was used as the gold standard for identification of CA-MRSA, use of the “lack of healthcare risk factors” criterion as a screening test produced similar results (table 5)

Table 5
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Table 5

Use of the “lack of healthcare risk factors” criterion as a screening test for identification of community-associated methicillin-resistant Staphylococcus aureus, in accordance with 2 theoretical gold standards

We did not consider that children <1 year of age had a healthcare-associated risk factor simply because all of these patients were born in the hospital. If all such children were considered to have a risk factor for HA-MRSA infection, then use of the “lack of healthcare risk factors” criterion would increase the number of patients designated as having HA-MRSA isolates recovered to 430 (69.8%). This designation would also decrease the sensitivity of this criterion as a screening test for clindamycin susceptibility to 46.1%, and it would decrease the sensitivity of the criterion as a screening test for the SCCmec IV criterion to 44.4%

Most of the 212 patient-isolate sets considered to be CA-MRSA by the “lack of healthcare risk factors” criterion would also have been considered CA-MRSA by any of the other 7 criteria examined (figure 1A). However, of the 404 isolates considered HA-MRSA (and the infections considered to be HA-MRSA infections) by the “lack of healthcare risk factors” criterion, 35.9%–67.1% would be considered CA-MRSA by the other criteria (figure 1B). Importantly, of the patients identified as having HA-MRSA infection by the “lack of healthcare risk factors” criterion, 166 (41.1%) had an isolate recovered that was clindamycin susceptible, and 190 (47.0%) had an isolate recovered that had a type IV SCCmec element

Of the 616 isolates recovered from unique patients, 304 isolates (49.4%) were clindamycin susceptible and carried both the PVL genes and the SCCmec type IV element. A slightly smaller group of 286 isolates (46.4%) had these 3 characteristics as well as the ST8 genotype. Thus, a large group of the MRSA isolates we analyzed shared all 4 of these criteria for CA-MRSA. Of these 286 isolates, 121 (42.3%) would be considered HA-MRSA by the “lack of healthcare risk factors” criterion

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Discussion

The CDC criteria that define CA-MRSA rely on a lack of exposure to the healthcare environment to identify cases of CA-MRSA infection. The criteria accurately predicted the identification of a subset of about one-third of the MRSA isolates identified in our surveillance that had the ST8 genotype and were clindamycin susceptible, SCCmec IV–bearing, and PVL+—isolate-specific traits most commonly believed to designate CA-MRSA isolates

The epidemiology of the remaining two-thirds of the MRSA isolates is more complex. The presence of the traditionally recognized HA-MRSA risk factors (i.e., exposure to the healthcare environment) identified nearly all of the patients from whom isolates were recovered that had the ST5 genotype and were clindamycin resistant, SCCmec II–bearing, and PVL−—attributes commonly believed to define HA-MRSA isolates. However, many patients who had exposure to the healthcare environment also had MRSA isolates recovered that had the ST8 genotype and were clindamycin susceptible, SCCmec IV–bearing, and PVL+. This apparent paradox was not due to the nosocomial spread of isolates with these traits at the University of Chicago, as these isolates represented only a small minority of those acquired >48 hours after hospitalization. Rather, our data indicate that the MRSA isolates with these traits must be circulating in the community and must be being transmitted to individuals living in the community, both those who have had exposure to the healthcare environment and those who have not had such exposure

Conversely, if the presence of the ST8 genotype, clindamycin susceptibility, the SCCmec IV element, and PVL positivity are assumed to define CA-MRSA isolates, only some patients from whom such isolates were recovered fulfilled the CDC’s definition for CA-MRSA infection. Use of the CDC definition, therefore, underestimates the community-based disease burden caused by MRSA isolates with these traits. For example, a healthcare risk factor–based screening tool for MRSA isolates in population-based surveillance in Atlanta, Baltimore, and 12 hospital laboratories in Minnesota determined that only 8%–20% of MRSA patient-isolate sets were CA-MRSA [38]. Had we used a similar definition, we would have excluded 164 isolates that were clindamycin-susceptible, 26.6% of our total. We would also have excluded 190 isolates that carried SCCmec IV, 30.8% of our total

What, then, is CA-MRSA? For a clinician contemplating the prescription of clindamycin for an SSTI that occurs in an ambulatory patient, there will be uncertainty about the wisdom of this choice if the patient does not fulfill the CDC definition of CA-MRSA infection, although many isolates from patients who do not satisfy this definition will be clindamycin susceptible. For an epidemiologist, a temporal definition (e.g., culture sample collected >48 hours after hospital admission) or a healthcare risk factor–based definition may designate a subset of patients whose MRSA isolates that might be of interest. However, we have demonstrated that such a definition will not correlate with a definition based on molecular typing or antimicrobial susceptibility. For a molecular biologist who is interested in the subset of isolates that are PVL+, SCCmec IV–bearing, or bear some other genetic trait, still other definitions may be of interest

We have shown that a history of exposure to the healthcare environment is poorly predictive of the genotypic and phenotypic characteristics of the MRSA isolates that cause infections. This is so because the epicenter of the MRSA epidemic is no longer in healthcare institutions; it has moved to the community, and these infections affect both individuals who have exposure to the healthcare environment and individuals who do not have such exposure. Therefore, a single definition of CA-MRSA cannot fulfill all purposes. Investigators who use the term must carefully expostulate the definition they use and explicitly consider the different meanings given to the term “CA-MRSA.”

Our data also demonstrate that a disproportionate burden of the MRSA isolates that had genotype ST8 and were clindamycin susceptible, SCCmec IV–bearing, PVL+ came from children, which is consistent with an earlier communication from our center [34]. This suggests that children with MRSA infections may be more likely to be exposed to a reservoir of MRSA strains with these traits, compared with adults. Further research is needed to assess the importance of this finding

By the CDC’s risk factor–based definition, a history of a previous MRSA infection classifies a patient-isolate set as HA-MRSA. We found that patients with such a history frequently had MRSA isolates recovered that had genotype ST8 and were clindamycin susceptible, SCCmec IV–bearing, and PVL+, thus demonstrating the lack of close association between this risk factor and recovery of MRSA strains that had genotype ST5 and were clindamycin-resistant, SCCmec II-bearing, and PVL−, which are historically associated with HA-MRSA infection

There are several limitations to our study. It was conducted at a single center in the midst of a changing epidemic of MRSA infections in the community. However, the south side of Chicago has been a focus of epidemic CA-MRSA disease for almost a decade [5]. We were not able to interview every patient. However, thorough reviews of the paper medical records were conducted for those who were not interviewed, and additional information was collected from the electronic records of all patients. It is unlikely that more complete interview data would have yielded a greater number of patients categorized by the “lack of healthcare risk factors” criterion as having HA-MRSA infection

Epidemiologists have promulgated criteria to define the nosocomial transmission of many pathogens. These criteria remain valuable to infection control activities in healthcare facilities. The CDC’s definition of CA-MRSA is an inverse criterion; it is now used to describe community transmission of a pathogen that was previously nearly exclusively limited to healthcare settings. In the late 1990s, early in the CA-MRSA epidemic, the CDC’s definition was useful because it helped to define a new epidemiological phenomenon. However, the “community versus nosocomial” paradigm is no longer optimal for distinguishing genotypically distinct groups of MRSA isolates

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Footnotes

  • Presented in part: Israeli Society for Clinical Pediatrics Conference, February 2007, Tel Aviv, Israel (abstract 21); and the 2007 Pediatric Academic Societies’ Annual Meeting, May 2007, Toronto, Canada (publication 8422.7)

    Potential conflicts of interest: R.S.D. is supported by grants from the National Institute of Allergy and Infectious Diseases (NIAID) (1R01AI067584-01A2) and the Centers for Disease Control and Prevention (CDC) (1 U01 CI000384-01), and he has received grant support from Sage Products, Pfizer, Sanofi Pasteur, and Chlorox. He has served on paid advisory boards for Clorox, Sanofi Pasteur, GlaxoSmithKline, Pfizer, and the MRSA National Faculty Meeting (sponsored by Astellas and Theravance), and he has received lecture fees from Nabi Biopharmaceuticals and Pfizer. The other authors report no relevant conflicts

    Financial support: CDC (R01 CCR523379 and R01 CI000373-01 to R.S.D and S.B., R01 CI000373-01 to M.Z.D.), the NIAID (R01 AI40481-01A1 to R.S.D and S.B., 1R01AI067584-01A2 to S.E.C. and M.A.H), and the Grant Healthcare Foundation (to R.S.D and S.B.)

  • M.Z.D. and D.G. contributed equally to this work

  • Received June 15, 2007.
  • Accepted October 10, 2007.
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  1. J Infect Dis. (2008) 197 (9): 1235-1243. doi: 10.1086/533502
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