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Methods

Setting. Johns Hopkins Hospital is a 946‐bed tertiary care hospital located in Baltimore, Maryland, that serves as a referral center for the mid‐Atlantic region. The Department of Hospital Epidemiology and Infection Control at Johns Hopkins Hospital has tracked the rate of MRSA colonization with active surveillance cultures among adult intensive care unit patients and inpatients with human immunodeficiency virus (HIV) infection. Culture samples are obtained at admission and weekly thereafter.

Study design. A total of 200 HCWs were recruited and followed up prospectively for 6 months during the period from December 2004 through May 2005. Eligible participants included medical and pediatric house staff, clinical infectious diseases fellows (a group included in the house staff category), and nursing staff from the inpatient HIV infection unit and the medical intensive care unit (MICU). This study was approved by the Johns Hopkins Hospital institutional review board. All participants provided informed consent. Once a month, anterior nares samples were obtained for culture from each participant with a premoistened, cotton‐tipped swab. Each participant completed baseline and exit questionnaires that addressed risk factors for MRSA colonization and infection and the participant's history of MRSA infections.

Microbiological methods. All swab samples were transported to the laboratory within 24 hours after collection. Specimens were inoculated into Chromagar SA media (BD Diagnostics). Susceptibility profiles were determined with agar dilution. MRSA isolates were tested for the presence of the Panton‐Valentine leukocidin (PVL) gene and the staphylococcal cassette chromosome (SCC) mecIV to assess whether any isolates were community‐associated MRSA (CA‐MRSA) strains, and isolates were tested for clonality by pulsed‐field gel electrophoresis (PFGE), as described elsewhere.^7,8 All testing was performed in the Johns Hopkins Hospital microbiology laboratory.

Statistical analysis. Risk factor analysis was performed with univariate logistic regression. All statistical analyses were performed using Stata software, version 8 (Stata).

Results

Of the eligible participants, 70 (64.8%) of 108 medical house staff, 43 (53.8%) of 80 pediatric house staff, 5 (100%) of 5 clinical infectious diseases fellows, 33 (86.8%) of 38 nursing staff from the HIV unit, and 49 (80.3%) of 61 nursing staff from the MICU chose to enroll in the study. The baseline characteristics of participants are listed in the Table. Participants were followed up for 997.2 person‐months, with a median follow‐up period of 5.3 months (interquartile range, 4.8‐5.4 months). Among participants, compliance with regular collection of culture samples was 95.5% after enrollment. The rate of colonization with methicillin‐susceptible S. aureus (MSSA) remained stable, at approximately 30% each month throughout the study period, which is consistent with previously published studies.

Table.  Baseline Characteristics of 200 Healthcare Workers Screened for Staphylococcus Aureus Colonization

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Patterns of colonization. Of 200 study participants, 117 (58.5%) never had cultures that yielded S. aureus. A total of 27 participants (13.5%) had cultures that consistently yielded MSSA from each sample obtained. Forty‐eight people (24%) were found to be intermittently colonized with MSSA, of which 24 were incident cases. Eight participants (6 [27.3%] of 8 nursing staff and 2 [1.7%] of 118 physicians) were found to be colonized with MRSA at least once during the study period. Four of these 8 participants were found to be colonized with MRSA at the beginning of the study, and the remaining 4 represented incident cases. Four of 8 HCWs colonized with MRSA were found to be persistently colonized (defined as 2 consecutive cultures that yield MRSA).

Incidence and prevalence of S. aureus. Among patients, the prevalence of infection and/or colonization with MRSA on admission between July 1, 2004, and July 1, 2005, was 33%‐37% in the inpatient HIV infection unit and 21%‐28% in the MICU. For HCWs, the baseline prevalence of colonization with S. aureus was 28% (95% confidence interval [CI], 22%‐34% [56 of 200 HCWs]), and the prevalence of colonization with MRSA was 2% (95% CI, 0.04%‐4.0% [4 of 200]). The incidence of S. aureus acquisition was 50 (95% CI, 32‐75) new colonizations per 100 person‐years, and the incidence of MRSA acquisition was 4.9 (95% CI, 1.3‐13) new colonizations per 100 person‐years. One HCW reported having had an MRSA infection before the study period. No HCWs reported developing an infection during the study period.

Molecular analysis. None of the MRSA isolates we analyzed contained the PVL gene, although 5 of 8 isolates contained SCCmecIV. All but 1 of the colonized study participants carried the same MRSA strain throughout the study, as determined by PFGE. The MRSA strains were shown to be related but not identical by PFGE and did not resemble any of the known CA‐MRSA strains (USA 300 or USA 400)⁷ (Figure).

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Figure.  Dendrogram and pulsed‐field gel electrophoresis (PFGE) patterns of methicillin‐resistant Staphylococcus aureus (MRSA) isolates from study participants, compared with patterns for common MRSA clones. The PFGE patterns of the community‐associated MRSA strains USA 300 and USA 400 are shown for comparison. Panton‐Valentine leukocidin (PVL) gene findings and SCCmec typing of strains are also shown.

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Risk factor analysis for S. aureus and MRSA colonization. On univariate risk factor analysis, male sex was associated with increased risk of S. aureus colonization, with an odds ratio of 2.43 (95% CI, 1.30‐4.56; ). Other factors, such as being a member of the house staff, receipt of antibiotic therapy, and/or medical conditions such as allergic rhinitis, asthma, or chronic sinusitis, were not statistically significant. With respect to MRSA colonization, house staff members had a markedly lower risk of colonization, compared with nursing staff, with an odds ratio of 0.22 (95% CI, 0.04‐1.11; ). Other potential risk factors were not statistically significant, most likely because of the low prevalence and incidence of MRSA colonization. Model building did not yield any additional risk factors.

Discussion

This study suggests that the risk of MRSA being transmitted to HCWs is low in a hospital where MRSA is endemic, as long as standard infection control practices are followed. We examined a cohort of HCWs with significant potential exposure to MRSA, and we anticipated that the rate of MRSA colonization would be approximately 10%‐50% for clinical staff, as reported elsewhere.⁵ The patterns of colonization were similar to those in recent national population studies; most HCWs (58.5%) were not found to be colonized with S. aureus during the study period.³ This finding suggests that intrinsic factors may determine S. aureus colonization status. Recent studies support genetic polymorphisms associated with S. aureus nasal carriage.⁹

This study has several limitations. Cultures of hand samples were not performed, which could cause potential underestimation of colonization rates. The anterior nares is the reservoir for hospital‐acquired MRSA, although it appears that this is not necessarily the case for CA‐MRSA.¹⁰ Selection bias could have occurred, resulting in an underestimate of MRSA colonization, if those who were at less risk of acquiring MRSA were more likely to volunteer for the study. PFGE did not show any HCW to be colonized with CA‐MRSA during this study, which may be because of the absence of CA‐MRSA colonization or because of low yield of CA‐MRSA in cultures of nasal swab samples.

MRSA is a persistent and ever‐growing problem for healthcare institutions, and CA‐MRSA adds another degree of complexity. Minimizing the emergence of this organism and containing its spread remain challenges that need to be addressed. For HCWs routinely exposed to this organism, the personal risk appears to be minimal.