ABSTRACT
To compare the culture screening protocols for methicillin-resistant Staphylococcus aureus (MRSA), a total of 300 duplicate nasal swabs (233 initial cultures and 67 weekly follow-up cultures) were collected consecutively from 233 patients in the Intensive Care Unit (ICU). One swab was plated directly on MRSA-Select agar (D-MRSA-Select) and observed at 24 hr. The duplicate swab was incubated in tryptic soy broth (TSB) with 6.5% NaCl for 24 hr, and then subcultured on MRSA-Select (B-MRSA-Select), BAP (B-BAP), and mannitol salt agar with 4 mg/L oxacillin (B-MSA(OXA)), and observed at 24 hr. MRSA was detected in 13.7% (32/233) of the initial and 22.4% (15/67) of the follow-up specimens. A patient was classified as MRSA-positive if any of the media grew colonies that were tested and confirmed to be MRSA. In the initial screening samples, the sensitivities of D-MRSA-Select, B-MRSA-Select, B-BAP, and B-MSA(OXA) were 78.1%, 84.4%, 78.1%, and 65.6%, respectively, and the specificities were 100%, 98.0%, 83.1%, and 93.5%, respectively. The sensitivities of all but the B-MRSA-Select protocol were significantly lower (p <0.05). In follow-up screening, the sensitivities of D-MRSA-Select, B-MRSA-Select, B-BAP, and B-MSA(OXA) were 66.7%, 86.7%, 66.7%, and 53.3%, respectively, and the specificities were 100%, 98.1%, 90.4%, and 90.4%, respectively. D-MRSA-Select protocol was considered useful in screening for MRSA because it was fast, highly specific, and showed sensitivity comparable to B-BAP. Salt-containing enrichment broth in conjunction with MRSA-Select (B-MRSA-Select) provides a promising way to increase sensitivity in initial and follow-up screening for MRSA.
Subject(s)
Agar/metabolism , Bacteriological Techniques/methods , Chromogenic Compounds/metabolism , Methicillin Resistance , Staphylococcus aureus/isolation & purification , Humans , Intensive Care UnitsABSTRACT
To compare the sensitivity of various protocols for methicillin-resistant Staphylococcus aureus (MRSA) surveillance, active surveillance for detecting MRSA nasal colonization was performed on 97 members of the medical staff and 218 patients in the Intensive Care Unit (ICU) of a university hospital. Duplicate nasal swabs were collected from each participant. One was plated directly on a blood agar plate (D-BAP) and observed at 24 and 48 hr. Another was incubated overnight in tryptic soy broth (TSB) with 6.5% NaCl, and subcultured on both BAP (B-BAP) and mannitol salt agar with 4 mg/L of oxacillin (B-MSAOXA). The MRSA colonization rate was similar in the medical staff and patient samples (16.5% vs 11.9%, p = 0.285). Among the medical staff members, the sensitivity of MRSA detection was the same (93.8%) in D-BAP and B-BAP. In the ICU patients, which are a high-risk group, the sensitivity of MRSA detection was improved by adding a pre-enrichment step (73.1% on D-BAP vs 96.2% on B-BAP). The simple direct plating protocol was sufficiently sensitive for the medical staff members, but pre-enrichment was an essential step to increase detection of MRSA in the ICU patients.
