MRSA screening – Comparison of chromogenic agar and enrichment broths

Abstract number: P2129

Bjørnholt J.V., Fostervold A., Ranheim T., Lauvrak V., Steinbakk M.

Objective: The detection of meticillin-resistant Staphylococcus aureus (MRSA) in clinical samples is important. This is particularly so during screening in a low prevalence setting, where extensive infection control measures are imposed by a positive finding. The NaCl concentration is important for mecA expression, we therefore compared our routine chromogenic agar with two variants of enrichment broth one with 0.5% NaCl and another with 2% NaCl, during two outbreaks in nursing homes.

Methods: The chromogenic agar (bioMérieux) were compared with a modified enrichment broth originally described by Wertheim et al. with 0.5% NaCl concentration (broth1) and an another identical variant with 2% NaCl concentration (broth 2). Screening samples received from the nursing homes were inoculated on chromogenic and blood agar and in broth, MRSA isolates were verified by presence of mecA and Sa442. All isolates were characterised with MLST, spa-typing, SCCmec type and presence/absence of PVL. McNemar and Sign test were used for testing differences in detection rates.

Results: Broth 1 (0.5% NaCl); 4931 samples were included. MRSA were found in 187 samples 3.9% (95% CI; 3.3–4.3). The main results are presented in table 1. Broth 1 and chromogenic agar detected 17% (95% CI; 12–22) more MRSA-isolates than Chromogenic agar alone.

Table 1

Broth 2 (2% NaCl); 1697 samples were included. MRSA were found in 104 samples 6.1% (95% CI; 1.5–10). The main results are presented in table 2. Broth 2 and chromogenic agar detected 31% (95% CI; 22–40) extra MRSA than chromogenic agar alone and had the same sensitivity as the Chromogenic agar.

Table 2

All isolates belonging to the outbreaks were MLST 8, spa-type 304, SCCmec IV and PVL neg. Oxacillin MIC ranged from 1 to 64 mg/l. Three isolates with oxacillin MIC < 2 mg/l were CO2 dependent and were detected on blood agar only.

Conclusion: The addition of an enrichment broth to chromogenic agar when screening for MRSA increases sensitivity significantly and 2% NaCl concentration seems better than 0.5%. MRSA isolates with low mecA expression (low Oxacillin MIC) were not detected by the selective media.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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