Evaluation of cefoxitin disks to detect methicillin-resistant coagulase negative staphylococci

Abstract number: 1733_1136

Mouton J.W., Bohne M.T., Klaassen C.H., Horrevorts A.M., de Valk H.A., Voss A.

Objectives: For S. aureus, the use of cefoxitin disks is the recommended method to screen for methicillin resistance (MetR). A similar method has been suggested for coagulase negative staphylococci (CNS) but it is not clear which method performs best. Specificity is compromised since CNS consists of multiple species and every species has its own characteristics, including the discriminatory effect on zone diameters. We investigated 4 different methods (2 disks and 2 media) to determine the cut-off zone diameters to achieve a high specificity and sensitivity to distinguish metR from metS CNS. These values were also determined at the species level.

Methods: 250 CNS were isolated from blood cultures or other relevant sites of infection and stored at -70°C. Strains were revived by plating on blood agar. The next day, colonies were suspended to obtain an adequate inoculum to result in semiconfluent growth (Iso Sensitest agar) or confluent growth (Mueller Hinton agar). A 10 and 30 mg cefoxitine disk (Oxoid) was placed on each 9 cm agar plate and incubated for 24 h at 35°C. Zone diameters (mm) were read in two directions to one decimal. Identification was performed by AFLP and partial 16S rRNA gene sequencing. The mecA gene was identified by a LightCycler method.

Results: 167 strains were metR and 83 metS. Of the four methods, the 30 mg disk on MH performed best, but there was considerable overlap in zone size between metR and metS strains. Setting the sensitivity at 100% resulted in a specificity of 67%. At the species level, the median zone diameter for metS S. epidermidis (N = 128) and S. hominis (N = 38) was 30.7 mm and 26.6 mm respectively. A 100% sensitivity as well as specifity at the species level was obtained at cut offs of 27.5 mm and 17 mm, respectively.

Conclusion: The method best discriminating between metR and metS strains was MH with confluent growth and a 30 mg disk but with a relatively poor specificity of 67%. Speciation increased both sensitivity and specificity to 100% for S. epidermidis and S. hominis.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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