Semi-selective broth improves screening for methicillin-resistant Staphylococcus aureus

Böcher, S.
Middendorf, B.
Westh, H.
Mellmann, A.
Becker, K.
Skov, R.
Friedrich, A. W.

¹National Center for Antimicrobials and Infection Control, Statens Serum Institut, Copenhagen, Denmark
²Institute of Hygiene, University Hospital Münster, Germany
³Department of Clinical Microbiology, Hvidovre Hospital, Denmark
⁴Faculty of Health, University of Copenhagen, Denmark
⁵Institute of Medical Microbiology, University Hospital Münster, Germany

Corresponding author. Tel: +45-3268-8348; Fax: +45-3268-3231; E-mail: [email protected]

Received September 4, 2009

Accepted December 30, 2009

Journal of Antimicrobial Chemotherapy 65(4):p 717-720, April 2010. | DOI: 10.1093/jac/dkq001

Objectives

To evaluate two enrichment broths for methicillin-resistant Staphylococcus aureus (MRSA) detection and compare results with direct plating.

Methods

Swabs from 1224 patients were re-analysed for MRSA in a central laboratory (Münster) using six methods. Swabs were suspended in 0.5 mL of non-selective enrichment broth (NB) and vortexed. Aliquots of 100 µL were inoculated on/into: (I) ChromID MRSA agar; (II) Columbia sheep blood (5%) agar (BA) and ChromID MRSA; (III, IV) NB incubated overnight followed by plating on BA and ChromID MRSA; and (V, VI) a semi-selective broth containing cefoxitin and aztreonam (TSB-SSI) incubated overnight followed by plating on BA and ChromID MRSA. In III–VI, 100 µL of the enriched broth was plated on each agar.

Results

The combined MRSA-positive rate was 21.5%. MRSA isolates detected by each method were: TSB-SSI, n = 223; NB, n = 205; BA and ChromID MRSA, n = 203; ChromID MRSA alone, n = 183. TSB-SSI detected more positive throat samples than the comparators and significantly reduced methicillin-susceptible S. aureus (MSSA) growth. The maximum sensitivity obtained was only 85%, possibly due to the study design using pre-used swabs and dilution of swab material. For 997 samples, results from Münster were compared with initial results. Peripheral laboratories identified 172 MRSA compared with Münster where 186, 186 and 204 MRSA were found for direct plating, NB and TSB-SSI broth, respectively.

Conclusions

TSB-SSI was superior to both NB and direct plating on ChromID MRSA and BA. Despite re-using swabs for the study, we showed that routine diagnostic screening could be significantly improved, using a semi-selective enrichment broth.