Optimization of detection and yield of methicillin-resistant Staphylococcus aureus phage type III-29.

To the Editor: In recent years, methicillinresistant Staphylococcus aureus (MRSA) of phage type III-29 has become the MRSA strain most frequently introduced to Dutch hospitals from other European countries.1 From July 1992 to July 1993, our 800bed regional teaching hospital experienced the continual presence of an MRSA III-29 strain that colonized or infected 29 patients hospitalized on the intensive care units (ICUs; n=11) or, later, on two surgical wards (n=18). The index patient had brought the strain with her following repatriation from a German hospital. On the ICU, eight patients became infected (wound or respiratory tract); two of these patients also had positive blood cultures. On the surgical wards, six patients had postoperative wound infections. All patients had one or more known risk factors for acquisition of MRSA.2 Despite vigorous efforts of the infection control department to contain the epidemic, new cases were detected regularly. Fast detection of new colonized patients was thus of the utmost importance, in order that strict isolation measures could be implemented as soon as possible. In this 1-year period, nearly 300 contact patients were screened for the presence of MRSA. Personnel of wards on which MRSA was present were screened weekly (nose only). Various methods to detect MRSA among patients and healthcare personnel have been advocated.2,3 Considering the relative yields of various culture sites (nose, perineal area, axilla, or wounds), detection of a MRSA-positive patient could be maximally increased (to >95%) by a combination of cultures from various anatomical sites.4 In addition, there are considerable variations in the culture techniques described for optimal detection of MRSA in clinical specimens, not only in the media employed5,6 but also in the processing of specimens. The British Working Party on epidemic MRSA3 mentions the use of a broth culture as an enrichment medium for enhancing the detection of MRSA, as it resulted in an improved recovery rate.7,8 Furthermore, culture on a solid selective medium is said to increase MRSA detection significantly by inhibiting interfering or contaminating bacterial flora.9 We chose to use a ceftazidime-containing agar plate because of the heavy colonization with Pseudomonas aeruginosa found in the index patient at admission. For the first set of culture specimens in which MRSA was detected from each patient involved in this MRSA epidemic, we report our bacteriological results with reference to the yield from various body sites and the use of a noninhibitory enrichment broth, plated out on a solid selective agar medium, for the culture of swabs.