Detection of methicillin-resistant Staphylococcus aureus (MRSA) in screening swabs by direct culture on a solid screening medium and broth enrichment culture separately and in combination

Abstract

Patient and staff contacts of methicillin-resistant Staphylococcus aureus (MRSA) carriers were screened during a hospital outbreak over a 45 day period from 16 February-1 April 2001 inclusive. Screening swabs were collected from patient nostrils, perineum, hands and throat. Wounds, ulcers and skin lesions were swabbed when present. An urethral swab and urine were collected when an indwelling urinary catheter was present. Screening swabs were collected from staff nostrils and hands. Broken skin areas were also swabbed. Swabs were individually plated onto methicillin aztreonam mannitol salt agar (MAMSA) and incubated in air at 36 °C for 20-24 hours. All the swabs were placed in 10 mL of selective enrichment broth consisting of nutrient broth containing methicillin 4 mg/L, aztreonam 2 mg/L and sodium chloride 25 g/L in a single 30 mL container and incubated at 36 °C for 20?24 hours. The broth culture was resuspended by manual shaking and one microlitre was plated onto MAMSA and incubated in air at 36 °C for 20?24 hours. A total of 2,130 sets of screening swabs were collected; 1,358 from patients and 772 from staff. Direct culture alone and enrichment culture alone each detected MRSA in 312 sets. Direct culture detected MRSA in some sets of swabs in which MRSA was missed by enrichment culture and vice versa. The combination of direct and enrichment cultures detected MRSA in 446 sets, which was 134 sets more than either method alone. As direct culture and enrichment culture each detected MRSA in only 70% of the sets of screening swabs compared with the combination of both methods, it is concluded that the laboratory should use both methods for processing MRSA screening swabs.