Comparison of three sampling methods for the quantification of methicillin-resistant Staphylococcus aureus on the surface of pig carcases

The objective was to determine the appropriate sampling method to quantify methicillin-resistant Staphylococcus aureus (MRSA) on the surface of pig carcases. Carcases were contaminated using brain-heart infusion broth containing MRSA at 5 log and 7 log cfu/ml. Samples were collected using three methods: a wet/dry double swab (100 cm2), an abrasive sponge (100 cm2) and a die cutter (2×5 cm2). Quantification of MRSA was done using decimal dilutions and cultivation in duplicate on chromogenic selective agar. The wet/dry double swab method was not sufficiently sensitive with 8 out of 10 samples being negative at low contamination levels. The abrasive sponge had reproducible results in all samples at both contamination levels. In low contamination areas 5 of the 10 die cutter samples were negative, while in the heavily contaminated areas, results of the die cutter and the abrasive sponge were comparable. It is concluded that with expected low contamination levels using the abrasive sponge is superior to the other two methods and should be used whenever contamination levels are not known.