Comparison of the agar contact method and the wet-dry double swabbing method for determining the total viable bacterial count on pig carcass surfaces

A practical and representative sampling method for microbiological examination of the slaughter process is useful for identifying abattoir-specific risk factors within the pig slaughter line. The aim of this study was to examine the suitability of an agar contact method (ACM), where the agar was homogenized before the microbiological processing, in comparison with the wet-dry double swabbing method (WDSM) for quantitative determination of total viable counts (TVC) on pig skin surfaces. In our experimental trial, pig skin pieces were artificially contaminated at 2 levels (3 log and 7 log cfu/ml) with a suspension of bacteria species commonly found on pig skin and cultivated in vitro. Within our field trial, pig carcasses were investigated at pre-chilling in an abattoir under standard processing conditions. For both sampling methods, TVC was determined, and statistical equivalence tests were calculated. Linear regression models showed the similarity of the sampling methods, with coefficient of determination (R²) > 90% and slope parameters of nearly 1 for both trials separately. Statistically significant equivalence between the 2 sampling methods was proven in both trials (with p < 0.0001 within an equivalence range of ± 0.5 log cfu/ml, respectively). The field trial revealed TVC on carcass surfaces sometimes at or below the lower detection limit for the ACM, while TVC from all carcasses were able to be determined by WDSM. Overall, low contamination levels were less reliably detectable by ACM than by WDSM. The ACM can be seen as an additional and suitable sampling procedure for pig skin and can contribute to the identification of abattoir specific risk factors for investigations of the hygienic status at process stages along the pig slaughter line.