An In Vitro System for the Comparison of Excision and Wet-Dry Swabbing for Microbiological Sampling of Beef Carcasses

An in vitro system for the comparison of wet-dry swabbing and surface tissue excision was developed to ascertain whether the commonly accepted statement of the advantage (in terms of bacterial recovery) of the tissue excision method is also legitimate when different kinds of bacteria are used. A total of 1,770 sections (2.5 by 10 cm) of bovine skin were individually inoculated on the subcutaneous fat side by spreading various suspensions of marker organisms (nalidixic acid-resistant Escherichia coli, vancomycin-resistant Enterococcus faecalis, and methicillin-resistant Staphylococcus aureus) at different concentrations and sampled by two standard methods: cotton wet-dry swabbing and excision. Most counts from cuts sampled by excision were significantly (P < 0.05) higher than the wet-dry swabs; however, no differences were observed between the control and the sampling method when sections were inoculated with bacterial solutions at a concentration of 10³ CFU/ml and sampled by excision. For sections inoculated with bacterial solutions at a concentration of 10³ CFU/ml, counts given as log CFU/25 cm2 ranged from 1.97 (S. aureus sampled by wet-dry swab) to 3.06 (S. aureus sampled by excision). For sections inoculated at a concentration of 10⁴, counts given as log CFU/25 cm2 ranged from 2.15 (E. faecalis sampled by wet-dry swab) to 3.19 (S. aureus sampled by excision). For sections inoculated at 10⁵, counts given as log CFU/25 cm2 ranged from 2.94 (E. faecalis, wet-dry swab) to 3.98 (S. aureus, excision), and for sections inoculated at 10⁶, counts given as log CFU/25 cm2 ranged from 3.53 (E. coli, wet-dry swab) to 4.69 (S. aureus, excision). The proposed system, which enabled a considerable amount of samples to be analyzed under controlled experimental conditions and a large number of data to be generated in a short time, demonstrated among the tested microorganisms that whereas the excision method recovered the highest number of bacteria, control means were always (with the exception of an inoculum of 10³/ml) significantly higher than means from either of the sampling methods. Our results indicate that particular attention should be paid to the diverse microflora that can contaminate carcasses in a given slaughterhouse and that it is not appropriate to generalize by saying that the destructive method is the reference technique for the bacteriological sampling of carcasses in slaughterhouses, especially when the contamination is higher than 10³ CFU/25 cm2.