Prevalence and location of L. monocytogenes contamination on beef carcasses after slaughter in Belgian slaughterhouses

Introduction Despite the efforts already made by the sector, beef slaughterhouses are still confronted with the foodborne pathogen Listeria monocytogenes in the production environment and on carcasses. The presence of the pathogen on carcasses may result in contaminations of the meat which may lead to economic losses and a potential risk for public health. The bacterial contamination occurs as a result of pathogen transfer from the environment, human handlings (cutting) and/or contact with the skin. This study on the prevalence and location of L. monocytogenes on beef carcasses after slaughter should be the basis for a better understanding of the complex introduction, persistence and variable contamination sources and routes of this pathogen. In addition, the possibility of using an indicator organism for L. monocytogenes contamination was investigated. Material and methods Sampling of 60 carcasses was carried out in two beef slaughterhouses. Carcass locations assumed to have an increased risk for contamination based on literature knowledge were swabbed just after slaughter and before cooling. The following carcass locations (400 cm2 each) were sampled: pelvic duct, split surface of neck, inside throat region, hind leg, flank (medial side), brisket center, inside foreleg and shoulder region. Listeria spp. and L. monocytogenes were detected and enumerated according to ISO11290-1 and 2. Moreover, total aerobic bacteria and Enterobacteriaceae were quantified. Results Listeria spp. and L. monocytogenes were detected in 46% (219/480) and 14% (68/480) of the swab samples respectively. Sixty-two percent (95% confidence interval from 43% to 80%) of the carcasses were found L. monocytogenes positive for at least one of the eight locations; more specifically the figures were 73% and 50% for the two slaughterhouses. Among the different locations, the brisket center line and the inside throat region represented the highest frequencies (20% and 17% respectively) for L. monocytogenes contamination. The shoulder showed the lowest contamination frequency (8%). However, the contamination rate at the sampled locations was not significantly different (P>0.05). Across all samples, 4% had counts for Listeria spp. between -1.30 and -0.30 log10 CFU/cm2. In five samples, L. monocytogenes numbers exceeded the limit of enumeration (above -1.3 log10 CFU/cm2). Combining the results of three carcass sites (chest, inside throat region and split surface of neck) resulted in the detection of 73% of the L. monocytogenes positive carcasses. Statistical analysis showed that detection of L. monocytogenes was significantly correlated with the total aerobic bacteria (P<0.01). Discussion Our results indicate a much higher prevalence of L. monocytogenes on carcasses in the investigated Belgian abattoirs (62%) compared to other studies (1-3%). A possible explanation is the fact of sampling eight sample sites on the carcass, which increases detection while classically only one location is sampled. The high prevalence of this pathogen in Belgian beef plants highlights the need of identifying contamination sources and routes. Characterization of the pathogen, environmental sampling and a follow-up of the slaughter operations from single carcasses will be carried out for a better understanding of the contamination.