Detection of Listeria monocytogenes in raw and pasteurized liquid whole eggs and characterization by PFGE

Listeria monocytogenes has been recognized as a human pathogen for decades and is known to be an important foodborne pathogen. There have been no documented foodborne L. monocytogenes illnesses due to the consumption of eggs or egg products, even though the bacterium has been isolated from faeces, body fluid, and oviducts of asymptomatic laying hens. In order to describe L. monocytogenes contamination of egg products, 144 liquid whole egg samples were collected from 3 different egg-breaking plants during 3 sampling periods. L. monocytogenes detection was performed on raw samples stored at 2 degrees C for two days (D+2) and on pasteurized samples stored at 2 degrees C at D+2 and at shelf-life date (SLD). L. monocytogenes was detected in 25 of the 144 raw egg samples collected, in 4 of the 144 pasteurized egg samples at D+2 and in 2 of the 144 ones analysed at SLD. Contamination of raw egg products appeared to be season dependant and was higher during summer and winter than during autumn. One hundred and ninety-six L. monocytogenes isolates were collected and serotyped; 3 serovars were demonstrated. The dominant serovar was L. monocytogenes 1/2a which was presented by 94.4% of the isolates. Typing of 196 L. monocytogenes isolates was carried out by macrorestriction of the genomic DNA with ApaI and AscI enzymes followed by pulsed field gel electrophoresis (PFGE). A large diversity was observed with 21 genotypes of L. monocytogenes, even for a given manufacturer. Nevertheless, most of the egg product samples were contaminated by one genotype, except for five samples which were contaminated by two or three distinct genotypes. The genotypes seem to be specific to each manufacturer. No cluster of L. monocytogenes was found to recur in the different plants over successive seasons.