Distribution of Listeria spp. in confectioners' pastries from western France: comparison of enrichment methods

Three hundred samples of pastry from 100 different suppliers in western France, including butter-cream, whipped dairy cream and custard filled cakes from each supplier, were collected and tested for the occurence of Listeria spp. in 25 g samples. Listeria spp. were detected in 21.7% of the samples: Listeria monocytogenes in 13.7%, Listeria innocua in 10% and Listeria seeligeri in 2.3%. Thirteen samples were contaminated with two species simultaneously. The frequency of contaminated samples was not related to the composition of the pastry filling used, but it seemed to increase with the number of aerobic contaminant microorganisms in the dairy cream-based samples. The contamination rate was dependent on the place of manufacture. The numbers of Listeria spp. and Listeria monocytogenes were estimated on positive samples at the 25 g level as follows: < 0.3/g, Listeria spp. in 47 samples, L. monocytogenes in 27; 0.3–30/g, Listeria spp. in 13, L. monocytogenes in nine; 30–300/g, L, monocytogenes in one; 300–3000/g; L. monocytogenes in three; 700 000/g, L. monocytogenes in one. Various detection methods were tested, including two enrichments broths tested in parallel: a modified LEB broth using 10 mg/l acriflavine-HCl and the UVM 1 broth, with incubation at 30° C and streaking onto PALCAM agar. The enrichment procedures were: ( a) primary enrichment of 25 g sample and plating after 48 h and 7 days; ( b) secondary enrichment by subculting the primary enrichment broths incubated for 24 h and 6 days, into fresh enrichment broth, then plating after 24 h incubation; ( c) pre-enrichment of 25 g sample for 24 h in the basal enrichment broths without inhibitors, followed by subculturing in complete broths which were plated after 24 h and 6 days incubation. In all cases, UVM performed better than the LEB broth. It was unnecessary to extend the primary enrichment period beyond 48 h. Secondary enrichments inoculated from 24-h incubated primary enrichments gave a slightly better isolation rate than primary enrichments. Secondary enrichments made from 6-day incubated primary enrichments gave no additional advantage. The pre-enrichment procedure had an efficiency higher than that obtained by primary enrichment.