A contribution of Listeria enrichment methodology - growth of Listeria monocytogenes under varying conditions concerning enrichment broth composition, cheese matrices and competing microbial flora

Reference methods for the detection of Listeria monocytogenes as proposed f rom various institutions (FDA, USDA, IDF; ISO) differ mainly in use of sele ctive broths of different composition and different procedures during enric hment. To comprehend the benefits of the individual procedures a harmonized enrichment method was recently proposed. The scope of this study was to de scribe the influence of parameters as cheese matrix composition and competi tive microbial flora on the enrichment of L. monocytogenes. Six commonly us ed enrichment broths were artificially inoculated with either (1) a L. mono cytogenes strain, or (2) with L, monocytogenes and competing flora, or (3) L. monocytogenes supplemented with sterile cheese matrix, or (4) L. monocyt ogenes, competing microflora and sterile cheese matrix. Samples were drawn after 6 h, 12 h, 24 h 48 h, 4 days and 7 days and subjected to enumeration of Listeria. Highest numbers for Listeria were demonstrated after 24 h incu bation in most media except in UVM II broth. Following a 48 h incubation pe riod a slow decrease of the Listeria cell count was observed, being evident most distinctly in the FDA broth as this lacks buffer systems. A 7-day enr ichment period was not found meaningful for any enrichment procedure. An im provement in Listeria enrichment as intended by the harmonized version prop osed by Hitchins was concluded. Results of HI broth in terms of productivit y and selectivity were seen to be satisfactory and equivalent to UVM broths The composition of the media basis is similar to cheaper FDA broth and bot h expensive and inappropriate indicator systems are avoided. (C) 1999 Acade mic Press.