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 from various institutions (FDA, USDA, IDF; ISO) differ mainly in use of selective broths of different composition and different procedures during enrichment. To comprehend the benefits of the individual procedures a harmonized enrichment method was recently proposed. The scope of this study was to describe the influence of parameters as cheese matrix composition and competitive microbial flora on the enrichment of L. monocytogenes. Six commonly used enrichment broths were artificially inoculated with either (1) a L. monocytogenes strain, or (2) with L, monocytogenes and competing flora, or (3) L. monocytogenes supplemented with sterile cheese matrix, or (4) L. monocytogenes, 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 incubation in most media except in UVM II broth. Following a 48 h incubation period 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 enrichment period was not found meaningful for any enrichment procedure. An improvement in Listeria enrichment as intended by the harmonized version proposed by Hitchins was concluded. Results of HI broth in terms of productivity 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 both expensive and inappropriate indicator systems are avoided. (C) 1999 Academic Press.