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Optimizing detection of heat-injured Listeria monocytogenes in pasteurized milk
A Y Teo1, G R Ziegler, S J Knabel
1Department of Food Science, The Pennsylvania State University, University Park 16802-2504, USA.
Abstract:
Optimal conditions for the detection of heat-injured cells of Listeria monocytogenes in modified Pennsylvania State University (mPSU) broth were determined using a response surface design generated by a computer program, EChip. Different combinations of incubation temperatures and lithium, magnesium, and D-serine concentrations were evaluated to determine the optimum conditions for the detection of heat-injured L. monocytogenes in filter-sterilized whole milk inoculated with selected problematic background microflora. A concentration of 212 mM lithium chloride completely inhibited the growth of Enterococcus faecium while permitting recovery and detection of L. monocytogenes. A concentration of 15.8 mM MgSO4 was found to be optimum for the recovery and detection of L. monocytogenes. A concentration of 140.2 mM D-serine was found to completely inhibit the germination of Bacillus subtilis var. globii spores but not recovery and detection of L. monocytogenes. Under optimum concentrations of LiCl, MgSO4, and D-serine and in the absence of background microflora, the effect of incubation temperature on percentage detection was described by a second-order polynomial model, and 28 degrees C was determined to be optimal. In the presence of background microflora, the effect of incubation temperature on percentage detection of heat-injured cells was described by a third-order polynomial model, and 30 degrees C was found to be optimal. Optimizing the levels of highly specific and selective agents, nutrients, and incubation temperature in one recovery enrichment system dramatically increased the Listeria/background microflora ratio. This resulting medium, optimized PSU (oPSU) broth, greatly improved the detection of heat-injured and nonheat-injured L. monocytogenes by both conventional and molecular methods (Oxoid's Listeria Rapid Test, Gen-Probe's Accuprobe Listeria monocytogenes Culture Identification Test, and Qualicon's BAX for screening Listeria monocytogenes).