Optimizing detection of heat-injured Listeria monocytogenes in pasteurized milk.
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).
Teo
A Y
AY
Department of Food Science, The Pennsylvania State University, University Park 16802-2504, USA.
Ziegler
G R
GR
Knabel
S J
SJ
D000818:Animals; D001431:Bacteriological Techniques; D003198:Computer Simulation; D003470:Culture Media; D016984:Enterococcus faecium; D005516:Food Microbiology; D005519:Food Preservation; D006358:Hot Temperature; D008089:Listeria monocytogenes / Q000187:drug effects / Q000302:isolation & purification*; D018021:Lithium Chloride / Q000494:pharmacology*; D008278:Magnesium Sulfate / Q000494:pharmacology*; D008892:Milk / Q000382:microbiology*; D012694:Serine / Q000494:pharmacology*; D013171:Spores, Bacterial; D013696:Temperature
D003470:Culture Media; D012694:Serine; D008278:Magnesium Sulfate; D018021:Lithium Chloride