The paper presents a new medium and a method for the rapid presumptive detection of Listeria spp. in cheese samples by electrical capacitance changes. The Bactometer based capacitance broth (LED medium) required a 30% change in signal within 30 h to identify presumptive positive results. Of 32 cheese samples tested, 10 were found to contain Listeria spp. using a Fraser broth screening method and 12 using the LED medium and method. The LED method also gave 16 fewer (total = 1) false presumptive positive results. Results show the LED method to be superior to Fraser broth in regard to both the number of false presumptive positive and confirmed false negative results detected. The method appears to be suitable as a reliable rapid screen for the presence of Listeria spp. in cheese.
Four secondary enrichment protocols (conventional methods: UVM II, Fraser 24 h and Fraser 48 h: Impedimetric method: Listeria electrical detection medium) were studied for their ability to isolate Listeria spp. and Listeria monocytogenes from fish and environmental samples collected along the processing chain of cold-smoked fish. From all methods, Listeria spp. and L. monocytogenes were respectively present in 56 and 34 of 315 samples analysed. Fraser broth incubated for 48 h gave the fewest false negative Listeria spp. results [4/56; (7.1%)], but concurrently only 15/34 (44.1%) samples were correctly identified as containing L. monocytogenes, Listeria electrical detection (LED) medium detected only 36/56 (64.3%) Listeria spp. positive samples. Despite this lower isolation rate, LED identified 20/34 (58.8%) L. monocytogenes positive samples correctly and gave fewer false positive results. The overall conclusion was that more than one isolation method is needed to accurately estimate L. monocytogenes contamination rates.
The development of a liquid medium for the detection of Listeria spp. by capacitance monitoring of food samples previously enriched in UVM 1 broth is described. Rapid growth of Listeria monocytogenes was shown to occur in liquid media with selectivity based on antibiotics found in Oxford agar. The final capacitance medium contained higher concentrations of the Oxford selective agents than Oxford agar and did not require the esculin/ferric ammonium citrate reaction to be observed. The medium relied upon the ability of Listeria spp. to induce a greater than 30% change in capacitance within 30 h. When run in parallel with the Listeria spp. test samples of a large food company, the method gave far fewer false-positive results than Fraser broth.
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