Enrichment in Fraser broth supplemented with catalase or Oxyrase®, combined with the microcolony immunoblot technique, for detecting heat-injured Listeria monocytogenes in foods

The microcolony immunoblot technique using monoclonal antibodies to Listeria monocytogenes was evaluated for its suitability to detect heat-injured cells. Pasteurized milk and filtrates of homogenized raw ground beef slurry and cabbage were inoculated with L. monocytogenes Scott A, heated, diluted, inoculated into Fraser broth (FB) supplemented with 400 μg of catalase ml−1 or 0.01 unit of Oxyrase® ml−1, and incubated at 30 °C for 6 h. Three inoculum populations (high, medium, and low) were used. The extent of injury was dependent on the heating menstruum. Forty percent of the cells were injured in beef slurry filtrate, whereas 79 and 94% were injured in milk and cabbage filtrate, respectively, when foods were heated at 52 °C for 20 min. Populations of viable cells were determined using the immunoblot technique and by surface plating on modified Oxford (mMOX) agar. Recovery of cells from heated foods was enhanced in FB supplemented with catalase or Oxyrase® compared to recovery in control broth. Essentially all unheated (control) cells could be detected within about 30 h using enrichment and the immunoblot technique; 54 h were required to easily detect colonies on mMOX. In most cases, the number of cells detected in heated milk or filtrates of homogenized beef after enrichment in FB supplemented with catalase or Oxyrase® was significantly higher than populations detected using unsupplemented FB; however, enrichment in FB supplemented with catalase or Oxyrase® did not significantly increase cell populations in heated cabbage filtrate. Within each heat treatment and level of inoculum, cell populations detected on mMOX agar after incubating plates for 48 h or on immunoblots after 24 h were not significantly different. Results indicate that the immunoblot technique in conjunction with enrichment in FB containing either catalase or Oxyrase® can be successfully used to detect healthy and heat-injured cells of L. monocytogenes in diverse types of foods within 34 h.