Alkali-stressed Listeria monocytogenes
Taormina, Peter Joseph
MetadataShow full item record
Listeria monocytogenes was more resistant to heat treatment following incubation for 45 min in tryptose phosphate broth (TPB) at pH 12.0 compared to pH 7.3. Cells survived at least 6 days in TPB at pH 9.0, 10.0, and 11.0 stored at 4 or 21ºC, and cells stored in TPB at pH 10.0 for 48 h were more resistant to treatment at 56ºC than were cells stored in TPB at pH 7.3. Cells were suspended in 1% solutions of eight food processing plant cleaners (pH 7.1 – 12.5) or in water (control) and incubated at 4ºC for 30 min or 48 h. In only three alkaline cleaning solutions (pH 11.6 – 12.4) were populations reduced significantly (P =0.05) after 30 min compared to reductions in water. After 48 h, populations were significantly higher in one cleaning solution (pH 10.4) than in water, while populations in six other cleaning solutions were reduced by >= 4.72 log10 CFU/ml. Cells exposed to cleaning solutions for 30 min became sensitive to 4.0 or 6.0 ppm free chlorine, and to 50 or 100 ppm benzalkonium chloride and cetylpyridinium chloride, common components of quaternary ammonium sanitizers. Cells exposed at 4ºC to 1% solutions of two alkaline cleaners or alkali-adapted in TPB (pH 10.0) at 37ºC for 45 min, then 4ºC for 48 h, were inoculated onto beef frankfurters containing high fat (16 g) and high sodium (550 mg) (HFHS) or low fat (8 g) and low sodium (250 mg) (LFLS) per 57-g serving. Frankfurters were surface-inoculated (2.0 log10 CFU/g), vacuum packaged, stored at –20, 4, or 12ºC, and analyzed for populations over time. Populations remained stable at -20ºC for up to 12 weeks. After storage at 4ºC for 6 weeks, populations of control cells and cells exposed to alkaline cleaners were ca. 6.0 log10 CFU/g of LFLS frankfurters and ca. 3.5 log10 CFU/g of HFHS frankfurters, but growth of alkali-adapted cells on both types of frankfurters was retarded. Growth at 12ºC was more rapid, but growth of alkali-adapted cells on HFHS and LFLS frankfurters after storage for 9 and 6 days, respectively, was also delayed. Alkaline stress of L. monocytogenes induces tolerance to heat, sensitivity to sanitizer, and changes in growth characteristics on frankfurters.