Anti-listerial potential of food-borne yeasts in red smear cheese

Screening of 175 yeasts in an agar plate co-cultivation assay revealed that five out of 31 species reduced Listeria monocytogenes by 4-5 log units, one exceptionally active Pichia norvegensis reduced Listeria by 7 log units. To test the anti-listerial activity of this Pichia strain on cheese, Tilsit cheese and smeared acid curd cheese (Harzer) were prepared. The Tilsit cheese surface was inoculated with a 3%-NaCl brine containing Brevibacterium linens, Microbacterium gubbeenense, Corynebacterium casei, Staphylococcus equorum, Debaryomyces hansenii, P. norvegensis and L. monocytogenes. Ripening was done at 13 °C and >95% relative humidity. On the Tilsit, but not on the Harzer cheeses, a decrease of listerial cell numbers by 1-2 log units was observed. The difference between high inhibition in agar plate co-cultivation versus cheese is probably due to a decreased expression of the unknown inhibitory substance due to lactate, but not by the low pH.