Fate of Listeria monocytogenes on fully ripened Greek Graviera cheese stored at 4, 12, or 25 degrees C in air or vacuum packages: in situ PCR detection of a cocktail of bacteriocins potentially contributing to pathogen inhibition.

Abstract

The behavior of Listeria monocytogenes on fully ripened Greek Graviera cheese was evaluated. Three batches (A, B, and C) were tested. Batches A and C were prepared with a commercial starter culture, while in batch B the starter culture was combined with an enterocin-producing Enterococcus faecium Graviera isolate. Cheese pieces were surface inoculated with a five-strain cocktail of L. monocytogenes at ca. 3 log CFU/cm2, packed under air or vacuum conditions, stored at 4, 12, or 25 degrees C, and analyzed after 0, 3, 7, 15, 30, 60, and 90 days. L. monocytogenes did not grow on the cheese surface, regardless of storage conditions. However, long-term survival of the pathogen was noted in all treatments, being the highest (P < 0.05) at 4 degrees C under vacuum conditions. Overall, the lower the storage temperature, the higher and longer the survival of L. monocytogenes was. Although enterocin A-specific PCR products were detected in situ in cheese batch B, inhibition of L. monocytogenes by the enterocin-producing strain was not enhanced compared with batches A and C, which also contained enterocin A, but in lower amounts. Additionally enterocins B, P, L50A, and L50B; lactococcin G; and plantaricin A genes were detected in all batches, suggesting that indigenous bacteriocin-producing lactic acid bacteria might contribute to Listeria inhibition in cheese. In conclusion, Graviera cheeses that may be accidentally contaminated in retail at the European Union maximal allowable level of 100 CFU/cm2 or g are at low risk regarding a potential outgrowth of L. monocytogenes, which, however, may survive for a long period during cheese storage.