Fate of Listeria monocytogeneson Fully Ripened Greek Graviera Cheese Stored at 4, 12, or 25°C in Air or Vacuum Packages: In Situ PCR Detection of a Cocktail of Bacteriocins Potentially Contributing to Pathogen Inhibition

The behavior of Listeria monocytogeneson 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 faeciumGraviera isolate. Cheese pieces were surface inoculated with a five-strain cocktail of L. monocytogenesat ca. 3 log CFU/cm2, packed under air or vacuum conditions, stored at 4, 12, or 25°C, and analyzed after 0, 3, 7, 15, 30, 60, and 90 days. L. monocytogenesdid 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°C under vacuum conditions. Overall, the lower the storage temperature, the higher and longer the survival of L. monocytogeneswas. Although enterocin A–specific PCR products were detected in situ in cheese batch B, inhibition of L. monocytogenesby 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 Listeriainhibition in cheese. In conclusion, Graviera cheeses that may be accidentally contaminated in retail at the European Union maximal allowable level of 100 CFU/cm2or g are at low risk regarding a potential outgrowth of L. monocytogenes, which, however, may survive for a long period during cheese storage.