Can lactic acid bacteria be an efficient tool for controlling Listeria monocytogenes contamination on cheese surface? The case of Gorgonzola cheese

Gorgonzola is an Italian blue-veined, mould-ripened PDO cheese obtained using pasteurised cow's milk. Like other mould cheeses, Gorgonzola rind represents a suitable substrate for the growth of Listeria. In this study, we proposed the use of lactic acid bacteria (LAB) to limit the growth of L. monocytogenes on Gorgonzola surface during ripening process. A total of 23 LAB strains were screened by agar disk diffusion test and microplate turbidometric growth inhibition assays for their antimicrobial activity against two L. monocytogenes and two L. innocua biotypes isolated from Gorgonzola rinds. The anti-Listeria activity differed among the LAB strains and it was Listeria biotype-dependent. Amongst them, Carnobacterium SCA, SCB, Lactobacillus plantarum NA18, Lb. sakei SCC, Lactococcus lactis FT27, N16 and SV77 were selected for their marked anti-Listeria activity and a challenge test was performed to evaluate their effect on the development of L. monocytogenes and L. innocua artificially inoculated on Gorgonzola rinds at 6 and 55 days of ripening and maintained at 4 and 8 °C. Five strains (Carnobacterium SCB, Lb. sakei SCC, Lc. lactis FT27, N16 and SV77) showed notable inhibition when low level of Listeria contamination (102 CFU/g) occurred during the first stage of ripening (6 days) and L. monocytogenes cells were maintained below the EC limit (< 102 CFU/g) for 60 days. Differently, when Listeria was inoculated on the cheese surface at the end of ripening process (after 50 days; pH: 6.7), only Lc. lactis FT27 exerted a significant inhibition on the growth of the two Listeria species if the cheese was strictly maintained at 4 °C. This research underlined that the susceptibility of Listeria biotypes to LAB antimicrobial activity is strain-dependent, thus a blend of different LAB strains could represent an effective tool to develop protective culture for blue-veined cheeses.