Use of surface-smear bacteria for inhibition ofListeria monocytogeneson the rind of smear cheese

Surface-smear micro-organisms isolated from Taleggio cheese were screened for their ability to inhibitListeria monocytogenes. Most of the isolates showing antilisterial activity (19% of the total) consisted of coryneform bacteria, mainly belonging to theMicrobacterium lacticumspecies. The inhibitory activity was observed also after growth at 5°C and pH 6·0. After cross-inhibition tests among inhibitory strains and between these and other pigmented, non-inhibitory strains, two bacterial mixtures were assayed as surface-smear starter of Taleggio cheese. At different ripening stages (1, 20 and 40 days), the cheese surface was contaminated with 2·5×102cfu cm−2L. monocytogenes(Ohio strain) and the growth evaluated over 15 days of incubation at 5°C. Contrary to the laboratory experiments,Listeriacould not be completely inhibited on the cheese surface. With 5·5–6·0 pH range of the cheese rind, (lower than usual values), the growth of surface-smear bacteria was delayed, or even stopped. Nevertheless, a listeriostatic effect was achieved on the surface of cheese samples contaminated at the end of ripening. This seemed to confirm the essential role played by surface-smear bacteria, within the rind microflora, in controlling the development of contaminant micro-organisms, and by competitive bacteria in reducing the overall risks associated with soft cheese. The mechanisms involved in the selection and the growth of competing bacteria on the cheese surface are also discussed.