High biodiversity and potent anti-listerial action of complex red smear cheese microbial ripening consortia
When Listeria monocytogenes EGDe (serovar 1/2a) was cultivated in cell-free supernatants prepared from red smear cheese microbial ripening consortia grown for 8 h in liquid medium, 8 out of 49 supernatants exhibited a bactericidal activity, sometimes even reducing the inoculum of L. monocytogenes from 5 × 107 CFU/ml to zero after 24 h of incubation. Another five consortia displayed a bacteriostatic capacity. No inhibition in supernatants was observed when the complex consortia were incubated for a 10-min period only, indicating that the activity depends on actively growing consortia. Consortia displayed a very high biodiversity (Simpson’s strain diversity index up to 0.97, species diversity up to 0.89). However, biodiversity did not correlate with anti-listerial activity. There was no obvious similarity between the anti-listerial consortia studied, and no general difference in comparison to non-inhibitory communities. The proportion of lactic acid bacteria (LAB) in the consortia ranged between 3 and 45%. Therefore, the presence of 23 different LAB bacteriocin genes was investigated using specific PCR primers, identifying one to five bacteriocin genes in several consortia. In situ transcription of lactococcin G mRNA on the cheese surface was demonstrated by RT-PCR in five samples, but this bacteriocin displayed no anti-listerial activity. Supernatants subjected to thermal and enzymatic treatment suggested the presence of heat-stable, non-proteinaceous molecules as well as heat-labile compounds which are sensitive to proteolytic digestion. Probably, substances other than LAB bacteriocins are responsible for the pronounced antilisterial action of some supernatants.
KeywordsListeria monocytogenes Red smear cheese Bactericidal activity Biodiversity Bacteriocin genes Lactic acid bacteria
This study received research funding from the European Community’s Sixth Framework Programme. TRUEFOOD (Traditional United European Food) is an Integrated Project financed by the European Commission under the 6th Framework Programme for RTD (contract number FOOD-CT-2006-016264), and in part by the German Ministry of Economics and Technology (via AiF) and the FEI (Forschungskreis der Ernährungsindustrie e.V., Bonn). Project AiF 14786 N. The information in this document reflects only the author’s views while the funding agencies, especially the High European Community, are not liable for any use that may be made of the information contained therein.
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