Potent antilisterial cell-free supernatants produced by complex red-smear cheese microbial consortia

The microbial surface ripening consortia of 49 soft cheeses were investigated with respect to their inhibition of Listeria monocytogenes. When L. monocytogenes EGDe (serovar 1/2a) was cultivated in cell-free supernatants obtained from consortia grown for 8 h in liquid medium, a strong bactericidal activity was observed in several cases. The cell-free supernatants of 2 of these consortia (I and II) reduced an initial L. monocytogenes inoculum of 5 × 107 cfu/mL to zero after 24 h of incubation. No inhibitory substances could be washed off the complex consortia when incubated for a 10-min period. A taxonomical analysis of the antilisterial consortia I and II using Fourier transform infrared spectroscopy yielded a considerable species diversity, with lactic acid bacteria increasing strongly during the 8-h cultivation. Therefore, 23 lactic acid bacteria bacteriocin genes were assayed using specific PCR primers, identifying 3 bacteriocin genes in both microbial communities. However, no transcription of these genes was found on cheese surfaces or in consortia propagated in liquid culture. Individual lactic acid bacteria isolates of consortia I and II displayed no or only weak inhibition of L. monocytogenes on solid medium. The complex cell-free supernatants I and II, in contrast, exhibited an unusually broad inhibitory spectrum, killing L. monocytogenes ssp., Bacillus spp., Staphylococcus aureus, as well as gram-negative bacteria such as Escherichia coli DH5α and Salmonella enterica serovar Typhimurium. Inhibition could not be abolished by heating to 100°C or by proteinase K treatment. Initial purification of an inhibitory substance from consortium I by solid-phase extraction and HPLC indicates the presence of rather small, extremely stable compounds, which, most probably, are not bacteriocins.