Abstract
BackgroundSurface contamination of smear cheese by Listeria spp. is of major concern for the industry. Complex smear ecosystems have been shown to harbor antilisterial potential but the microorganisms and mechanisms involved in the inhibition mostly remain unclear, and are likely related to complex interactions than to production of single antimicrobial compounds. Bacterial biodiversity and population dynamics of complex smear ecosystems exhibiting antilisterial properties in situ were investigated by Temporal temperature gradient gel electrophoresis (TTGE), a culture independent technique, for two microbial consortia isolated from commercial Raclette type cheeses inoculated with defined commercial ripening cultures (F) or produced with an old-young smearing process (M).ResultsTTGE revealed nine bacterial species common to both F and M consortia, but consortium F exhibited a higher diversity than consortium M, with thirteen and ten species, respectively. Population dynamics were studied after application of the consortia on fresh-produced Raclette cheeses. TTGE analyses revealed a similar sequential development of the nine species common to both consortia. Beside common cheese surface bacteria (Staphylococcus equorum, Corynebacterium spp., Brevibacterium linens, Microbacterium gubbeenense, Agrococcus casei), the two consortia contained marine lactic acid bacteria (Alkalibacterium kapii, Marinilactibacillus psychrotolerans) that developed early in ripening (day 14 to 20), shortly after the growth of staphylococci (day 7). A decrease of Listeria counts was observed on cheese surface inoculated at day 7 with 0.1-1 × 102 CFU cm-2, when cheeses were smeared with consortium F or M. Listeria counts went below the detection limit of the method between day 14 and 28 and no subsequent regrowth was detected over 60 to 80 ripening days. In contrast, Listeria grew to high counts (105 CFU cm-2) on cheeses smeared with a defined surface culture.ConclusionsThis work reports the first population dynamics study of complex smear ecosystems exhibiting in situ antilisterial activity. TTGE revealed the presence of marine lactic acid bacteria that are likely related to the strong Listeria inhibition, as their early development in the smear occurred simultaneously with a decrease in Listeria cell count.
Highlights
Surface contamination of smear cheese by Listeria spp. is of major concern for the industry
Profile 5 corresponded to both Brachybacterium sp. and Arthrobacter arilaitensis, profile 12 to Staphylococcus equorum, Staphylococcus epidermidis and Facklamia tabacinasalis, and profile 16 to both Lactococcus lactis and Marinilactibacillus psychrotolerans (Figure 1)
It was used as reference for species-level identification in temperature gradient gel electrophoresis (TTGE) fingerprints obtained by the culture independent approach
Summary
Surface contamination of smear cheese by Listeria spp. is of major concern for the industry. Bacterial biodiversity and population dynamics of complex smear ecosystems exhibiting antilisterial properties in situ were investigated by Temporal temperature gradient gel electrophoresis (TTGE), a culture independent technique, for two microbial consortia isolated from commercial Raclette type cheeses inoculated with defined commercial ripening cultures (F) or produced with an old-young smearing process (M). Yeasts’ deacidification properties create a favorable environment for the populations, mainly staphylococci followed by coryneforms These two shifts in the microbial community structure of the smear have been observed in multiple studies [6,7,8]. Fingerprinting techniques can be used to generate data on main populations of such ecosystems These methods are fast and can give a more exhaustive view of the biodiversity in cheese but they are greatly influenced by the quality of DNA extraction protocols and bias may be introduced by the PCR amplification step [11]. TTGE/DGGE has been applied to study dominant bacteria of dairy products, enabling detection of species accounting for at least 1 to 10% of the total flora, depending on the amplification efficiency of the PCR step for a given species [4,12]
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