Effects of Proteus vulgaris growth on the establishment of a cheese microbial community and on the production of volatile aroma compounds in a model cheese.

The definitive version is available at ww3.interscience.wiley.comInternational audienceAims: To investigate the impact of Proteus vulgaris growth on a multispecies ecosystem and on volatile aroma compound production during cheese ripening. Methods and Results: The microbial community dynamics and the production of volatile aroma compounds of a nine-species cheese ecosystem were compared with or without the presence of P. vulgaris in the initial inoculum. Proteus vulgaris was able to colonize the cheese surface and it was one of the dominant species, representing 37% of total isolates at the end of ripening with counts of 9Æ2 log10 CFU g)1. In the presence of P. vulgaris, counts of Arthrobacter arilaitensis, Brevibacterium aurantiacum and Hafnia alvei significantly decreased. Proteus vulgaris influenced the production of total volatile aroma compounds with branched-chain aldehydes and their corresponding alcohols being most abundant. Conclusions: Proteus vulgaris was able to successfully implant itself in a complex cheese ecosystem and significantly contributed to the organoleptic properties of cheese during ripening. This bacterium also interacted negatively with other bacteria in the ecosystem studied. Significance and Impact of the Study: This is the first time that the impact of a Gram-negative bacteriu

Investigation of Associations of Yarrowia lipolytica, Staphylococcus xylosus, and Lactococcus lactis in Culture as a First Step in Microbial Interaction Analysis▿ †

The interactions that may occur between microorganisms in different ecosystems have not been adequately studied yet. We investigated yeast-bacterium interactions in a synthetic medium using different culture associations involving the yeast Yarrowia lipolytica 1E07 and two bacteria, Staphylococcus xylosus C2a and Lactococcus lactis LD61. The growth and biochemical characteristics of each microorganism in the different culture associations were studied. The expression of genes related to glucose, lactate, and amino acid catabolism was analyzed by reverse transcription followed by quantitative PCR. Our results show that the growth of Y. lipolytica 1E07 is dramatically reduced by the presence of S. xylosus C2a. As a result of a low amino acid concentration in the medium, the expression of Y. lipolytica genes involved in amino acid catabolism was downregulated in the presence of S. xylosus C2a, even when L. lactis was present in the culture. Furthermore, the production of lactate by both bacteria had an impact on the lactate dehydrogenase gene expression of the yeast, which increased up to 30-fold in the three-species culture compared to the Y. lipolytica 1E07 pure culture. S. xylosus C2a growth dramatically decreased in the presence of Y. lipolytica 1E07. The growth of lactic acid bacteria was not affected by the presence of S. xylosus C2a or Y. lipolytica 1E07, although the study of gene expression showed significant variations

The effect of reduced sodium chloride content on the microbiological and biochemical properties of a soft surface-ripened cheese

Many health authorities have targeted salt reduction in food products as a means to reduce dietary sodium intake due to the harmful effects associated with its excessive consumption. In the present work, we evaluated the effect of reducing sodium chloride (NaCl) content on the microbiological and biochemical characteristics of an experimental surface-ripened cheese. A control cheese (1.8% NaCl) and a cheese with a reduced NaCl content (1.3% NaCl) were sampled weekly over a period of 27 d. Reducing NaCl content induced microbial perturbations such as the lesser development of the yeast Debaryomyces hansenii and the greater development of the gram-negative bacterium Hafnia alvei. This was accompanied by changes in proteolytic kinetics and in profiles of volatile aroma compounds and biogenic amine production. Finally, the development of the spoilage microorganism Pseudomonas fragi was significantly higher in the cheese with a reduced salt content

Safety assessment of Gram-negative bacteria associated with traditional French cheeses

International audienceTwenty Gram-negative bacterial (GNB) strains were selected based on the biodiversity previously observed in French traditional cheeses and their safety was assessed considering various safety criteria. For the majority of tested GNB strains, only gastric stress at pH 2 (vs pH 4) resulted in low survival and no regrowth after an additional simulated gastro-intestinal stress. Presence of milk was shown to be rarely protective. The majority of strains was resistant to human serum and had a low level of adherence to Caco-2 cells. When tested for virulence in Galleria mellonella larvae, GNB strains had LD 50 values similar to that of safe controls. However, four strains, Hafnia paralvei 920, Proteus sp. (close to P. hauseri) UCMA 3780, Providencia heimbachae GR4, and Morganella morganii 3A2A were highly toxic to the larvae, which suggests the presence of potential virulent factors in these strains. Noteworthy, to our knowledge, no foodborne intoxication or outbreak has been reported so far for any of the GNB belonging to the genera/species associated with the tested strains. The role of multiple dynamic interactions between cheese microbiota and GIT barriers could be key factors explaining safe consumption of the corresponding cheeses