Growth and adaptation of microorganisms on the cheese surface

Cet article a été publié une nouvelle fois dans le numéro 362 http://femsle.oxfordjournals.org/content/362/1/1.20#sec-1Microbial communities living on cheese surfaces are composed of various bacteria, yeasts and molds that interact together, thus generating the typical sensory properties of a cheese. Physiological and genomic investigations have revealed important functions involved in the ability of microorganisms to establish themselves at the cheese surface. These functions include the ability to use the cheese's main energy sources, to acquire iron, to tolerate low pH at the beginning of ripening and to adapt to high salt concentrations and moisture levels. Horizontal gene transfer events involved in the adaptation to the cheese habitat have been described, both for bacteria and fungi. In the future, in situ microbial gene expression profiling and identification of genes that contribute to strain fitness by massive sequencing of transposon libraries will help us to better understand how cheese surface communities function

Sulfur metabolism in hemiascomycetes yeast

Sulfur metabolism is a central function of the cell. It has been extensively studied in the model yeast Saccharomyces cerevisiae. A comparative genomic study carried out across the hemiascomycetes clade has shown that S. cerevisiae displayed specificities not shared by the other yeast species. For instance, an O-acetylserine pathway was shown to be present in many yeast species. The complex regulatory pathways seem also to be conserved, with the exception of MET28, whose presence seems to be restricted to S. cerevisiae and related species. In order to explore this pathway in two distant yeast species, Kluyveromyces lactis and Yarrowia lipolytica, transcriptomic and metabolomic studies have been carried out in different conditions of sulfur supply. These high-throughput techniques allowed confirmation of the data of the comparative genomics but also the investigation of new components and new functions linked to sulfur metabolism, for instance, the role of the O-acetylserine pathway in cysteine biogenesis and the role of the aminotransferases in the degradation of methionine were confirmed. The screening of the pools of metabolic intermediates affected by the sulfur supply allowed the identification of new components of the pathway in Y. lipolytica such as taurine and hypotaurine, which seemed to play a role of sulfur storage. These methods also allowed the identification of the set of transporters involved in sulfur metabolism. Eventually, the comparison of these results with the data accumulated in the model S. cerevisiae highlighted the large-scale conservation of this pathway but also the large diversity in the regulated steps inside the pathway

Physiological and biochemical responses of Yarrowia lipolytica to dehydration induced by air-drying and freezing

Organisms that can withstand anhydrobiosis possess the unique ability to temporarily and reversibly suspend their metabolism for the periods when they live in a dehydrated state. However, the mechanisms underlying the cell's ability to tolerate dehydration are far from being fully understood. The objective of this study was to highlight, for the first time, the cellular damage to Yarrowia lipolytica as a result of dehydration induced by drying/rehydration and freezing/thawing. Cellular response was evaluated through cell cultivability determined by plate counts, esterase activity and membrane integrity assessed by flow cytometry, and the biochemical composition of cells as determined by FT-IR spectroscopy. The effects of the harvesting time (in the log or stationary phase) and of the addition of a protective molecule, trehalose, were investigated. All freshly harvested cells exhibited esterase activity and no alteration of membrane integrity. Cells freshly harvested in the stationary phase presented spectral contributions suggesting lower nucleic acid content and thicker cell walls, as well as longer lipid chains than cells harvested in the log phase. Moreover, it was found that drying/rehydration induced cell plasma membrane permeabilization, loss of esterase activity with concomitant protein denaturation, wall damage and oxidation of nucleic acids. Plasma membrane permeabilization and loss of esterase activity could be reduced by harvesting in the stationary phase and/or with trehalose addition. Protein denaturation and wall damage could be reduced by harvesting in the stationary phase. In addition, it was shown that measurements of loss of membrane integrity and preservation of esterase activity were suitable indicators of loss and preservation of cultivability, respectively. Conversely, no clear effect of freezing/thawing could be observed, probably because of the favorable operating conditions applied. These results give insights into Y. lipolytica mechanisms of cellular response to dehydration and provide a basis to better understand its ability to tolerate anhydrobiosis

Overview of a surface-ripened cheese community functioning by meta-omics analyses

Cheese ripening is a complex biochemical process driven by microbial communities composed of both eukaryotes and prokaryotes. Surface-ripened cheeses are widely consumed all over the world and are appreciated for their characteristic flavor. Microbial community composition has been studied for a long time on surface-ripened cheeses, but only limited knowledge has been acquired about its in situ metabolic activities. We applied metagenomic, metatranscriptomic and biochemical analyses to an experimental surface-ripened cheese composed of nine microbial species during four weeks of ripening. By combining all of the data, we were able to obtain an overview of the cheese maturation process and to better understand the metabolic activities of the different community members and their possible interactions. Furthermore, differential expression analysis was used to select a set of biomarker genes, providing a valuable tool that can be used to monitor the cheese-making process