Cold adaptation of the cheese ripening bacterium Protectionniste freudenreichii
Résumé
Propionibacterium freudenreichii is known to play a key role in the formation of cheese flavor throughout Swiss-type cheese ripening and especially during cold storage of cheese. At every stage of cheese manufacture, P. freudenreichii has to face several stresses-generating conditions and especially a cold-induced stress when Swiss cheeses are transferred from a warm (24°C) ripening room to a cold (4°C) room. The aim of this study was to investigate the adaptation and survival of P. freudenreichii at cold temperature by means of the first global gene expression profile for this species. The temporal transcriptomic response of P. freudenreichii was analyzed during its growth phase at 30°C and then during further incubation at 4°C for 9 days, always preventing any exhaustion of the main carbon source (lactate). As the cells transitioned from the warm to the cold condition, most of the down-expressed genes were involved in cell division, protein turnover, translation, transcription and DNA replication and repair. During incubation at cold temperature, P. freudenreichii adopted multiple strategies for maintaining its viability. It activated a two-component quorum-sensing system (a two-component system sensor kinase and luxR) that can induce population-wide changes in gene expression. It used polyphosphate supplies as energy sources by activating genes coding for Nudix hydrolases and pyrophosphatases. It accumulated carbon supplies by up-regulating genes of lactate, alanine and serine conversion to pyruvate, of aspartate conversion to fumarate, of gluconeogenesis and of glycogen synthesis. Thus, even if its metabolic activity is slowed down at cold temperature, P. freudenreichii remains active, which could explain its ability to continue the production of aroma compounds in cheese during their ripening at low temperature. Understanding the metabolism of P. freudenreichii during cold storage could constitute a real asset for a better optimization of Swiss cheese ripening process thus decreasing the carbon footprint of Swiss-type cheese manufacture
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