Abstract
Propionibacterium freudenreichii is used as a ripening culture in Swiss cheese manufacture. It grows when cheeses are ripened in a warm room (about 24°C). Cheeses with an acceptable eye formation level are transferred to a cold room (about 4°C), inducing a marked slowdown of propionic fermentation, but P. freudenreichii remains active in the cold. To investigate the P. freudenreichii strategies of adaptation and survival in the cold, we performed the first global gene expression profile for this species. The time-course transcriptomic response of P. freudenreichii CIRM-BIA1T strain was analyzed at five times of incubation, during growth at 30°C then for 9 days at 4°C, under conditions preventing nutrient starvation. Gene expression was also confirmed by RT-qPCR for 28 genes. In addition, proteomic experiments were carried out and the main metabolites were quantified. Microarray analysis revealed that 565 genes (25% of the protein-coding sequences of P. freudenreichii genome) were differentially expressed during transition from 30°C to 4°C (P<0.05 and |fold change|>1). At 4°C, a general slowing down was observed for genes implicated in the cell machinery. On the contrary, P. freudenreichii CIRM-BIA1T strain over-expressed genes involved in lactate, alanine and serine conversion to pyruvate, in gluconeogenesis, and in glycogen synthesis. Interestingly, the expression of different genes involved in the formation of important cheese flavor compounds, remained unchanged at 4°C. This could explain the contribution of P. freudenreichii to cheese ripening even in the cold. In conclusion, P. freudenreichii remains metabolically active at 4°C and induces pathways to maintain its long-term survival.
Highlights
Micro-organisms have a determining contribution to the formation of the typical flavor and texture characteristics of each cheese variety
To better understand how P. freudenreichii CIRM-BIA1T copes with coldinduced stress when cheeses are transferred to the cold room, an omic approach was applied
During Swiss cheese ripening, P. freudenreichii grows in the absence of carbohydrates since all the lactose of milk is converted to lactate by lactic starters within the first day of cheesemaking [5]
Summary
Micro-organisms have a determining contribution to the formation of the typical flavor and texture characteristics of each cheese variety. Studies have focused on the metabolism of non-growing bacteria under the sub-optimal conditions prevailing in cheese. They showed that bacterial metabolism is markedly modified by cheese conditions, like starvation and low temperatures. P. freudenreichii grows during the ripening in the warm room with populations reaching stable levels over 109 colony-forming units (cfu)/g [1]. It converts the lactate produced by lactic acid bacteria into propionate, acetate and CO2.
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