The use of targeted mRNA expression as biomarker to track growth and activity of Lactobacillus helveticus in broth and in Swiss cheese
Résumé
Introduction: Biomarker is defined as a biological characteristic that is objectively measured and evaluated as an indicator of biological processes.
Purpose: The aim of this work was to monitor Lactobacillus helveticus mRNA expression levels via biomarkers associated to metabolic activity (tuf gene) and stress response (groL gene) during culture in broth and throughout Swiss cheese manufacture.
Methods: Three small scale Swiss cheese were manufactured from microfiltered milk inoculated with Lactobacillus helveticus ITG LH56, Streptococcus thermophilus, Propionibacterium freudenreichii and Lactobacillus paracasei. In parallel, three independent cultures of L. helveticus ITG LH56 were performed in MRS at 37°C. Population was quantified by numeration on MRS agar medium and qPCR. Quantifications of tuf and groL transcripts were expressed as percentage of the total number of targeted transcripts, and considered as indicators of metabolic activity and stress response, respectively.
Results: In pure culture, L. helveticus growth occurred with a percentage of mRNA expression levels associated to tuf gene>70%. In contrast, late-exponential and stationary phases were associated to an increasing expression of groL, which reached a maximum of 30% at the beginning of stationary phase. During cheese manufacture, L. helveticus reached maximal population during the acidification step with groL mRNA expression levels higher than 40% yielding a peak at the end of the cold room storage. Entering cold (10 days at 12°C) and warm storage (7 days at 24°C) was associated with tuf expression of 50% even though culturable bacterial counts were decreasing, suggesting that cells were metabolically active and may thus be involved in the formation of aroma compounds.
Incidence: This study reports how recording of targeted mRNA expression levels along cheese manufacture and ripening process can give information on a metabolic activity of interest, or more globally, on cell physiological state. The use of biomarkers enables accurate quantifications to better understand and improve online industrial fermentation processes