Metabolic activity and technological role of Lactococcus lactis throughout cheese ripening

Fermented dairy products are an important part of traditional diet and the study of microbial ecology associated with dairy fermentations is fundamental to understand the bases of important traits of dairy products. The cheese microbiota play a critical and pivotal role in the development of the unique characteristics of each cheese. Traditionally, methods based on cultivation on selective media followed by phenotypic and/or molecular characterization led to the evidence of the role and activity, in cheese manufacturing and ripening, of starter lactic acid bacteria, producing sufficient lactic acid during cheese manufacture to reduce the pH of milk and the non-starter lactic acid bacteria, which grow later, during cheese ripening, with an impact on flavour development. In the recent years, culture-independent methods, based on protocols where total DNA or RNA is directly extracted from the substrate, are supporting the hypothesis of the presence of species belonging to starter populations during late ripening. In particular, metabolically active populations of L. lactis were found by some authors in various ripened cheeses (Belen-Florez et al., 2006; Rantsiou et al., 2008; Ulve et al., 2008; Dolci et al., 2010). Thus, it should be considered the possibility that starter populations are present in viable but non-culturable (VNC) state during cheese ripening and, for this reason, culture-dependent methods were not able to highlight this insight. Culture-independent methods based on DNA analysis detect, in cheese samples, the presence of microorganisms without distinguishing between viable and dead cells owing to DNA stability. On the contrary, RNA is a better indicator of microbial vitality and activity as it is degraded rapidly upon cell death. In recent years, the analysis of microbial transcriptome by means of both RT-qPCR and microarray technologies and, more recently, the metatrascriptomic approach, represents a rich molecular toolbox for the study of microbial gene expression. RT-qPCR (Ulve et al., 2008; Ndoye et al., 2010; Cretenet et al., 2011) and microarrays (Xie et al., 2004, Taibi et al., 2011) have been applied for the study of L. lactis gene expression in vitro in the first technological stages during model cheese manufacturing. Information on the vitality and the possible technological role of L. lactis throughout cheese ripening is still missing. Thus, the first objective of this project is to assess the vitality of L. lactis during cheese ripening; afterwords, the aim will be to investigate the metabolic activity and the technological role of this microorganism during ripening. Overall, these information will lead to the final objective that is to correlate L. lactis metabolism with the cheese quality. The transfer of this knowledge to dairy industry could lead to the selection of new L. lactis strains with specific metabolic traits and, thus, able to impart the desired characteristics to the final products.