Dynamic analysis of the Lactococcus lactis transcriptome in cheeses made from milk concentrated by ultrafiltration reveals multiple strategies of adaptation to stresses

Abstract

Lactococcus lactis is used extensively for the production of various cheeses. At every stage of cheese fabrication, L. lactis has to face several stress-generating conditions that result from its own modification of the environment as well as externally imposed conditions. We present here the first in situ global gene expression profile of L. lactis in cheeses made from milk concentrated by ultrafiltration (UF-cheeses), a key economical cheese model. The transcriptomic response of L. lactis was analyzed directly in a cheese matrix, starting from as early as 2 h and continuing for 7 days. The growth of L. lactis stopped after 24 h, but metabolic activity was maintained for 7 days. Conservation of its viability relied on an efficient proteolytic activity measured by an increasing, quantified number of free amino acids in the absence of cell lysis. Extensive downregulation of genes under CodY repression was found at day 7. L. lactis developed multiple strategies of adaptation to stressful modifications of the cheese matrix. In particular, expression of genes involved in acidic- and oxidative-stress responses was induced. L. lactis underwent unexpected carbon limitation characterized by an upregulation of genes involved in carbon starvation, principally due to the release of the CcpA control. We report for the first time that in spite of only moderately stressful conditions, lactococci phage is repressed under UF-cheese conditions.

FIG. 1.

Fermentation profiles of L. lactis strain LD61 under UF-cheese-making conditions. The kinetics of L. lactis growth (•), pH decrease (▪), and temperature shift (dotted line) are shown.

FIG. 2.

Gene expression profiles of some phage-related genes evaluated by quantitative RT-PCR during the process of making UF-cheese with L. lactis LD61.