Comparative transcriptome analysis of Lactococcus lactis subsp. cremoris strains under conditions simulating Cheddar...

Gene expression in response to technological variations can influence fermentation and flavor generation in Cheddar cheese, and can vary from one lactococcal strain to another, perceived as differences in starter performance. The aim of this study was to determine the influence of cheese cooking temperature at 38 °C and salting on the transcriptional profiles of four closely related strains of L. lactis subsp. cremoris under simulated conditions of Cheddar cheese manufacture. Two responses could be distinguished, a core gene expression, corresponding to the common response of all strains and strain-specific response during the Cheddar simulating process. For the core gene expression after heating of inoculated milk at 38 °C, two groups of differentially expressed genes were identified: i) stress response and ii) carbohydrate and amino acid metabolism. The response to combined stresses of heat, acid and salt resulted in: i) general decrease of functions linked to cell division and metabolism, ii) specific responses related to stress such as the induction of genes coding for chaperones and proteases and iii) expression of prophage lytic systems for certain strains. Strain-specific responses were mainly observed in three of the four tested strains. These responses were the induction of genes related to osmotic stress or the release of CodY repression leading to the activation of oligopeptide transporters as well as the bcaT gene, related to amino acid degradation for the production of flavor. Comparing transcriptomes provides a core expression profile that contributes to understanding gene expression responses to environmental variations. The strain-specific responses identify predictive markers for the transcriptional state of starter strains before they enter the cheese ripening phase.