Exploiting the lactococcal cell envelope stress response for proficient dairy starters

Abstract

Lactococcus lactis is widely used as dairy starter for cheese manufacture and is also regarded as an alternative cell factory for the production of multiple food- and non-food related commodities. In both roles, L. lactis is confronted with several biotic and abiotic stresses that often have an impact on the cell wall. This outermost cell structure is crucial for survival and, subsequently, its integrity determines the success of any biotechnological process that relies on bacterial viability. We have hypothesized that by triggering the cell envelope stress response for an extended period, beneficial mutations of technological interest might be selected and fixed. To test this hypothesis, adaptive evolution experiments were carried out with several L. lactis strains exposed to sublethal concentrations of the cell wall active bacteriocin Lcn972. After a stabilization step, evolved clones were phenotypical and genetically characterized. While fitness and dairy traits were hardly affected, different inter- and intra-strain-dependent phenotypes were observed, involving changes in phage susceptibility, surface properties and cross-resistance to cell wall antibiotics. Draft genomes revealed that mutations were preferentially found in genes involved in cell envelope biogenesis, active transport and regulatory functions, unmasking novel protective mechanisms against cell wall damage.