Knockout of three-component regulatory systems reveals that the apparently constitutive plantaricin-production phenotype shown by Lactobacillus plantarum on solid medium is regulated via quorum sensing.

It has been found that many bacteriocins from lactic acid bacteria (LAB) are only produced in broth cultures when specific growth conditions are achieved and a dedicated three-component regulatory system, involved in a quorum sensing (QS) mechanism, is switched on. Surprisingly, bacteriocin production in LAB occurs in an apparently constitutive manner on solid media. This study addresses the question of constitutive versus regulated bacteriocin production on solid media in two different QS-regulated plantaricin-producing strains: Lactobacillus plantarum NC8 and L. plantarum WCFS1. Construction of knockout mutants for their respective regulatory operons revealed that bacteriocin production is controlled through a QS mechanism in both strains, on solid as well as in liquid media. These results could be extensible to other bacteriocins from LAB which are only produced on agar plates and not in broth cultures. Our findings suggest that QS-regulated bacteriocin production in LAB has evolved for competing on solid supports rather than in liquid media. In practice, this could be of major importance in vegetable fermentations, where the solid substrate itself provides an enormous surface where bacteria can attach to and produce biofilms. Therefore, QS-regulated bacteriocinogenic LAB growing in biofilms are under the optimum conditions to produce bacteriocins. Selection of strains to be used as starter cultures for vegetable fermentations should take into account these facts.