Quorum-Sensing Regulation of Constitutive Plantaricin by Lactobacillus plantarum Strains under a Model System for Vegetables and Fruits

Abstract

ABSTRACTThis study aimed at investigating the regulatory system of bacteriocin synthesis byLactobacillus plantarumstrains in vegetables and fruits in a model system. Sterile and neutralized cell-free supernatant (CFS) fromL. plantarumstrains grown in MRS broth showedin vitroantimicrobial activities toward various indicator strains. The highest activity was that ofL. plantarumC2. The antimicrobial activity was further assayed on vegetable and fruit agar plates (solid conditions) and in juices (liquid conditions). A regulatory mechanism of bacteriocin synthesis via quorum sensing was hypothesized. The synthesis of antimicrobial compounds seemed to be constitutive under solid conditions of growth on vegetable and fruit agar plates. In contrast, it depended on the size of the inoculum whenL. plantarumC2 was grown in carrot juice. Only the inoculum of ca. 9.0 log CFU ml−1produced detectable activity. The genesplnA,plnEF,plnG, andplnHwere found in allL. plantarumstrains. The genesplnJKandplnNwere detected in only three or four strains. Reverse-phase high-performance liquid chromatography purification and mass spectrometry analysis revealed the presence of a mixture of eight peptides in the most active fraction of the CFS fromL. plantarumC2. Active peptides were encrypted into bacteriocin precursors, such as plantaricins PlnJ/K and PlnH and PlnG, which are involved in the ABC transport system. A real-time PCR assay showed an increase in the expression ofplnJKandplnGduring growth ofL. plantarumC2 in carrot juice.