After introduction of the mutagenesis plasmids into competent L. plantarum WCFS1, cells were plated on MRS containing 10 μg·ml−1 chloramphenicol. After 48 h, double-crossover deletion mutants were initially selected by colony PCR using primer pairs M plus O and N plus P (named 87 [34]) for ctsR and K plus O and L plus P for hrcA (see Table S2).