Production of bacteriocins by lactic acid bacteria is in some cases regulated by a quorum sensing mechanism that involves a secreted bacteriocin-like peptide pheromone. In the case of Lactobacillus plantarum C11, this pheromone, the 26-mer plantaricin A (PlnA), has the interesting property of having both bacteriocin and pheromone activities. To gain insight into how PlnA functions as a pheromone and as a bacteriocin, the L- and D-enantiomers of an N-terminally truncated form of PlnA were synthesized (PlnA-22L and PlnA-22D; PlnA-22L has full biological activity). With circular dichroism, it was shown that the two peptides are unstructured in aqueous solution, but they adopt mirror-image amphiphilic helical structures in the presence of trifluoroethanol and membrane-mimicking entities such as micelles of dodecylphosphocholine and negatively charged Ole2GroPGro liposomes, but not in the presence of zwitterionic Ole2GroPCho liposomes. Thus, the negative charge on the membrane is important for structuring of the (positively charged) PlnA peptides. In terms of in vivo antimicrobial activity, PlnA-22L and PlnA-22D behaved almost identically. Likewise, the peptides dissipated the membrane potential and the transmembrane pH gradient in sensitive cells equally effectively. PlnA-22L induced bacteriocin production in L. plantarum C11 (i.e., displayed pheromone activity), the level of induction being clearly dose-dependent. PlnA-22D did not display pheromone activity, but, at high concentrations, was able to inhibit the pheromone activity of PlnA-22L. The results indicate that the antimicrobial activity of PlnA does not require chiral interactions and is mediated through the formation of a strongly amphiphilic alpha-helical structure. In contrast, PlnA's pheromone activity is dependent on a chiral interaction between the amphiphilic helix (PlnA-22L) and a receptor protein. One may speculate that PlnA is an evolutionary intermediate between a true bacteriocin and a pheromone.