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Listeria monocytogenes  ( Lm ) is an important food-borne human pathogen that is able to strive under a wide range of environmental conditions. Its accessory gene regulator ( agr ) system was shown to impact on biofilm formation and virulence and has been proposed as one of the regulatory mechanisms involved in adaptation to these changing environments. The  Lm agr  operon is homologous to the  Staphylococcus aureus  system, which includes an  agrD -encoded autoinducing peptide that stimulates expression of the  agr  genes via the AgrCA two-component system and is required for regulation of target genes. The aim of the present study was to identify the native autoinducing peptide (AIP) of  Lm  using a luciferase reporter system in wildtype and  agrD  deficient strains, rational design of synthetic peptides and mass spectrometry. Upon deletion of  agrD , luciferase reporter activity driven by the P II  promoter of the  agr  operon was completely abolished and this defect was restored by co-cultivation of the  agrD -negative reporter strain with a producer strain. Based on the sequence and structures of known AIPs of other organisms, a set of potential  Lm  AIPs was designed and tested for P II -activation. This led to the identification of a cyclic pentapeptide that was able to induce P II -driven luciferase reporter activity and restore defective invasion of the  agrD  deletion mutant into Caco-2 cells. Analysis of supernatants of a recombinant  Escherichia coli  strain expressing AgrBD identified a peptide identical in mass and charge to the cyclic pentapeptide. The  Lm agr  system is specific for this pentapeptide since the AIP of  Lactobacillus plantarum , which also is a pentapeptide yet with different amino acid sequence, did not induce P II  activity. In summary, the presented results provide further evidence for the hypothesis that the  agrD  gene of  Lm  encodes a secreted AIP responsible for autoregulation of the  agr  system of  Lm . Additionally, the structure of the native  Lm  AIP was identified.

Identification of the agr Peptide of Listeria monocytogenes