Structure, activity and evolution of the group I thiolactone peptide quorum-sensing system of Staphylococcus aureus

In Staphylococcus aureus, the agr locus is responsible for controlling viru lence gene expression via quorum sensing. As the blockade of quorum sensing offers a novel strategy for attenuating infection, we sought to gain novel insights into the structure, activity and turnover of the secreted staphyl ococcal autoinducing peptide (AIP) signal molecules. A series of analogues (including the L-alanine and D-amino acid scanned peptides) was synthesized to determine the functionally critical residues within the S. aureus group I AIP. As a consequence, we established that (I) the group I AIP is inacti vated in culture supernatants by the formation of the corresponding methion yl sulphoxide; and (ii) the group I AIP lactam analogue retains the capacit y to activate agr, suggesting that covalent modification of the AgrC recept or is not a necessary prerequisite for agr activation. Although each of the D-amino acid scanned AIP analogues retained activity, replacement of the e ndocyclic amino acid residue (aspartate) located C-terminally to the, centr al cysteine with alanine converted the group I AIP from an activator to a p otent inhibitor. The screening of clinical S. aureus isolates for novel AIP groups revealed a variant that differed from the group I AIP by a single a mino acid residue (aspartate to tyrosine) in the same position defined as c ritical by alanine scanning. Although this AIP inhibits group I S. aureus s trains, the producer strains possess a functional agr locus dependent on th e endogenous peptide and, as such, constitute a fourth S. aureus AIP pherom one group (group IV). The addition of exogenous synthetic AIPs to S. aureus inhibited the production of toxic shock syndrome toxin (TSST-1) and entero toxin C3, confirming the potential of quorum-sensing blockade as a therapeu tic strategy.