An antimicrobial peptide that targets DNA repair intermediates in vitro inhibits Salmonella growth within murine macrophages.

The hexapeptide WRWYCR was previously identified on the basis of its ability to inhibit bacteriophage lambda integrase-mediated recombination by trapping and preventing resolution of the Holliday junction intermediate. This peptide inhibits several unrelated DNA repair enzymes that bind to and process Holliday junctions and branched DNA substrates. WRWYCR and its d stereoisomer, wrwycr, are bactericidal against both Gram-positive and Gram-negative bacteria, causing the accumulation of DNA breaks, chromosome segregation defects, and the filamentation of cells. DNA repair is a novel target of antibiotics. In the present study, we examined the ability of the peptides to inhibit the growth of Salmonella in mammalian cells. J774A.1 macrophage-like cells and murine peritoneal macrophages were infected with Salmonella enterica serovar Typhimurium and grown in the presence or absence of peptide. We found that peptide wrwycr reduced the number of Salmonella cells recovered after 24 h growth in J774A.1 cells by 100 to 1,000 times, depending on the multiplicity of infection. The peptide also inhibited Salmonella growth in peritoneal macrophages, and although higher doses were required, these were not toxic to the host cells. The apparent lower level of potency of the peptide paralleled the lower level of replication of Salmonella and the lower level of permeation of the peptide in the peritoneal macrophages than in the J774.1 cells. Treatment with peptide wrwycr elicited the SOS response in a significant fraction of the intracellular bacteria, as would be expected if the mechanism of bacterial killing was the same in pure culture and in host cells. These results represent a proof of principle of the antimicrobial activities of compounds that target DNA repair.