Microbial Cationic Peptides as a Natural Defense Mechanism\nagainst Insect Antimicrobial Peptides

Abstract

Bacteria\nproduce a plethora of specialized metabolites (SM), with\nthe ecological function of most of them not known. A major group of\nSM are peptides derived from nonribosomal peptide synthetases (NRPS).\nIn entomopathogenic bacteria of the genus <i>Xenorhabdus</i>, PAX (peptide-antimicrobial-<i>Xenorhabdus</i>) were described\nas NRPS-derived lipopeptides, which show antimicrobial activities\nagainst bacteria and fungi. We analyzed the production of PAX in <i>Xenorhabdus doucetiae</i> and found the majority bound to the\ncells. We derivatized PAX with fluorophores and show binding to cells\nwhen added externally using super-resolution microscopy. Externally\nadded PAX in <i>X. doucetiae</i> and <i>E. coli</i> as well as inducible PAX production in <i>X. doucetiae</i> showed a protective effect against various antimicrobial peptides\n(AMPs) from insects, where they are used as a defense mechanism against\npathogens. Because AMPs are often positively charged, our results\nsuggest a PAX-induced repulsive force due to positive charge at the\nbacterial cell wall.