Microbial Cationic Peptides as a Natural Defense Mechanism against Insect Antimicrobial Peptides.

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