Xenorhabdus antimicrobial products: Genetic regulation of biosynthesis and perspectives of application

Antimicrobial production of bacterial symbionts (EPB) of entomopathogenic nematodes (EPN)   is positively controlled by the hfq gene. Deletion of hfq turns off the expression of numerous biosynthetic gene clusters (BCG) encoding for biosynthetic cooperating enzymes responsible for the production of antimicrobial-active end-products. If any of the hfq-controlled BGC operons are reactivated in a hfq  mutant, then the strain will produce only one type of antimicrobial. The so-called “easy Promoter Activated Compound Identification” (easyPACId) approach (Bode et al., 2019) revolutionized the search for EPB-produced drug candidate molecules. We also use this remarkably reproducible technique for drughunting in Xenorhabdus szentirmaii (EMC), and X. budapestensis (EMA) discovered and first characterized in our labs. We recently published that the cell-free conditioned media (CFCM) of the wild-type strains of both species severely inhibit the chytrid fungus Batrachochytrium dendrobatidis (Bd) both in vitro and in vivo. (Bd is the causative agent of the epidemic amphibian disease, chytridiomycosis). The EMC-CFCM proved to efficiently reduce the efficiency of artificial Bd-infection load on juvenile common toads (Bufo bufo) without any harmful side effects. The  CFCM of each of the  hfq-deleted (EMA, and EMC) strains performed  a significantly (P<0.005), but not completely, lower antifungal activity than those of the respective wild-type strain. We reactivated several BGCs one by one in our Δhfq  mutants and tested their antagonistic potentials on different Gram-positive and Gram-negative bacteria, and also eukaryotic pathogens of clinical, veterinary, and plant pathogenic significance.  We also focus on the strength and the cell-specificity of cytotoxic side effects in the organisms to be protected. The aim is to identify drug-candidate bioactive ingredients.