Entomopathogenic bacteria of the genus Xenorhabdus are known to be symbiotically associated with soil dwelling nematodes of the Steinernematidae family. These bacteria are transported by their nematode hosts into the hemocoel of the insect larvae, where they proliferate and produce insecticidal proteins, inhibitors of the insect immune system and antimicrobial molecules. In this study, we describe the discovery of a new family (PAX) of five antimicrobial compounds produced by fermentation of the Xenorhabdus nematophila F1 strain and purified by cation exchange chromatography and reversed phase chromatography. The chemical structure of PAX 3, a lysine-rich cyclolipopetide, was obtained from the analysis of homo and heteronuclear 2D NMR and confirmed by MS-MS experiments. The five members of the PAX family showed significant activity against plants and human fungal pathogens and moderate activity against few bacteria and yeast. No cytotoxicity was observed on CHO or insect cells.

Xenorhabdicin, the phage tail-like bacteriocins of Xenorhabdus nematophilus, and phage head particles, elements produced together after mitomycin induction in X. nematophilus lysogenic strain F1 cultures, were separated by DEAE chromatography, examined by transmission electron microscopy, and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Electrophoresis of xenorhabdicin showed two major subunits of 43 and 20 kDa corresponding to the sheath and the inner core, respectively. At least five other minor subunits of 67, 54, 35, 28, and 16 kDa were also characterized. Electrophoresis of the phage head capsids showed a major 40-kDa subunit and two minor 50-and 34-kDa subunits. Bactericidal activity recorded against closely related bacterial species and spontaneously produced by X. nematophilus resides in the xenorhabdicin particles and is another antimicrobial barrier to save the symbiotic association.

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Induction by mitomycin or high-temperature treatment resulted in the production of bacteriocins and phages in both phases of Xenorhabdus nematophilus A24, indicating lysogeny. Phage DNA purified from X. nematophilus A24 hybridized to several fragments of DraI-digested A24 chromosomal DNA, confirming that the phage genome was incorporated into the bacterial chromosome. Bacteriocins and phages were detected in cultures of most other Xenorhabdus spp. after mitomycin or high-temperature treatment. Xenorhabdus luminescens K80 was not lysed by these treatments, and no phages were seen associated with this strain. However, bacteriocins were detected in limited quantities in all Xenorhabdus cultures, including X. luminescens K80, without any induction. X. nematophilus A24 bacteriocins were antagonistic for other Xenorhabdus species but not for A24 or other strains of X. nematophilus.