Bacteria of the Genus Xenorhabdus, a Novel Source of Bioactive Compounds

Abstract

The genus Xenorhabdus of the family Enterobacteriaceae, are mutualistically associated with entomopathogenic nematodes of the genus Steinernema. Although most of the associations are species-specific, a specific Xenorhabdus sp. may infect more than one Steinernema sp. During the Xenorhabdus-Steinernema life cycle, insect larvae are infected and killed, while both mutualists produce bioactive compounds. These compounds act synergistically to ensure reproduction and proliferation of the nematodes and bacteria. A single strain of Xenorhabdus may produce a variety of antibacterial and antifungal compounds, some of which are also active against insects, nematodes, protozoa, and cancer cells. Antimicrobial compounds produced by Xenorhabdus spp. have not been researched to the same extent as other soil bacteria and they may hold the answer to novel antibacterial and antifungal compounds. This review summarizes the bioactive secondary metabolites produced by Xenorhabdus spp. and their application in disease control. Gene regulation and increasing the production of a few of these antimicrobial compounds are discussed. Aspects limiting future development of these novel bioactive compounds are also pointed out.

Keywords: Xenorhabdus; antimicrobial peptides; antimicrobial properties; bioactive compounds; secondary metabolites.

FIGURE 1

The Steinernema life cycle. The infective juvenile (IJ) nematodes infect an insect host and recover to the feeding phase (J3). J3 nematodes molt into fourth phase (J4) juveniles, which in turn develop into male and female adults. These adults reproduce and lay eggs. The eggs hatch as first phase juveniles (J1) which feed and molt to second, third, and fourth juvenile phases (J2–J4), and ultimately into adults. After one to three generations, when nutrients are depleted, second phase juveniles develop into IJs (special third phase juveniles). Each of the IJs host Xenorhabdus bacteria in their receptacle. These IJs then leave the cadaver and await a new prey.

FIGURE 2

Xenorhabdus bioactive compounds. Bioactive compounds with unknown structures include the antibacterial xenoprotec, bicornituns C and D, and the two bacteriocins, xenorhabdicin, and xenocin. Compiled from Boemare et al. (1992), Paik et al. (2001), Hwang et al. (2003), El-Hag and El-Sadawy (2008), Gaultieri et al. (2012).