Does the Future of Antibiotics Lie in Secondary Metabolites Produced by Xenorhabdus spp.? A Review
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Does the Future of Antibiotics Lie in Secondary Metabolites Produced by Xenorhabdus spp.? A Review E. Booysen 1 & L. M. T. Dicks 1
# Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The over-prescription of antibiotics for treatment of infections is primarily to blame for the increase in bacterial resistance. Added to the problem is the slow rate at which novel antibiotics are discovered and the many processes that need to be followed to classify antimicrobials safe for medical use. Xenorhabdus spp. of the family Enterobacteriaceae, mutualistically associated with entomopathogenic nematodes of the genus Steinernema, produce a variety of antibacterial peptides, including bacteriocins, depsipeptides, xenocoumacins and PAX (peptide antimicrobial-Xenorhabdus) peptides, plus additional secondary metabolites with antibacterial and antifungal activity. The secondary metabolites of some strains are active against protozoa and a few have anti-carcinogenic properties. It is thus not surprising that nematodes invaded by a single strain of a Xenorhabdus species are not infected by other microorganisms. In this review, the antimicrobial compounds produced by Xenorhabdus spp. are listed and the gene clusters involved in synthesis of these secondary metabolites are discussed. We also review growth conditions required for increased production of antimicrobial compounds. Keywords Xenorhabdus . Secondary metabolites . Antimicrobial peptides . Infection control
Introduction Bacteria live in confined spaces and have to compete for nutrients, electron acceptors, adhesion to substrates and physical space. In nutrient-rich niches that support the growth of millions of species, competition becomes fierce and calls for the developing of unique survival strategies. These may include the production of secondary metabolites such as organic acids, fatty acids, hydrogen peroxide, ethanol and carbon dioxide, or the secretion of antibiotics, toxins and antimicrobial peptides. Antagonistic compounds of low molecular weight diffuse through biofilms, destroy communities and create zones inhospitable to competitors [1]. Since resources and growth conditions in microenvironments often change rapidly, microbial communities have to adapt metabolically and structurally to survive [2]. Not all species adapt equally well to changes and communities may consist of individual micro communities, each with unique characteristics and growth requirements. Perhaps the most
* L. M. T. Dicks [email protected] 1
Department of Microbiology, Stellenbosch University, Stellenbosch 7600, South Africa
evident form of community changes and cell re-orientation is observed among aerobic bacteria. Obligate aerobes such as Pseudomonas fluorescens compete for oxygen-rich areas by increasing the production of extracellular polysaccharides (EPS) to “force” cells to the surface [3]. In the case of Bacillus subtilis, EPS-producing cells undergo controlled cell death, which decreases lateral pressure between cells in the micro community. The lowering in
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