Cing Kiim, “Functions of the Diverse Antimicrobial Compounds Produced by Xenorhabdus szentirmaii”
Mentor: Steven Forst, Biological Sciences

Xenorhabdus szentirmaii are symbiotic bacteria that form mutualistic relationships with the insect-killing nematode Steinernema rarum. The nematode enters the intestine of an insect, perforates the midgut wall and moves to the body cavity where it releases X. szentirmaii into the insect blood. Here the bacteria transition to the pathogenic phase and secrete toxins that kill the insect. X. szentirmaii produces a wide range of antibiotics against closely related species and insect gut-derived microbes using non-ribosomal peptide synthetases (NRPS). NRPS enzymes require a phosphopantetheinyl group that is added by a transferase (PPTase) encoded by the ngrA gene. Therefore, a ngrA mutant strain cannot produce antibiotic activity. Twelve different NRPS gene clusters of the X. szentirmaii genome were identified and corresponding mutant strains were constructed previously. To determine specific NRPS gene clusters involved in antibiotic production, we performed antibiotic overlay assays of individual mutant strains against various insect gut-derived microbes. Mutant strains Xsz3, Xsz7, Xsz8, and Xsz9 showed dramatic reduction of antibiotic activity in the overlay assay experiments against Staphylococcus saprophyticus and Aerococcus viridans. We are currently conducting in vitro competition experiments of aforementioned mutants using a defined medium that is based on the nutrient composition of insect hemolymph (Grace’s insect medium). We have shown that wild-type X. szentirmaii suppresses growth of S. saprophyticus while S. saprophyticus outgrows the ngrA mutant strain. Our preliminary findings show that the mutant strains Xsz3 and Xsz8 are outcompeted by S. saprophyticus suggesting that these NRPS gene clusters produce biologically important antibiotic compounds. Co-culturing replicates complex ecological interactions that bacteria encounter in their natural habitats. Antibiotic-resistance is a major threat to global health. Studying antibiotic production in the natural environment will provide information that will help us better understand the vast area of microbial ecology and interspecies competition as well as facilitate the discovery of novel antibiotics.

Synchronous Presentation April 16th at 2:00pm

Phone Conference ID: 566 873 371#