Isolation and characterization of antibiotics produced by the nematode symbiont Xenorhabdus szentirmaii
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Each year, it is estimated that 1.7 million healthcare-acquired infections occur in the United States with 70% of the bacteria causing these infections being resistant to at least one antibiotic used to treat them. There are several bacterial pathogens that are so resistant to prescribed antibiotics that no treatments are available. Novel antibiotic compounds are needed to fight these infections, but antibiotic development has yielded few novel drugs in the last decade. Screening natural products to isolate antibiotic compounds is one of the most promising ways to locate compounds with novel structures, mechanisms of action, and drug-like properties. In this work, crude extracts were obtained from culture supernatants of Xenorhabdus szentirmaii using organic solvents and resins. These extracts were prepared at a concentration of 128 mg/ml and screened against a library of pathogens by disk diffusion assay to determine the presence of antibiotic activity. The crude extracts were found to have activity as low as 4 mg/ml, and further bioassay-guided isolation was performed to purify active compounds. Two compounds were purified and their structures elucidated. The first compound, xenofuranone A, a previously described cytotoxin, was found to have antibiotic activity of 8 ?g/ml against the clinical isolate of Clostridium difficile NAP-1. Xenofuranone A had MIC values >128 ?g/ml against all other bacteria assayed. The second compound isolated, 1,5-dihydroxy-9,10-anthraquinone, a fungal pigment commonly known as anthrarufin, was active against all Gram-positive bacteria tested (MICs 0.625-8 ?g/ml), but it only showed activity against two Gram-negative organisms, Neisseria sicca (MIC = 4 ?g/ml) and Acinetobacter calcoaceticus (MIC = 8 ?g/ml). All other Gram-negative MICs were >128 ?g/ml. These findings suggest that xenofuranone A possesses an inhibitory activity specific to C. difficile. Anthrarufin was found to have a wide spectrum antibiotic activity against Gram-positive organisms. The isolation of anthrarufin marks the first isolation of this compound from bacteria.