An antibacterial pyrazole derivative from Burkholderia glumae, a bacterial pathogen of rice

Abstract Burkholderia glumae , a bacterial pathogen on rice, produced compounds in liquid culture that, in agar diffusion assays, gave strong inhibitory action against Erwinia amylovora , the bacterium responsible for fire blight disease of apple and pear trees. Products were isolated from culture medium by cation exchange and then purified by bioassay-guided chromatographic methods. Two major products were obtained, one of which was not active when fully purified. Each product showed a single ninhydrin-staining spot on TLC and a single HPLC peak. The non-active product was deduced from NMR, MS, and chemical data, to be the tripeptide l -alanyl- l - homo serinyl- l -aspartate. The NMR data for the active product demonstrated that it contained the same tripeptide, but functionalised at the β-carboxyl of the C-terminal aspartate, by a moiety that provided an additional 98 mass units to the parent tripeptide. Various data led to the interpretation that this moiety was a highly unusual oxygenated pyrazole structure, and thus the bioactive product was deduced to be 3-[ l -alanyl- l - homo serinyl- l -aspartyl-β-carboxy]-4-hydroxy-5-oxopyrazole. This compound was found to inhibit the growth of a number of different bacterial species.