Burkholderia thailandensis methylated hydroxy-alkylquinolines: biosynthesis and antimicrobial activity in co-culture experiments

The bacterium Burkholderia thailandensis produces an arsenal of secondary metabolites that have diverse structures and roles in the ecology of this soil-dwelling bacterium. In liquid co-culture experiments, B. thailandensis secretes a previously unstudied antimicrobial that nearly eliminates another soil bacterium, Bacillus subtilis. To identify the antimicrobial, we used a transposon mutagenesis approach. This screen identified antimicrobial-defective mutants with insertions in the hmqA, C and F genes involved in biosynthesis of a family of quinolones called 3-methyl-4-hydroxy-2-alkenylquinolines (HMAQs), which are related to the Pseudomonas aeruginosa 4-hydroxy-2-alkylquinolines (HAQs). Insertions also occurred in a previously uncharacterized gene BTH_II1576. We confirm that BTH_II1576 is involved in generating N-oxide HMAQ (HMAQ-NO) in B. thailandensis and that HMAQ-NOs are sufficient to eliminate B. subtilis in co-cultures. Moreover, synthetic HMAQ-NO is ~50-fold more active than HMAQ. Both the methyl group and the length of the carbon side chain account for high activity of HMAQ-NO against B. subtilis. The results provide new information on the biosynthesis and activities of HMAQs and reveal new insight into how these molecules might be important for the ecology of B. thailandensis.