Enhanced rhamnolipid production in Burkholderia thailandensis transposon knockout strains deficient in polyhydroxyalkanoate (PHA) synthesis

Microbially produced rhamnolipids have significant commercial potential; however, the main bacterial producer, Pseudomonas aeruginosa, is an opportunistic human pathogen, which limits biotechnological exploitation. The non-pathogenic species Burkholderia thailandensis produces rhamnolipids; however, yield is relatively low. The aim of this study was to determine whether rhamnolipid production could be increased in Burkholderia thailandensis through mutation of genes responsible for the synthesis of the storage material polyhydroxyalkanoate (PHA), thereby increasing cellular resources for the production of rhamnolipids. Potential PHA target genes were identified in B. thailandensis through comparison with known function genes in Pseudomonas aeruginosa. Multiple knockout strains for the phbA, phbB and phbC genes were obtained and their growth characteristics and rhamnolipid and PHA production determined. The wild-type strain and an rhamnolipid (RL)-deficient strain were used as controls. Three knockout strains (ΔphbA1, ΔphbB1 and ΔphbC1) with the best enhancement of rhamnolipid production were selected for detailed study. ΔphbB1 produced the highest level of purified RL (3.78 g l⁻¹) compared to the wild-type strain (1.28 g l⁻¹). In ΔphbB1, the proportion of mono-rhamnolipid was also increased compared to the wild-type strain. The production of PHA was reduced by at least 80% in all three phb mutant strains, although never completely eliminated. These results suggest that, in contrast to Pseudomonas aeruginosa, knockout of the PHA synthesis pathway in Burkholderia thailandensis could be used to increase rhamnolipid production. The evidence of residual PHA production in the phb mutant strains suggests B. thailandensis possesses a secondary unelucidated PHA synthesis pathway.