A puzzling case of antibiotics leading to increased virulence in Burkholderia thailandensis

Burkholderia thailandensis , a nonpathogenic strain, has been used as a model organism to study the mechanism of gene regulation and virulence in pathogenic strains ( B. pseudomallei and B. mallei ). B. pseudomallei and B. mallei are the causative agents of melioidosis and glanders respectively, fatal diseases, which occur in animals as well as humans. These bacteria in general are resistant to a large number of commonly used antibiotics, creating a hurdle in treatment of the diseases. One of the most commonly prescribed medicines for treatment of glanders and melioidosis is co‐trimoxazole, (a cocktail of trimethoprim and sulfamethoxazole). Surprisingly, exposure of Caenorhabditis elegans to B. thailandensis grown on a sub‐lethal dosage of trimethoprim led to an increase in death of C. elegans by ~ 40%. This observed increase in virulence is in part due to an increase in production of malleilactone, as trimethoprim leads to ~50‐fold increase in malA expression and ~10‐fold increase of malR (an activator of malA ). We have previously shown that genes involved in production of various secondary metabolites including malleilactone are under direct/indirect regulation of the global regulator MftR (Major facilitator transport Regulator), but MftR is not a direct target of trimethoprim. ChiP data show that malR is directly regulated by MftR, suggesting that some compound accumulated under the effect of trimethoprim could act as ligand for MftR altering its binding to malR . Our data indicate that under effect of trimethoprim, the purine salvage pathway is disturbed indicated by decrease in purine catabolism (urate level decreases by ~ 0.6‐fold) while xanthine (a ligand for MftR) level increases by ~2‐fold. Allopurinol, an Xdh inhibitor, leads to similar upregulation of malA and malR as trimethoprim, supporting the notion that xanthine accumulation is sufficient to cause upregulation of the virulence genes. Thus, our results indicate that xanthine accumulated under the effect of trimethoprim alters binding of MftR to malR leading to increased expression of malA making B. thailandensis more virulent. Further, Δ mftR strain showed ~ 46% increase in death of C. elegans compared to wild type supporting the theme of MftR‐mediated regulation of the mal gene cluster. Our data shows that sublethal dosage of trimethoprim has unintended consequence of increasing virulence in Burkholderia via interrupting purine salvage pathway.