Contributions of two UDP-glucose dehydrogenases to viability and polymyxin B resistance of Burkholderia cenocepacia

Burkholderia cenocepacia is highly resistant to antimicrobial peptides and we hypothesized that the conversion of UDP-glucose to UDP-glucuronic acid, a reaction catalysed by the enzyme UDP-glucose dehydrogenase (Ugd) would be important for this resistance. The genome of B. cenocepacia contains three predicted ugd genes: ugd(BCAL2946), ugd(BCAM0855) and ugd(BCAM2034), all of which were individually inactivated. Only inactivation of ugd(BCAL2946) resulted in increased sensitivity to polymyxin B and this sensitivity could be overcome when either ugd(BCAL2946) or ugd(BCAM0855) but not ugd(BCAM2034) was expressed from plasmids. The growth of a conditional ugd(BCAL2946) mutant, created in the Deltaugd(BCAM0855) background, was significantly impaired under non-permissive conditions. Growth could be rescued by either ugd(BCAL2946) or ugd(BCAM0855) expressed in trans, but not by ugd(BCAM2034). Biochemical analysis of the purified, recombinant forms of Ugd(BCAL2946) and Ugd(BCAM0855) revealed that they are soluble homodimers with similar in vitro Ugd activity and comparable kinetic constants for their substrates UDP-glucose and NAD(+). Purified Ugd(BCAM2034) showed no in vitro Ugd activity. Real-time PCR analysis showed that the expression of ugd(BCAL2946) was 5.4- and 135-fold greater than that of ugd(BCAM0855) and ugd(BCAM2034), respectively. Together, these data indicate that the combined activity of Ugd(BCAL2946) and Ugd(BCAM0855) is essential for the survival of B. cenocepacia but only the most highly expressed ugd gene, ugd(BCAL2946), is required for polymyxin B resistance.