Inactivation of bpsl1039-1040 ATP-binding cassette transporter reduces intracellular survival in macrophages, biofilm formation and virulence in the murine model of Burkholderia pseudomallei infection

Burkholderia pseudomallei, a gram-negative intracellular bacillus, is the causative agent of a tropical infectious disease called melioidosis. Bacterial ATP-binding cassette (ABC) transporters import and export a variety of molecules across bacterial cell membranes. At present, their significance in B. pseudomallei pathogenesis is poorly understood. We report here characterization of the BPSL1039-1040 ABC transporter. B. pseudomallei cultured in M9 medium supplemented with nitrate, demonstrated that BPSL1039-1040 is involved in nitrate transport for B. pseudomallei growth under anaerobic, but not aerobic conditions, suggesting that BPSL1039-1040 is functional under reduced oxygen tension. In addition, a nitrate reduction assay supported the function of BPSL1039-1040 as nitrate importer. A bpsl1039-1040 deficient mutant showed reduced biofilm formation as compared with the wild-type strain (P = 0.027) when cultured in LB medium supplemented with nitrate under anaerobic growth conditions. This reduction was not noticeable under aerobic conditions. This suggests that a gradient in oxygen levels could regulate the function of BPSL1039-1040 in B. pseudomallei nitrate metabolism. Furthermore, the B. pseudomallei bpsl1039-1040 mutant had a pronounced effect on plaque formation (P < 0.001), and was defective in intracellular survival in both non-phagocytic (HeLa) and phagocytic (J774A.1 macrophage) cells, suggesting reduced virulence in the mutant strain. The bpsl1039-1040 mutant was found to be attenuated in a BALB/c mouse intranasal infection model. Complementation of the bpsl1039-1040 deficient mutant with the plasmid-borne bpsl1039 gene could restore the phenotypes observed. We propose that the ability to acquire nitrate for survival under anaerobic conditions may, at least in part, be important for intracellular survival and has a contributory role in the pathogenesis of B. pseudomallei.