Susceptibility of Clinical Isolates of Burkholderia pseudomallei to a Lipid A Biosynthesis Inhibitor

Burkholderia pseudomallei is the causative agent of melioidosis, a serious infection associated with high mortality and relapse. Current antimicrobial therapy using ceftazidime (CAZ) is often ineffective. Inhibitors of LpxC, the enzyme responsible for lipid A biosynthesis, have potential antimicrobial activity against several Gram-negative bacteria in vivo, but their activity against B. pseudomallei is unclear. Herein, we investigated the susceptibility of B. pseudomallei clinical isolates to LpxC-4, an LpxC inhibitor, and LpxC-4 in combination with CAZ. Time-kill assays for bactericidal activity were conducted for B. pseudomallei K96243, revealing growth inhibition and bactericidal effect at LpxC-4 concentrations of 2 μg/mL and 4 μg/mL, respectively. No significant synergistic effect was observed with the combination of LpxC-4 and CAZ. LpxC-4 susceptibility was tested on three groups of clinical isolates:1) CAZ- and trimethoprim-sulfamethoxazole (SXT)-susceptible (N = 71), 2) CAZ-resistant (N = 14), and 3) SXT-resistant (N = 23) isolates, by broth microdilution. The minimum concentration of LpxC-4 required to inhibit the growth of 90% of organisms was 2 μg/mL for all isolates. The median minimum inhibitory concentration of both the CAZ/SXT-susceptible and CAZ-resistant groups was 1 μg/mL (interquartile range [IQR] = 1-2 μg/mL), compared with 2 μg/mL (IQR = 2-4 μg/mL) for the SXT-resistant group. Cell morphology was observed after drug exposure by immunofluorescent staining, and a change from rod-shaped to cell wall-defective spherical cells was observed in surviving bacteria. LpxC-4 is a potent bactericidal agent against B. pseudomallei and warrants further testing as a new antibiotic to treat melioidosis.