(A) Plaque forming and (B) invasion efficiencies of B. pseudomallei K96243 wild-type, 6H4 mutant or 6H4/pME1039 complemented strains in infect HeLa cells. Plaque-forming efficiency was calculated as: number of plaques/bacterial CFU added per well. Percent invasion was determined as: (number of intracellular bacteria post infection/number of CFU added) × 100. B. pseudomallei K96243 wild-type (black bar), 6H4 mutant (white bar) and the 6H4/pME1039 complemented (checked bar) strains were used to infect (C) HeLa and (D) J774A.1 macrophage cells. Intracellular loads of bacteria were enumerated at 4, 6, 8, and 24 h p.i. Asterisks indicate significant differences (P < 0.05, t-test) between wild-type and its derivative strains. Results are presented as standard errors of the means for experiments done in triplicate, with three independent experiments.