Virulence of Burkholderia pseudomallei does not correlate with biofilm formation.

Burkholderia pseudomallei is the causative agent of melioidosis but currently the pathogenesis of the disease is still poorly understood. One of the virulent factors of gram-negative bacteria is the ability to produce biofilm to evade host defense. As B. pseudomallei has also been reported to develop the biofilm [1], in the present study, we therefore, quantified the biofilm formation in 50 strains of B. pseudomallei and compared with 50 strains of its avirulent counterpart Burkholderia thailandensis using a modified microtiter-plate test. The results showed that the quantity of biofilm produced by B. pseudomallei was statistically higher (P< 0.01) than that of B. thailandensis (means and SEs of the corrected OD630 were 2.17+/-0.29 and 0.59+/-0.05, respectively). Transmission electron micrographs of the B. pseudomallei strain with high biofilm formation exhibited microcolonies of bacterial cells surrounded by dense extracellular slime matrix comparing with only trace quantity in the low biofilm-producing strain or the biofilm mutants generated by Tn5-OT182 mutagenesis. However, no correlation could be observed between the biofilm formation and virulence, judging from the LD50 values in BALB/c mice. The data obtained with these naturally occurring Burkholderia species and the biofilm mutants are incompatible with the possibility that the biofilm plays a role in the pathogenesis of B. pseudomallei infection.