Resistance of planktonic and biofilm-grown Burkholderia cepacia complex isolates to the transition metal gallium

Objectives: The Burkholderia cepacia complex is a group of pathogens that can cause severe pulmonary infections in cystic fibrosis (CF) patients. The aim of the present study was to investigate the in vitro activity of gallium against planktonic and biofilm-grown B. cepacia complex isolates. Methods: Six B. cepacia complex isolates (belonging to three different species) as well as Pseudomonas aeruginosa PAO1 were included in the present study. MICs of Ga(NO3)(3) for planktonic cells were determined using a broth microdilution method. Biofilms were formed in 96-well microtitre plates, and the fraction of surviving cells following Ga(NO3)(3) treatment was determined using resazurin as a marker for cell viability. The antimicrobial effect of Ga(NO3)(3) was assessed in the presence (50 mu M) and absence of Fe3+. Results: When tested against planktonic cells, the MICs of Ga(NO3)(3) in the absence of Fe3+ were 64 mg/L for all B. cepacia complex strains investigated. However, the addition of 50 mu M Fe3+ in the presence of 64 mg/L Ga(NO3)(3) resulted in increased growth for all B. cepacia complex strains investigated. In sessile cells, resistance to Ga(NO3)(3) and the extent of the protective effect of 50 mu M Fe3+ against Ga(NO3)(3) appear to be strain-dependent: the Burkholderia cenocepacia strains investigated are insensitive to Ga(NO3)(3) in the presence of 50 mu M Fe3+, whereas the presence of Fe3+ has no protective effect for both Burkholderia multivorans strains investigated. Conclusions: As maximal tolerable Ga3+ levels in plasma are estimated to be similar to 200 mu M and considering the high levels of Fe3+ in the lungs of people with CF, our data suggest that the added value of a Ga(NO3)(3) treatment of B. cepacia complex-infected patients may be limited.