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

Abstract

Sir, We thank Lechuga-Ballesteros et al. for their interest in our study and for their comments. We agree that the MIC values of gallium(III) reported in our study are of the same order of magnitude as those previously reported for some other Gram-negative bacteria. Nevertheless, the available data suggest that the MIC values of gallium(III) in an iron-depleted medium for the limited number of Burkholderia cepacia complex strains investigated so far ( 17 mg/L and not 4–18 mg/L as erroneously asserted by Lechuga-Ballesteros et al.) are markedly higher than those for the majority of Pseudomonas aeruginosa isolates recovered from cystic fibrosis (CF) patients [86% (81/94) of all isolates investigated by Kaneko et al. had an MIC 0.7 mg/L]. Whether this difference would hold up if a larger number of B. cepacia complex and P. aeruginosa isolates were investigated under identical conditions (e.g. in terms of growth media) remains to be determined. A second remark concerns our ‘simplistic view’ about the protective effect of Fe3þ. We completely agree with Lechuga-Ballesteros et al. that the in vitro evaluation of the competition by Fe3þ cannot be extrapolated to the in vivo situation, where the free Fe3þ concentration is low. However, the data in Table 1 of our publication clearly indicate that the addition of Fe3þ (final concentration: 50 mM) hardly affects the antimicrobial activity of gallium(III) towards Burkholderia multivorans and Burkholderia dolosa biofilms, while it abolishes this effect for Burkholderia cenocepacia biofilms. Indeed, treating 28 h old B. cenocepacia LMG 16656 or LMG 18828 biofilms with 32 or 64 mg/L Ga(NO3)3 in the presence of 50 mM Fe3þ did not result in meaningful reductions in the number of viable sessile cells (reductions between 0.1% and 10.3%). However, reductions in viable cell numbers in B. multivorans and B. dolosa biofilms in the presence of Fe3þ were hardly different from those in the absence of Fe3þ. While the relevance of these differences for the in vivo situation is at present unclear, they are real and cannot be ignored. In addition, the observation that B. cenocepacia can use ferritin as an iron source suggests that not only free Fe3þ concentrations should be taken into account, but also that iron bound to ferritin (and possibly also to other carriers) may play a role in countering the antimicrobial effect of gallium(III) for at least this B. cepacia complex species. The fact that most B. cepacia complex lineages infecting a large number of patients (including the ET12, PHDC and Midwest clones) belong to B. cenocepacia stresses the potential clinical relevance of these observations. A final remark concerns our conclusion that the added value of Ga(NO3)3 to treat CF patients infected with B. cepacia complex may be limited. We can assure Lechuga-Ballesteros et al. and the readers of JAC that we hope to be proved wrong; however, at this point, there are no published and peer-reviewed data supporting the claims (i) that gallium can be used to treat B. cepacia complex infections in an animal model and (ii) that high doses (400 mM) of gallium can be administered through nebulization without adverse effects. Until data on the safety and efficacy of nebulized Ga(NO3)3 to treat CF lung infections caused by B. cepacia complex organisms become available, it seems wiser to tone down speculations about its potential use to treat these infections in CF patients.