Iron chelation directed against biofilms as an adjunct to conventional antibiotics

Abstract

TO THE EDITOR: We read with interest the study by MoreauMarquis et al. demonstrating the bactericidal effect of two iron-chelating compounds directed against Pseudomonas aeruginosa when combined with the aminoglycoside antibiotic tobramycin, following their recent demonstration of abnormal cystic fibrosis epithelial cell iron hardling (1, 2). In similar experiments to those of Moreau-Marquis and colleagues, we assessed the effects of the synthetic iron chelator 2,2-dipyridyl (2DP) on minimal inhibitory concentrations (MICs) of ceftazidime and meropenem. We used a broth microdilution technique in medium containing low (1 M) or high (10 M) amounts of iron (FeCl3) and examined growth and biofilm formation under aerobic and anaerobic conditions. The inclusion of anaerobic experiments is critical, as these conditions are likely encountered by P. aeruginosa in the cystic fibrosis (CF) lung environment, and anaerobic growth within biofilms is thought to be a critical factor in persistence (4, 5). Our data demonstrate that the iron requirement for growth and biofilm formation is much higher when bacteria are grown anaerobically compared with aerobically (O’May, unpublished observations). Under aerobic conditions, iron had little effect on MICs, but when P. aeruginosa strains were grown anaerobically in the presence of supplemental iron, the MICs were significantly increased to the point where the strains were considered to be resistant. The antimicrobial resistance of strains to ceftazidime and meropenem was significantly decreased (up to 4-fold) under both aerobic and anaerobic growth conditions by the addition of 2DP at levels that did not inhibit P. aeruginosa growth. Interestingly, under anaerobic conditions, the concentration of 2DP required to achieve the same reduction in MIC was much lower than under aerobic conditions. In experiments using a flow cell biofilm model in which bacteria were grown on glass surfaces and stained for viability (live P. aeruginosa stains green and dead bacteria stain red; BacLight), we have demonstrated that a combination of 2DP and tobramycin results in areas of denuded