The Antibacterial Activity of Ga³⁺Is Influenced by Ligand Complexation as Well as the Bacterial Carbon Source

Abstract

Gallium ions have previously been shown to exhibit antibacterial and antibiofilm properties. In this study, we report differential bactericidal activities of two gallium complexes, gallium desferrioxamine B (Ga-DFOB) and gallium citrate (Ga-Cit). Modeling of gallium speciation in growth medium showed that DFOB and citrate both can prevent precipitation of Ga(OH) 3 , but some precipitation can occur above pH 7 with citrate. Despite this, Ga-Cit 90% inhibitory concentrations (IC 90 ) were lower than those of Ga-DFOB for clinical isolates of Pseudomonas aeruginosa and several reference strains of other bacterial species. Treatment with Ga compounds mitigated damage inflicted on murine J774 macrophage-like cells infected with P. aeruginosa PAO1. Again, Ga-Cit showed more potent mitigation than did Ga-DFOB. Ga was also taken up more efficiently by P. aeruginosa in the form of Ga-Cit than in the form of Ga-DFOB. Neither Ga-Cit nor Ga-DFOB was toxic to several human cell lines tested, and no proinflammatory activity was detected in human lung epithelial cells after exposure in vitro. Metabolomic analysis was used to delineate the effects of Ga-Cit on the bacterial cell. Exposure to Ga resulted in lower concentrations of glutamate, a key metabolite for P. aeruginosa, and of many amino acids, indicating that Ga affects various biosynthesis pathways. An altered protein expression profile in the presence of Ga-Cit suggested that some compensatory mechanisms were activated in the bacterium. Furthermore, the antibacterial effect of Ga was shown to vary depending on the carbon source, which has importance in the context of medical applications of gallium.

In Vitro and In Vivo Antimicrobial Activities of Gallium Nitrate against Multidrug-Resistant Acinetobacter baumannii

“…The effect of Ga(III) on A. baumannii growth in M9-DIP was determined for all strains in a microtiter plate assay and expressed as IC 50 and IC 90 after 24 and 48 h growth (see Table S2 in the supplemental material). Ga(III) inhibited A. baumannii growth in a dose-and strain-dependent manner.…”

“…For Fe(III)-Ga(III) competition experiments, the same procedure was performed, except that FeCl 3 was added in a range of 0.4 to 50.0 M. Microtiter plates were incubated for up to 48 h at 37°C and growth (OD 600 ) was measured in a Wallac 1420 Victor3 V multilabel plate reader. For each strain, the Ga(NO 3 ) 3 concentrations that inhibited growth by 50% and 90% (IC 50 and IC 90 , respectively) in M9-DIP and human serum were calculated at 24 and/or 48 h using GraphPad Prism software (version 5.0; GraphPad Software, San Diego, CA). The effect of Fe(III) on Ga(III) activity was expressed as a percentage relative to bacterial growth at 48 h in M9-DIP without Fe(III) and Ga(III).…”

“…However, Ga(III) cannot be physiologically reduced, and when it replaces Fe(III) in redox enzymes, a number of essential biological reactions are inhibited, including those responsible for DNA and protein synthesis and energy production (8). Given the competitive nature of Ga(III)-dependent inhibition of bacterial metabolism, the antibacterial activity of Ga(III) is reversed by Fe(III), influenced by ligand complexation, and strongly reduced in iron-rich media (5,6,18,31,50).…”

Influence of chelation strength and bacterial uptake of gallium salicylidene acylhydrazide on biofilm formation and virulence of Pseudomonas aeruginosa

“…These contrasting effects are possibly connected to the levels of intracellular Fe and the signaling pathways connected to Fe acquisition and metabolism of the bacteria, as a larger uptake of the Fe(III)-antagonist, Ga(III), was observed in presence of Ga-ME0329. Ga has been shown to interfere with Fe metabolism and hence perturb multiple metabolic pathways in P. aeruginosa [25,37]. It may be speculated that Ga-related changes in metabolic status influence expression of T3SS, since the regulation of T3SS has been shown to also be subjected to metabolite control [38].…”

“…Ga uptake by P. aeruginosa from media with Ga-citrate, Ga-ME0163 or Ga-ME0329 was performed as previously described [25]. Briefly, PAO1 was grown to an optical density (at 600 nm, OD600) of 1.1 in Fe-depleted Iso-Sensitest 20%.…”