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

Abstract

b Multidrug-resistant Acinetobacter baumannii poses a tremendous challenge to traditional antibiotic therapy. Due to the crucial role of iron in bacterial physiology and pathogenicity, we investigated iron metabolism as a possible target for anti-A. baumannii chemotherapy using gallium as an iron mimetic. Due to chemical similarity, gallium competes with iron for binding to several redox enzymes, thereby interfering with a number of essential biological reactions. We found that Ga(NO 3 ) 3 , the active component of an FDA-approved drug (Ganite), inhibits the growth of a collection of 58 A. baumannii strains in both chemically defined medium and human serum, at concentrations ranging from 2 to 80 M and from 4 to 64 M, respectively. Ga(NO 3 ) 3 delayed the entry of A. baumannii into the exponential phase and drastically reduced bacterial growth rates. Ga(NO 3 ) 3 activity was strongly dependent on iron availability in the culture medium, though the mechanism of growth inhibition was independent of dysregulation of gene expression controlled by the ferric uptake regulator Fur. Ga(NO 3 ) 3 also protected Galleria mellonella larvae from lethal A. baumannii infection, with survival rates of >75%. At therapeutic concentrations for humans (28 M plasma levels), Ga(NO 3 ) 3 inhibited the growth in human serum of 76% of the multidrug-resistant A. baumannii isolates tested by >90%, raising expectations on the therapeutic potential of gallium for the treatment of A. baumannii bloodstream infections. Ga(NO 3 ) 3 also showed strong synergism with colistin, suggesting that a colistin-gallium combination holds promise as a last-resort therapy for infections caused by pan-resistant A. baumannii.

Gallium Nitrate Is Efficacious in Murine Models of Tuberculosis and Inhibits Key Bacterial Fe-Dependent Enzymes

“…We have previously shown that Ga is protective in murine lung infection models with both Pseudomonas aeruginosa (16) and Francisella tularensis (17). Others have reported various forms of Ga to be effective in a murine P. aeruginosa burn infection model (39) and in infections caused by Rhodococcus equi (40) and Acinetobacter baumannii (5).…”

“…A series of studies by our laboratory and others demonstrated that Ga possesses antimicrobial potency against a variety of human pathogens (3,5,6,(16)(17)(18). Disruption of bacterial iron acquisition/utilization has been shown to correlate with the ability of Ga to inhibit M. tuberculosis growth as well as that of other organisms (3,5,6,16,17).…”

“…Our laboratory and others have shown that disruption of iron metabolism inhibits the growth of M. tuberculosis and other bacteria regardless of whether they are growing extracellularly or within human macrophages (4)(5)(6). The ability of pathogens to obtain iron from host sources such as transferrin, lactoferrin, ferritin, and heme or other ironcontaining proteins is considered to be a key virulence factor (4,5).…”

“…Our laboratory and others have shown that disruption of iron metabolism inhibits the growth of M. tuberculosis and other bacteria regardless of whether they are growing extracellularly or within human macrophages (4)(5)(6). The ability of pathogens to obtain iron from host sources such as transferrin, lactoferrin, ferritin, and heme or other ironcontaining proteins is considered to be a key virulence factor (4,5). In animal models, the pathogenicity and virulence of many pathogens, including M. tuberculosis and Mycobacterium avium, are considerably enhanced as iron availability is increased in the host (7)(8)(9).…”

Role of Iron Uptake Systems in Pseudomonas aeruginosa Virulence and Airway Infection

“…Human serum samples (2.5 ml each) were collected from 130 healthy donors following submission, comprehension, and subscription of a written informed consent. Sera were pooled, filtered, and heat inactivated (30 min at 56°C) as previously described (43). Serum chemistry was as follows: total iron, 0.78 g/ml; ferritin, 0.21 g/ml; transferrin, 2.63 mg/ml; and total iron binding capacity, 3.16 g/ml (28% transferrin saturation, equivalent to 45.6 M unsaturated iron binding sites).…”