Nitric Oxide from IFNγ-Primed Macrophages Modulates the Antimicrobial Activity of β-Lactams against the Intracellular Pathogens <i>Burkholderia pseudomallei</i> and Nontyphoidal <i>Salmonella</i>

<div><p>Our investigations show that nonlethal concentrations of nitric oxide (NO) abrogate the antibiotic activity of β-lactam antibiotics against <i>Burkholderia pseudomallei, Escherichia coli</i> and nontyphoidal <i>Salmonella enterica</i> serovar Typhimurium. NO protects <i>B. pseudomallei</i> already exposed to β-lactams, suggesting that this diatomic radical tolerizes bacteria against the antimicrobial activity of this important class of antibiotics. The concentrations of NO that elicit antibiotic tolerance repress consumption of oxygen (O₂), while stimulating hydrogen peroxide (H₂O₂) synthesis. Transposon insertions in genes encoding cytochrome <i>c</i> oxidase-related functions and molybdenum assimilation confer <i>B. pseudomallei</i> a selective advantage against the antimicrobial activity of the β-lactam antibiotic imipenem. Cumulatively, these data support a model by which NO induces antibiotic tolerance through the inhibition of the electron transport chain, rather than by potentiating antioxidant defenses as previously proposed. Accordingly, pharmacological inhibition of terminal oxidases and nitrate reductases tolerizes aerobic and anaerobic bacteria to β-lactams. The degree of NO-induced β-lactam antibiotic tolerance seems to be inversely proportional to the proton motive force (PMF), and thus the dissipation of ΔH⁺ and ΔΨ electrochemical gradients of the PMF prevents β-lactam-mediated killing. According to this model, NO generated by IFNγ-primed macrophages protects intracellular <i>Salmonella</i> against imipenem. On the other hand, sublethal concentrations of imipenem potentiate the killing of <i>B. pseudomallei</i> by NO generated enzymatically from IFNγ-primed macrophages. Our investigations indicate that NO modulates the antimicrobial activity of β-lactam antibiotics.</p>