A declining pipeline of clinically useful antibiotics has made it imperative to develop more effective antimicrobial therapies, particularly against difficult-to-treat Gram-negative pathogens. Silver has been used as an antimicrobial since antiquity, yet its mechanism of action remains unclear. Here, we show that silver disrupts multiple bacterial cellular processes, including disulfide bond formation, metabolism and iron homeostasis. These changes lead to increased production of reactive oxygen species (ROS) and increased membrane permeability of Gram-negative bacteria, that can potentiate the activity of a broad range of antibiotics against Gram-negative bacteria in different metabolic states, as well as restore antibiotic susceptibility to a resistant bacterial strain. We show both in vitro and in a mouse model of urinary tract infection that the ability of silver to induce oxidative stress can be harnessed to potentiate antibiotic activity. Additionally, we demonstrate in vitro and in two different mouse models of peritonitis that silver sensitizes Gram-negative bacteria to the Gram-positive specific antibiotic, vancomycin, thereby expanding the antibacterial spectrum of this drug. Finally, we used silver and antibiotic combinations in vitro to eradicate bacterial persister cells, and show both in vitro and in a mouse biofilm infection model, that silver can enhance antibacterial action against biofilms. This work shows that silver can be used to enhance the action of existing antibiotics against Gram-negative bacteria thus strengthening the antibiotic arsenal for fighting bacterial infections.

The mammalian gut ecosystem has significant influence on host physiology1–4, but the mechanisms that sustain this complex environment in the face of different stresses remain obscure. Perturbations to this ecosystem, such as through antibiotic treatment or diet, are currently interpreted at the level of bacterial phylogeny5–7. Less is known about the contributions of the abundant population of phage to this ecological network. Here, we explore the phageome as a potential genetic reservoir for bacterial adaptation by sequencing murine fecal phage populations following antibiotic perturbation. We show that antibiotic treatment leads to the enrichment of phage-encoded genes that confer resistance via disparate mechanisms to the administered drug as well as genes that confer resistance to antibiotics unrelated to the administered drug, and we demonstrate experimentally that phage from treated mice afford aerobically cultured naïve microbiota increased resistance. Systems-wide analyses uncover post-treatment phage-encoded processes related to host colonization and growth adaptation, indicating that the phageome broadly enriches for functionally beneficial genes under stress-related conditions. We also show that antibiotic treatment expands the interactions between phage and bacterial species, leading to a more highly connected phage-bacterial network for gene exchange. Our work implicates the phageome in the emergence of multidrug resistance and indicates that the adaptive capacity of the phageome may represent a community-based mechanism for protecting the gut microflora, preserving its functional robustness during antibiotic stress.

Prolonged antibiotic treatment can lead to detrimental side effects in patients, including ototoxicity, nephrotoxicity, and tendinopathy, yet the mechanisms underlying the effects of antibiotics in mammalian systems remain unclear. It has been suggested that bactericidal antibiotics induce the formation of toxic reactive oxygen species (ROS) in bacteria. We show that clinically relevant doses of bactericidal antibiotics—quinolones, aminoglycosides, and β-lactams—cause mitochondrial dysfunction and ROS overproduction in mammalian cells. We demonstrate that these bactericidal antibiotic–induced effects lead to oxidative damage to DNA, proteins, and membrane lipids. Mice treated with bactericidal antibiotics exhibited elevated oxidative stress markers in the blood, oxidative tissue damage, and up-regulated expression of key genes involved in antioxidant defense mechanisms, which points to the potential physiological relevance of these antibiotic effects. The deleterious effects of bactericidal antibiotics were alleviated in cell culture and in mice by the administration of the antioxidant N-acetyl-L-cysteine or prevented by preferential use of bacteriostatic antibiotics. This work highlights the role of antibiotics in the production of oxidative tissue damage in mammalian cells and presents strategies to mitigate or prevent the resulting damage, with the goal of improving the safety of antibiotic treatment in people.