Think zinc: exploiting bacterial stress responses to visualize immune-mediated antimicrobial responses against bacterial pathogens (#22)
Macrophages and other innate immune cells employ a suite of antimicrobial responses for direct destruction of invading microorganisms. Detailed knowledge of such pathways may ultimately guide the development of immune-directed anti-infective agents to combat multidrug-resistant bacterial pathogens. Innate immune cells use direct metal ion poisoning as an antimicrobial weapon against several bacterial pathogens, but the molecular mechanisms involved are quite poorly understood. Here we investigated macrophage-mediated zinc toxicity against Gram-negative bacteria, and developed novel tools for visualizing this response. We show that, in primary human macrophages, toll-like receptor signalling initiates the formation of zinc-containing vesicles that are delivered to intracellular non-pathogenic E. coli K-12 strain MG1655. Intramacrophage survival of MG1655 is compromised when genes that defend against zinc stress are deleted. In contrast, inactivation of these genes in EC958, a representative uropathogenic E. coli strain of the globally-disseminated multidrug-resistant ST131 clone, does not affect bacterial survival within macrophages. To further investigate this phenomenon, we developed and validated a series of highly specific bacterial zinc stress reporter systems to visualize this antimicrobial response. With these tools, we show that the majority of EC958 evades the toll-like-receptor-inducible zinc toxicity response to survive within macrophages. In an intraperitoneal challenge model in mice, zinc toxicity was deployed against intracellular MG1655 for effective clearance, whereas the majority of EC958 escaped this response and disseminated to the liver and spleen. In summary, the development of bacterial zinc stress reporter strains has enabled us to effectively track host-directed zinc toxicity in vitro and in vivo. In the future, these tools should enable detailed characterization of the molecular pathways that the host uses to deploy zinc toxicity, as well as mechanisms by which pathogens evade this response.