Copper in Infectious Disease: Both Sides of the Penny

What the review examines

Copper occupies a paradoxical position in biology. As a redox-active transition metal it is an indispensable cofactor for enzymes of respiration, oxygen handling, and antioxidant defense, yet those same redox properties make free copper acutely toxic, especially to unicellular microbes. This 2021 review by Edward M. Culbertson and Valeria C. Culotta of Johns Hopkins University synthesizes how the vertebrate host turns copper's toxicity into a weapon while still using copper as a nutrient, capturing the metaphor of using 'both sides of the penny.'

The authors situate copper within the broader framework of nutritional immunity, the set of host strategies that manipulate the availability of transition metals such as iron, zinc, manganese, and copper to control invading microbes. Unlike iron and zinc, which the host typically withholds from pathogens (metal starvation), copper is frequently used in the opposite direction, delivered in a concentrated burst to poison the microbe. The review spans bacterial, fungal, and viral infection, noting that copper surfaces have been used as antimicrobials since roughly 400 BCE and that copper inactivates pathogens ranging from Escherichia coli to SARS-CoV-2.

The macrophage copper burst and phagosomal intoxication

The central host mechanism is copper-mediated killing inside the macrophage phagosome. Upon activation by interferon-gamma (IFN-gamma), macrophages upregulate the high-affinity copper importer CTR1 at the plasma membrane, increasing cellular copper uptake, and simultaneously induce the copper-transporting P-type ATPase ATP7A. Activated ATP7A traffics from the Golgi to vesicles that fuse with or lie adjacent to the pathogen-containing phagosome, pumping copper into the compartment where the microbe is trapped.

The consequences are quantitatively striking. In IFN-gamma-stimulated macrophages challenged with Mycobacterium avium, phagosomal copper rises roughly ten-fold, reaching concentrations on the order of 180 micromolar, far above what most microbes can tolerate. Genetic evidence ties this directly to killing: silencing ATP7A allows significantly greater survival of E. coli inside macrophages, and that defect is reversed by supplementing copper, confirming that copper transport, not some indirect effect, drives the bactericidal activity.

Copper intoxication kills through several routes: displacement of metal cofactors in essential microbial enzymes (mismetallation), damage to iron-sulfur clusters, and generation of reactive oxygen species via Fenton-like chemistry. Copper thus complements the oxidative and nitrosative bursts of the phagosome rather than acting alone.

How pathogens fight back: bacterial and fungal copper defenses

Because copper poisoning is such a common host tactic, successful pathogens carry dedicated copper-resistance machinery. In bacteria the frontline defense is a copper-exporting P-type ATPase, CopA, that pumps excess cytoplasmic copper out of the cell; a copper-sensing regulon controls its expression. Loss of CopA renders bacteria such as E. coli and Salmonella hypersensitive to macrophage killing, mirroring the host ATP7A story on the microbial side. The review frames host ATP7A and pathogen CopA as functionally antagonistic transporters facing off across the phagosomal membrane.

Fungal pathogens deploy an analogous but distinct toolkit. In Candida albicans, copper handling is tuned to the infection niche: the fungus expresses the copper-exporting pump Crp1 and copper-binding metallothioneins to resist copper overload early in kidney infection, then shifts toward the copper importer Ctr1 as the local environment becomes copper-limited later in infection. Cryptococcus neoformans similarly relies on metallothioneins to survive the copper-rich lung environment. This dynamic reflects that the host does not simply intoxicate: in some tissues it withholds copper, forcing pathogens to switch between detoxification and scavenging.

Copper as nutrient: the other side of the penny

Copper is not only a poison the host wields; it is also a nutrient the host must supply to its own immune machinery. Copper-dependent enzymes underpin immune function, including the copper-zinc superoxide dismutase (SOD1) that manages oxidative stress and the multicopper ferroxidase ceruloplasmin that circulates copper and links copper status to iron metabolism. Ceruloplasmin behaves as an acute-phase protein, rising during inflammation, which is one reason serum copper increases during many infections.

This dual dependency explains a long-standing clinical observation: copper deficiency impairs host defense. Copper-deficient animals show reduced bactericidal and candidacidal activity of phagocytes and heightened susceptibility to infection, underscoring that both the toxic and the nutritional faces of copper are required for a competent immune response. The host must maintain copper homeostasis (metallostasis) precisely enough to arm its own enzymes while retaining the ability to unleash copper as a localized poison.

Where copper fits the metal-microbiome-disease axis

This review focuses on acute host-pathogen encounters rather than the commensal microbiome, so its direct claims should not be overstated. That said, the same copper-handling systems it describes are central to how metals shape microbial communities more broadly. Nutritional immunity, copper efflux and import pumps, metallothioneins, and copper-driven mismetallation are the very mechanisms through which copper availability selects which microbes thrive, whether in a phagosome or in the gut lumen.

For the metal-microbiome-disease axis, the take-home is mechanistic rather than epidemiological: copper is a strong selective pressure on microbial life, and disturbances in copper handling can tilt the balance between host and microbe. Chronic dietary or environmental copper exposure, copper dysregulation in disease, and antibiotic-style copper effects on commensals are plausible ways this chemistry could reshape microbial ecology, but establishing that a given copper exposure alters the human microbiome and thereby drives disease requires dedicated studies beyond the scope of this pathogen-focused review. The value of this paper to the axis is that it details, with molecular precision, why copper is a lever on the microbiome at all.