Nutritional Immunity

Definition and origin

Nutritional immunity is the collection of host mechanisms that restrict a pathogen's access to nutrient metals, or deliberately overload the pathogen with toxic metal, in order to defend against infection. The concept captures a fundamental vulnerability of microbes: transition metals such as iron (Fe), zinc (Zn), and manganese (Mn) are indispensable cofactors for hundreds of enzymes, yet the same metals become lethal in excess.

The term was coined by microbiologist Eugene D. Weinberg in 1975 to describe the host's attempt to withhold growth-essential iron from bacterial, fungal, and protozoan invaders. For decades the idea was largely iron-centric. It was substantially broadened by M. Indriati Hood and Eric P. Skaar in their 2012 Nature Reviews Microbiology review, which reframed nutritional immunity around the full suite of transition metals at the pathogen–host interface and incorporated metal intoxication—poisoning microbes with copper and zinc—as a second, offensive dimension of the same defense.

How it works mechanistically

Metal withholding (sequestration) removes free metal from the extracellular and intracellular niches a pathogen occupies. In serum, transferrin binds iron with extremely high affinity; at mucosal surfaces and in neutrophil granules, lactoferrin does the same; and intracellular ferritin locks iron in storage. The hormone hepcidin governs systemic iron by triggering degradation of the iron exporter ferroportin, lowering circulating iron during inflammation. Within the phagosome of a macrophage, the transporter NRAMP1 (SLC11A1) pumps iron and manganese away from ingested bacteria.

For zinc and manganese, the dominant host effector is calprotectin, an abundant S100A8/S100A9 heterodimer released by neutrophils that can make up a large fraction of neutrophil cytoplasmic protein. Calprotectin chelates Mn and Zn at inflamed and infected sites, and the related protein S100A7 (psoriasin) sequesters zinc on skin surfaces. The iron-withholding response is reinforced by lipocalin-2 (siderocalin/LCN2), which captures bacterial siderophores before they can deliver iron back to the microbe.

Metal intoxication is the mirror image: instead of starving the pathogen, phagocytes deliver a toxic bolus of copper or zinc into the phagosome. Macrophages use the copper-transporting ATPase ATP7A to load copper into the phagolysosome, where it damages microbial iron–sulfur clusters and other targets, while zinc can be pumped in to disrupt manganese-dependent metabolism. Pathogens counter both arms of this pressure with dedicated systems—high-affinity siderophores and metallophores (for example enterobactin, yersiniabactin, and the staphylococcal metallophore staphylopine), import transporters such as ZnuABC (Zn) and MntH/MntABC (Mn), and metal-efflux pumps such as the mycobacterial copper exporter CtpV—all tuned by metal-sensing regulators including Fur, Zur, and MntR.

Concrete examples

The Staphylococcus aureus–calprotectin conflict is a canonical example: calprotectin starves the pathogen of manganese, crippling its manganese-dependent superoxide dismutase and heightening its sensitivity to oxidative killing, while S. aureus deploys the Mn importer MntABC to fight back. Acinetobacter baumannii and Klebsiella pneumoniae are likewise restricted by calprotectin-mediated Zn/Mn limitation during pneumonia.

In the iron arena, Escherichia coli and Salmonella secrete the siderophore enterobactin, which the host neutralizes with lipocalin-2; pathogenic strains evade this by producing salmochelin, a glycosylated 'stealth' siderophore that lipocalin-2 cannot bind. Mycobacterium tuberculosis exemplifies the intoxication axis, exporting copper via CtpV and detoxifying it to survive the copper burst inside macrophages. Fungal pathogens such as Candida albicans and Aspergillus fumigatus face the same metal-limited environment and rely on their own siderophores and high-affinity uptake systems to grow in the host.

Why it matters

Nutritional immunity is a first-line, broadly acting component of innate defense, and its regulators double as clinically important biomarkers and drug targets. Hepcidin-driven iron withholding during chronic inflammation produces the anemia of inflammation (anemia of chronic disease), directly linking this defense to human hematologic disease. Fecal and serum calprotectin are widely used markers of intestinal and systemic inflammation, reflecting the same neutrophil metal-sequestration response.

Because pathogens depend so heavily on metal acquisition, these systems are actively exploited therapeutically. 'Trojan horse' siderophore–antibiotic conjugates, such as the FDA-approved cephalosporin cefiderocol, hijack bacterial iron-uptake machinery to smuggle antibiotics into the cell. More broadly, nutritional immunity sits at the center of the metal–microbiome–disease axis: dietary and host-controlled metal availability shapes which microbes colonize a niche, contributing to colonization resistance in the gut and to the outcome of infection—making it a core concern of microbial metallomics.