Biofilm

Metal Concentration in the EPS Matrix#

The biofilm EPS matrix (polysaccharides, proteins, eDNA) binds and concentrates metal ions, creating local metal reservoirs partially shielded from host metal restriction. Enterococcus faecium massively upregulates EPS production under cadmium stress, and this EPS sequesters metals in the biofilm matrix Inter-Kingdom Metal Shielding.

Metal concentration creates spatial gradients: periphery cells face host metal restriction while interior cells access matrix-concentrated metals.

Biofilms as Barriers to Host Metal Restriction#

The EPS matrix physically limits diffusion of host metal-sequestering proteins (Calprotectin (S100A8/A9), Lactoferrin) into the biofilm interior. This means biofilm-embedded bacteria can access metals that would be unavailable to planktonic cells in the same environment. The biofilm structure thus represents a collective strategy to overcome Nutritional Immunity (Metal Sequestration).

Staphylococcus aureus#

Urease genes are significantly upregulated in biofilm-embedded cells compared to planktonic cells.^{[1]Role of Nickel in Microbial PathogenesisRobert J. Maier, Stéphane L. Benoit · 2019Open reference 1 ↓} Ammonia and bicarbonate generated by nickel-dependent urease buffer the local biofilm pH, creating a favorable microenvironment for bacterial survival.

Biofilm formation on implanted medical devices depends partly on urease activity, linking Nickel metabolism to device-associated chronic infections.

Proteus mirabilis#

Urease-driven alkalinization causes struvite (MgNH4PO4) and apatite (Ca10(PO4)6CO3) crystal formation within biofilms on urinary catheters.^{[1]Role of Nickel in Microbial PathogenesisRobert J. Maier, Stéphane L. Benoit · 2019Open reference 1 ↓} These crystalline biofilms physically obstruct catheter lumens and provide a mineralized scaffold extremely resistant to antibiotic penetration and host immune clearance.

This is arguably the most dramatic example of a metal-dependent virulence factor (nickel (Ni)-urease) driving biofilm pathology.

Candida-Bacteria Mixed-Kingdom Biofilms#

Candida albicans frequently forms polymicrobial biofilms with bacterial species in oral, vaginal, and wound infections.^{[2]Metal Nanoparticles to Combat Candida albicans Infections: An UpdatePaulo Henrique Fonseca do Carmo, Maira Terra Garcia, Livia Mara Alves Figueiredo-Godoi et al. · 2023Open reference 2 ↓} Mixed-kingdom biofilms are more resistant to antimicrobials than single-species biofilms due to metabolic cooperation and physical architecture.

Metal nanoparticles (silver (Ag), Au, iron (Fe)-oxide, and nickel (Ni)-containing bimetallic NPs) have been investigated as anti-biofilm agents targeting these mixed communities through ROS generation, membrane disruption, and enzyme inactivation.^{[2]Metal Nanoparticles to Combat Candida albicans Infections: An UpdatePaulo Henrique Fonseca do Carmo, Maira Terra Garcia, Livia Mara Alves Figueiredo-Godoi et al. · 2023Open reference 2 ↓}

Metallophore Sharing#

In polymicrobial biofilms, one species' metallophore can supply metals to another—siderophores are "public goods" captured by any cell with the appropriate receptor.

Staphylopine (S. aureus) and pyoverdine (P. aeruginosa) chelate different metals with different efficiencies; co-infection within a biofilm provides a more complete metal acquisition profile than either pathogen alone.

Synergistic Urease Activity#

In mixed Proteus mirabilis and Providencia stuartii catheter biofilms, urease activity is synergistically enhanced beyond what either species produces alone.^{[1]Role of Nickel in Microbial PathogenesisRobert J. Maier, Stéphane L. Benoit · 2019Open reference 1 ↓}

This inter-species metal-enzyme cooperation amplifies virulence in polymicrobial infections.

Clinical Significance#

Device-associated infections: biofilms on catheters, prosthetic joints, and implants are notoriously difficult to treat because antibiotics cannot penetrate the EPS matrix effectively. Chronic wounds: polymicrobial biofilms with metal-concentrating properties resist both host immunity and topical treatments.

Gut biofilms: mucosal biofilms in IBD may shield pathobionts from host metal restriction, contributing to persistent Metal-Driven Inflammation. Treatment approaches: disrupting metal supply to biofilms (metal chelation, blocking metallophore receptors) is a proposed adjunct to conventional antibiotic therapy.