Biofilm

Structured microbial communities encased in a self-produced extracellular polymeric substance (EPS) matrix. Biofilms are the predominant mode of bacterial and fungal growth in chronic infections, device-associated infections, and the gut. From a metallomics perspective, biofilms create distinct metal microenvironments that shield microbes from host nutritional immunity and concentrate metals for microbial use.

Metal Dynamics in Biofilms

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, 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.

Urease and Biofilm Formation

Staphylococcus aureus

  • Urease genes are significantly upregulated in biofilm-embedded cells compared to planktonic cells [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].
  • 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 (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].
  • Mixed-kingdom biofilms are more resistant to antimicrobials than single-species biofilms due to metabolic cooperation and physical architecture.
  • Metal nanoparticles (Ag, Au, Fe-oxide, and Ni-containing bimetallic NPs) have been investigated as anti-biofilm agents targeting these mixed communities through ROS generation, membrane disruption, and enzyme inactivation [2].

Biofilm Metal Cooperation

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].
  • 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 inflammation.
  • Treatment approaches: disrupting metal supply to biofilms (metal chelation, blocking metallophore receptors) is a proposed adjunct to conventional antibiotic therapy.

Key Sources

Connections

References (4)

  1. ★Robert J. Maier, Stéphane L. Benoit (2019). Role of Nickel in Microbial Pathogenesis. Inorganics. doi:10.3390/inorganics7070080
  2. Paulo Henrique Fonseca do Carmo, Maira Terra Garcia, Livia Mara Alves Figueiredo-Godoi et al. (2023). Metal Nanoparticles to Combat Candida albicans Infections: An Update. Microorganisms. doi:10.3390/microorganisms11010138
  3. Akbari MS, Doran KS, Burcham LR (2022). Metal Homeostasis in Pathogenic Streptococci. Microorganisms. doi:10.3390/microorganisms10081501
  4. Karen Pendergrass (2026). Nickel as a Catalytic Driver of Necrotizing Enterocolitis: Dietary Nickel, Microbial Metallomics, and the Activation of Nickel-Dependent Virulence Pathways in the Preterm Gut. Zenodo Preprint. doi:10.5281/zenodo.18200348

Related Articles