📄 chemistry

Synergistic Microbiologically Influenced Corrosion of SLM TC4 by Co-culture of Streptococcus mutans and Veillonella dispar via Riboflavin-Mediated Extracellular Electron Transfer

This study reveals that the synergistic co-culture of *Streptococcus mutans* and *Veillonella dispar* significantly accelerates the microbiologically influenced corrosion of selective laser melted TC4 oral implants through a riboflavin-mediated extracellular electron transfer mechanism, which compromises the passive film and alters cellular responses.

Original authors: yilan wang, kuiwei zheng, zhenyu liu, hongxiang yuan, xiaohua fu, xing zhou

Published 2026-06-30

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CC BY 4.0

Original authors: yilan wang, kuiwei zheng, zhenyu liu, hongxiang yuan, xiaohua fu, xing zhou

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Rusty Implant in a Busy City

Imagine a dental implant (made of a strong metal alloy called TC4) sitting in your mouth. Usually, we think of rust as something that happens to old cars left out in the rain. But in your mouth, "rust" (corrosion) can be caused by tiny living things: bacteria.

This study looked at what happens when two specific types of bacteria, Streptococcus mutans (the "acid-maker") and Veillonella dispar (the "acid-eater"), team up on a dental implant. The researchers wanted to see if these two bacteria working together cause more damage than when they work alone.

The Setup: Building a "Biofilm City"

Think of the bacteria not as individual soldiers, but as a city builder.

The Solo Builders: When S. mutans works alone, it builds a small, messy neighborhood. When V. dispar works alone, it builds a tiny, sparse village.
The Power Couple: When they work together, they build a massive, dense skyscraper complex called a biofilm. This "city" is thicker, stickier, and packed with more residents (cells) and "construction waste" (extracellular polymeric substances, or EPS) than either could build alone.

The Attack: How the Bacteria Eat the Metal

The researchers found that this bacterial "city" attacks the metal implant in two main ways, but one was a surprise.

1. The Acid Trap (The Expected Attack)
Usually, we think bacteria cause rust by spitting out acid (like lemon juice on metal). S. mutans is famous for making acid. However, in this study, the team noticed something strange: the overall environment actually became less acidic when the two bacteria worked together. V. dispar ate the acid produced by S. mutans.

The Takeaway: Acid alone wasn't the main reason the metal was getting eaten. Something else was going on.

2. The Electrical Theft (The Real Culprit)
The study discovered a hidden mechanism called Extracellular Electron Transfer (EET).

The Analogy: Imagine the metal implant is a battery. The bacteria are like thieves who want to steal the electricity (electrons) from that battery to power their own bodies.
The "Wire": To steal the electricity, the bacteria need a wire. The study found that a molecule called Riboflavin (a form of Vitamin B2) acts as this wire. It shuttles electrons from the metal surface directly into the bacteria.
The Result: When the researchers added extra Riboflavin to the mix, the bacteria stole electricity even faster, and the metal corroded much more quickly. This proved that the bacteria were literally "plugging in" to the metal to drain its energy.

The Damage Report

The researchers measured the damage after two weeks and found:

The "Pits": The metal surface developed deep craters (pits). The "Power Couple" (dual-species) created pits that were nearly twice as deep as those made by the solo bacteria.
The Shield Breaker: The metal has a natural invisible shield (a passive film) that protects it from rust. The bacteria managed to chip away at this shield, turning the protective coating into a weaker, damaged version.
The Leak: Because the shield was broken, dangerous metal ions (Titanium, Aluminum, and Vanadium) leaked out of the implant into the surrounding fluid.

Is It Dangerous to Cells?

The researchers tested the "leaked" fluid on human cells in a dish.

The Result: Surprisingly, the fluid didn't kill the cells immediately. In fact, it seemed to make the cells grow a bit faster.
The Warning: Just because the cells didn't die doesn't mean it's safe. The study suggests that the constant release of metal ions and the presence of these aggressive bacteria could cause long-term inflammation or other issues that aren't immediately visible, similar to how a slow leak in a pipe eventually causes structural damage even if the water doesn't flood the house right away.

The Conclusion

This study reveals that when these two specific bacteria team up, they don't just make acid; they form a super-dense city that acts like a corrosion machine. They use a molecular "wire" (Riboflavin) to steal electrons from the dental implant, causing it to rust and break down much faster than we previously thought.

In short: Two bacteria working together are far more dangerous to dental implants than one, not just because of acid, but because they have learned how to "steal electricity" from the metal to fuel their own growth.

Technical Summary: Synergistic Microbiologically Influenced Corrosion of SLM TC4 by Co-culture of Streptococcus mutans and Veillonella dispar via Riboflavin-Mediated Extracellular Electron Transfer

Problem Statement
Dental implants, particularly those fabricated from Ti-6Al-4V (TC4) alloy using Selective Laser Melting (SLM), are susceptible to failure due to peri-implantitis and microbiologically influenced corrosion (MIC). While previous research has largely focused on single-strain biofilms or metabolite-driven corrosion (M-MIC), the oral environment typically hosts complex, synergistic multi-species communities. The specific mechanisms by which co-cultures of oral bacteria, such as the acid-producing Streptococcus mutans and the nitrate-reducing Veillonella dispar, interact to accelerate the corrosion of additively manufactured TC4 implants remain underexplored. Furthermore, the potential role of extracellular electron transfer (EET) in oral MIC has been largely overlooked.

Methodology
The study employed an in vitro model to investigate the corrosion behavior of SLM-processed TC4 specimens under anaerobic conditions at 37°C over 14 days. The experimental design compared three conditions: a sterile control, mono-species biofilms (S. mutans or V. dispar), and a dual-species co-culture.

Biofilm Characterization: Confocal Laser Scanning Microscopy (CLSM) was used to assess biofilm thickness, viability, and the distribution of cells, proteins, and polysaccharides (EPS). Field Emission Scanning Electron Microscopy (FESEM) examined surface morphology and pitting.
Electrochemical Analysis: Open Circuit Potential (OCP), Linear Polarization Resistance (LPR), Electrochemical Impedance Spectroscopy (EIS), and Potentiodynamic polarization tests were conducted to determine corrosion rates, charge transfer resistance ( $R_{ct}$ ), and corrosion current densities ( $i_{corr}$ ).
Mechanistic Probing: To test the hypothesis of EET-mediated corrosion, riboflavin (an electron shuttle) was added to V. dispar cultures. pH levels were monitored to distinguish between acid-induced corrosion and other mechanisms.
Surface and Ion Analysis: X-ray Photoelectron Spectroscopy (XPS) analyzed the composition and depth profile of passive films (Ti, Al, V oxides). Flame Atomic Absorption Spectroscopy (FAAS) quantified the release of Ti, Al, and V ions.
Biocompatibility Assessment: Cytotoxicity was evaluated using L-929 fibroblast cells via CCK-8 assays and cytoskeletal staining to observe cell adhesion and proliferation in the presence of corrosion extracts.

Key Results

Biofilm Synergy: The dual-species biofilm was significantly thicker, denser, and contained more EPS than mono-species biofilms. S. mutans provided adhesion sites and nutrients, while V. dispar consumed metabolic acids, alleviating microenvironmental acidosis and promoting further EPS synthesis by S. mutans.
Accelerated Corrosion: The co-culture induced the most severe corrosion. The corrosion current density ( $i_{corr}$ ) in the dual-species group (40.7 nA·cm⁻²) was approximately four times higher than the sterile control and significantly higher than mono-species groups. The charge transfer resistance ( $R_{ct}$ ) was lowest in the dual-species group, indicating the poorest corrosion resistance.
Pitting and Passive Film Degradation: CLSM revealed that the dual-species biofilm caused the deepest pitting (maximum depth 9.4 µm). XPS analysis showed a significant reduction in the relative content of protective TiO₂ in the passive film under dual-species conditions, with increased detection of lower oxidation states (Ti₂O₃, TiO) and metallic Ti, Al, and V.
EET Mechanism: Contrary to expectations of acid-driven corrosion, the overall pH of the dual-species system increased, ruling out acidification as the primary driver. However, the addition of riboflavin to V. dispar cultures significantly reduced $R_{ct}$ and increased corrosion rates, confirming that V. dispar utilizes riboflavin as an electron shuttle to facilitate Extracellular Electron Transfer (EET-MIC).
Ion Release and Cytotoxicity: The dual-species corrosion solution resulted in the highest release of Ti, Al, and V ions. Despite the increased ion release, cytotoxicity assays (RGR > 75%) indicated that the corrosion extracts remained non-toxic to L-929 cells, though they stimulated cell growth and proliferation more strongly than sterile controls.

Significance and Claims
The authors claim that this study provides new insights into the mechanisms of oral MIC, specifically highlighting the synergistic effect of S. mutans and V. dispar on SLM TC4 implants. The paper establishes that:

Multi-species biofilms create a more aggressive corrosion environment than single strains due to structural complexity and metabolic interdependence.
EET-MIC, mediated by riboflavin and electroactive Veillonella, is a critical mechanism in oral corrosion, distinct from traditional acid-induced corrosion.
The degradation of the passive film (reduction of TiO₂) and subsequent metal ion release are accelerated by these synergistic interactions.
The developed in vitro corrosion-cytotoxicity model offers a framework for evaluating the long-term clinical performance and biocompatibility of medical metals in complex microbial environments.

The study concludes that understanding these synergistic and electroactive mechanisms is essential for developing corrosion-resistant oral materials and strategies for high-risk patients, noting that the potential carcinogenic risks associated with accelerated metal ion release due to microbial activity warrant further investigation.