Electrobiocorrosion: How bacteria cause iron to rust

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Microbial corrosion of iron is one of the mechanisms whereby structural damage occurs. The anaerobic microbial iron corrosion occurs due to conductive pili creating the process of electrobiocorrosion.

 

Iron not only rusts on contact with oxygen and water. Some bacteria can decompose iron anaerobically. The sediment-dwelling bacterium Geobacter sulfurreducens uses electrically conductive protein threads for this purpose. The bacteria produce magnetite from the iron, which promotes further corrosion in a positive feedback loop. 

 

 

G. sulfurreducens is a rod-shaped microbe with a Gram-negative cell wall. Geobacter is known as a type of bacteria that is able to conduct levels of electricity, and the species G. sulfurreducens is also known as “electricigens” due to their ability to create an electric current and produce electricity.

 

Geobacter does not use atmospheric oxygen for respiration; instead, it draws energy from the transfer of electrons from iron, forming magnetite in the process. However, the mechanism by which Geobacter corrodes iron metal has been uncertain – until now.

 

The mechanism of action of electrobiocorrosion has been pinpointed by researchers based at the Northeastern University in Shenyang, China. This has shown how electrically conductive pili, thin filaments which grow out of the bacteria, play an important role in this mechanism.

 

Geobacter forms "e-pili" from conductive proteins. These e-pili act like electric wires, conducting electricity.

 

The researchers left two strains of Geobacter to grow on a stainless-steel surface until biofilms formed. One of the two strains formed conductive e-pili, while the other still produced pili, but had been genetically modified so that the pili were formed from less conductive proteins.

 

The researchers next observed that the bacterial strain that grew e-pili fared significantly better on the steel plate. This bacterium grew more and made deeper pits in the metal, demonstrating how much metal it was consuming. The researchers also measured a corrosion current, a direct sign of the oxidation of iron.

 

 

It was concluded that the bacteria with the e-pili formed a sort of "electrical connection" to the metal. Bacteria located further away in the biofilm, not in direct contact with the metal, were also able to supply themselves with electrons using e-pili.

 

As magnetite is formed during the corrosion of iron, and this mineral also conducts electricity, the scientists also investigated its influence on microbial corrosion. It was noted that not only did adding magnetite to the biofilm increase the growth of Geobacter, it also led to a stronger corrosion current measured at the surface of the metal.

 

The research has significant corrosion implications and for attempts to improve corrosion protection.

 

See: Yuting Jin, Enze Zhou, Toshiyuki Ueki, et al.  Accelerated Microbial Corrosion by Magnetite and Electrically Conductive Pili through Direct Fe0‐to‐Microbe Electron Transfer. Angewandte Chemie International Edition, 2023; DOI: 10.1002/anie.202309005 

 

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)