Electron transport is crucial to the generation of energy in animals and microorganisms, but the methods by which these various life forms transfer electrons during cellular respiration vary greatly. Understanding these mechanisms may enable the use of bacteria in developing new conductive materials. Some bacteria respire through a process called extracellular electron transfer (EET)—a mechanism through which these microorganisms transfer electrons from the inside of the cell to the compounds in the environment. It has been known that some bacteria use proteins with hemes, an iron-containing compound found in the center of a heterocyclic, organic compound called porphyrin, for EET. However, it seems that many bacteria employ a different mechanism for EET, according to a recent study published in Nature.

“Previously studied EET systems were based on heme, but this mechanism is based on flavins,” says Dan Portnoy, senior author of the study and microbiologist at the University of California, Berkeley, noting that one flavin, called riboflavin, or more commonly known as vitamin B2, is abundant in nutrient-rich environments, like the human host. Riboflavin and other flavin-related compounds (i.e., redox-active compounds) serve as conduits for electron transfer.

“This study is the first to identify molecular mechanistic details of how extracellular electron transfer can occur in Gram-positive bacteria,” says Jeffrey Gralnick, a professor at the University of Minnesota who was not involved in this work. Gram-positive bacteria are bacteria encased by a thick cell wall encased within a special sugary protein. “While we did have some indication that Gram-positive bacteria could carry out extracellular electron transfer, the mechanism was not known.”

Portnoy’s research team examined several different bacteria, including the Gram-positive fermentative bacteria called Listeria monocytogenes. L. monocytogenes is a foodborne pathogen. The bacteria often coexist with decaying plant matter without offering or gaining benefit by its residence but exhibit pathogenic activity when they infect mammalian hosts. To address how EET is accomplished in this bacteria, the research team identified mutants that resulted in L. monocytogenes’ exhibiting decreased extracellular iron and electrochemical activity, implicating proteins that use flavins for moving electrons out of the cell.

To determine whether flavins could actually execute electron shuttling activity, the researchers evaluated how exogenous riboflavin in the presence of the flavin byproduct flavin mononucleotide (FMN) and the redox cofactor flavin adenine dinucleotide (FAD) affected EET activity. They found that injecting FMN into an L. monocytogenes-inoculated chamber increased the electric current. Additionally, while they observed high baseline levels of flavin-related nonresponsive activity in cells immersed in soluble ferric iron, the presence of flavins significantly enhanced the concentration-dependent reduction of insoluble ferric oxide.

The researchers wrote that such activity supports their conclusion that they can recruit environmental flavins to “shuttle electrons to outlying acceptors.”

Interestingly, genes for the newly identified proteins are present in many microbial species, including other pathogens, as well as bacteria used for food fermentation and probiotics. The research team confirmed that some of these other bacterial species also possess iron-reduction activity, suggesting that this new EET system is fairly common.

“The major take-home is that while we used to think that EET was, more or less, an activity favored by these exotic mineral-respiring bacteria, we’ve now identified different types of bacteria that possess the activity, but accomplish it in a mechanistically different way,” says Sam Light, a postdoctoral fellow at the University of California, Berkeley and lead author of the study.

Gralnick says that evidence presented in this study is important not only because it helps scientists understand the ubiquity of EET among bacteria but because it sheds new light on the methods used by Gram-positive bacteria for growth and sustenance in oxygen-deficient environments. He says this information lays a foundation for new research that may one day fuel efforts to make novel conductive materials derived from redox-active proteins.

Read the abstract in Nature.