Direct Exchange of Electrons Within Aggregates of an Evolved Syntrophic Coculture of Anaerobic Bacteria

<jats:title>Wired for Life</jats:title> <jats:p> Syntrophic bacteria live on the metabolic by-products of a partner species. The exchange of the by-products accompanies a flow of electrons in the opposite direction that helps some species grow in conditions that would otherwise be unfavorable. In mixed anaerobic cultures of two related <jats:italic>Geobacter</jats:italic> species, <jats:bold> Summers <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1413" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1196526">1413</jats:related-article> ) observed that one species evolved to promote the transfer of electrons directly to the other, in large aggregated cell clusters, without coupling to common anaerobic by-products such as hydrogen or formate. Selection pressures in nine parallel populations all resulted in a point mutation that truncated a protein involved in the production of small hairlike projections involved in intercellular communication—pili—and indirectly increased the expression of a <jats:italic>c</jats:italic> -type multiheme cytochrome responsible for extracellular electron transfer. The evolved aggregates were conductive, suggesting that the direct exchange of electrons between partner species is a possible alternative route to anaerobic syntrophy rather than interspecies hydrogen transfer; indeed, deleting a gene that encodes a hydrogenase involved in hydrogen transfer conferred a growth advantage in the cocultures. </jats:p>