Elec­tri­fy­ing Bac­te­ria!

by mem­bers of the In­te­gra­tive Mi­cro­bi­ol­ogy grad­u­ate course (win­ter quar­ter 2008), Uni­ver­sity of Cal­i­for­nia at San Diego/San Diego State Uni­ver­sity*

Many a live wire would be a dead one ex­cept for his con­nec­tions.
—Wil­son Mizner, Play­wright

Imag­ine that there is an alien – on some dis­tant planet – who can reach out with a liv­ing ap­pendage of its own and use byprod­ucts of its own me­tab­o­lism to trans­fer an elec­tric cur­rent out­side of its own body. If you think that is bizarre, look no fur­ther than our planet – and think small. Metal-re­duc­ing bac­te­ria do ex­actly that. If a lit­tle con­vinc­ing was nec­es­sary to think that bac­te­ria are the most ex­cit­ing and ver­sa­tile crea­tures in this world, then their abil­ity to pro­duce an elec­tric cur­rent might just do it.

EM im­age show­ing the pho­to­syn­thetic cyanobac­terium Syne­chocys­tis PCC 6803 mak­ing elec­tric­ity us­ing light (en­ergy source) and wa­ter (elec­tron source) in a mi­crobial fuel cell. Cour­tesy of Yuri Gorby, J .C. Ven­ter In­sti­tute

Elec­tric­ity-pro­duc­ing bac­te­ria ("elec­tri­ci­gens") have the re­mark­able abil­ity to es­tab­lish di­rect con­nec­tions with an in­sol­u­ble ex­tra­cel­lu­lar elec­tron ac­cep­tor (min­er­als) via bac­te­r­ial nanowires. When we speak of nanowire, what is most likely to be in people's mind is the lab-made car­bon nanowire. Now we have nat­ural nanowires made by bac­te­ria, thanks to sev­eral re­cent stud­ies. Just what are bac­te­r­ial nanowires? They are pili, pro­tein ap­pendages that ex­tend from the cells. Un­til this new and un­usual role was re­vealed, bac­te­r­ial pili were some­thing you learned about in your Mi­cro­bi­ol­ogy course or gleaned from a friend's text–book. They were said to be in­volved in con­ju­ga­tion, at­tach­ment, and motil­ity, but who would have thought they could pos­si­bly be ca­pa­ble of con­duc­tiv­ity? Pili can also fa­cil­i­tate long-range elec­tron trans­fer across thick biofilm lay­ers, such as are formed on elec­trodes. In one of the elec­tri­ci­gens stud­ied, She­wanella onei­den­sis, the pili are about 10 nm in di­am­e­ter, form­ing bun­dles as wide as 150 nm that can grow to hun­dred of mi­crons in length.

But it still came as a sur­prise when, in 2005, mem­bers of Derek Lovley's lab dis­cov­ered that the pili of Geobac­ter sulfurre­ducens are elec­tri­cally con­duc­tive. G. sulfurre­ducens pro­duces pili only dur­ing growth on Fe (III) ox­ide, not on sol­u­ble Fe (III), and loses the abil­ity to grow on in­sol­u­ble fer­ric ox­ide when its pilin do­main pro­teins are knocked out.

Since their orig­i­nal dis­cov­ery, nanowires have been dis­cov­ered in many other bac­te­ria, and in some that do not re­duce met­als, such as oxy­genic pho­to­syn­thetic cyanobac­terium with car­bon diox­ide lim­i­ta­tion, and a ther­mophilic fer­ment­ing bac­terium. This is how re­markable bac­te­ria are: they are ca­pa­ble of respir­ing aer­o­bi­cally and anaer­o­bi­cally us­ing a va­ri­ety of com­pounds, in­clud­ing met­als, as ter­mi­nal elec­tron ac­cep­tors.

Elec­tri­cally con­duc­tive nanowires ex­tend many times the length of the bac­terium She­wanella onei­den­sis. Photo by R. Bencheikh and B. Arey, cour­tesy of 'New Sci­en­tist'

And now we can even see how these or­gan­isms as­so­ciate with their en­vi­ron­ment, in­clud­ing metal ox­ides, to carry out the nec­es­sary elec­tron trans­fer. Nanowire pili are es­sen­tial in biofilm forma­tion and are also re­quired for cell ag­gre­ga­tion. This is an in­ter­est­ing idea be­cause form­ing com­munities and ac­cu­mu­lat­ing a high enough cell den­sity al­lows for higher rates of elec­tron trans­fer. Al­low­ing for such an ex­ten­sive net­work of mi­cro­bial nanowires even al­lows for elec­tri­cal trans­fer across long dis­tances. "Imag­ine pack­ing a room full of peo­ple," Dr. Yuri Gorby told our class, "then elim­i­nat­ing all of the oxy­gen in it, so that no one ex­cept the one per­son stand­ing in the door­way could breath out­side air. If these peo­ple in the room were like some bac­te­ria, just by ex­tend­ing their arms out to touch a neigh­bor so that every­one in the room would have a phys­i­cal con­nec­tion to one an­other, the­o­ret­i­cally, no one would suf­fo­cate as long as the one per­son at the door­way could breath."

Re­searchers have har­nessed probe atomic force mi­croscopy and its sis­ter method, scan­ning tun­nel­ing mi­croscopy, to mea­sure elec­tri­cal cur­rents on the 10 nA (nano Amps) scale. The sam­ple is spread over a per­fectly flat graphite sur­face and a small con­duc­tive probe scans the sur­face. When the probe reaches an el­e­vated sec­tion, such as a pilus, it scans that sec­tion for elec­tric ac­tiv­ity, thus mea­sur­ing both the height and con­duc­tiv­ity of the ob­ject it en­coun­ters. A com­puter then sim­u­lates an im­age of the sur­face from the probe's mea­sure­ments. This im­age can be su­per­im­posed with the elec­tri­cal mea­sure­ments, al­low­ing us to "see" elec­tri­cally con­duc­tive pili.

Elec­tron-con­duct­ing nanowires are hot top­ics in mi­cro­bial sci­ence, bring­ing to­gether the fields of physics, chem­istry, and mi­cro­bi­ol­ogy to ex­plain the phe­nom­e­non of elec­tron trans­fer through bac­te­r­ial cell-sur­face pili. But how do these cells "send" their elec­trons through these wires? Much of the mech­a­nism of elec­tron flow re­mains to be elu­ci­dated; how­ever, it is now sug­gested that the c‑type cy­tochromes, key mol­e­cules in the elec­tron trans­port chain, are re­spon­si­ble for trans­fer­ring their elec­tron to the pili; in­deed, mu­tants lack­ing these com­po­nents were only poor con­duc­tors.

With the need for al­ter­na­tive en­ergy sources be­com­ing in­creas­ingly ap­par­ent, it should come as no sur­prise that pos­si­ble mi­crobe-based so­lu­tions have gar­nered their fair share of at­ten­tion. Mi­cro­bial fuel cells are an emerg­ing tech­nol­ogy that will likely make its first ap­pear­ances in re­mote (in­con­ve­nient to main­tain), low-power ap­pli­ca­tions such as en­vi­ron­men­tal sen­sors or small-scale wa­ter treat­ment de­vices. When­ever hu­man be­ings be­lieve that we have in­vented some­thing new, it's just a mat­ter of time be­fore we dis­cover that Na­ture and evo­lu­tion have al­ready in­vented it. Spi­ders weave, ants build high­ways, and now it seems that bac­te­ria have man­aged to net­work them­selves bet­ter than Mi­crosoft.

In the words of one of us: "The bac­te­r­ial world is not done sur­pris­ing us. That is prob­a­bly why I chose mi­cro­bi­ol­ogy to be my field of study..."

*Kjeld Aamodt, Brandi Bai­ley, Mar­tin Chuang, Jen­nifer Cis­son, Tal Danino, Bar­rett De­ris, Shirin Doroudgar, David Er­ick­son, Kim­ber­lee Fis­cher, Britt Fla­herty, Sheng Hui, Anne Lamsa, Liu Hao Yi, Vic­to­ria Leonidova, Tracy Mc­Dole, Oc­tavio Mon­dragon-Palomino, Lau­rel Mon­ti­celli, Il­ham Naili, Su­jaya Nayak, Cha­lon­grat Noree, Paula Pat­ter­son, Rachel Pat­ter­son. Jen­nifer Tamai, Sharon Tori­goe, Bai­ley Zhao. The au­thors con­tributed some­what un­equally to this pa­per, but all con­tributed sub­stan­tially.