On Numeracy, Bacterial Nanowires And Electrical Circuits

Our local library is all abuzz these days over a traveling exhibition that chronicles the life and works of America’s grand statesman, philanthropist and polymath Benjamin Franklin.  The tercentenary exhibition, bearing the title In Search Of A Better World, began in 2008 on a journey that, when finished, will have hit 40 libraries in 31 states across the USA (1).  Two copies of the 1000-square foot series of display panels that have been carefully assembled in a manner that all age groups and interest niches will find engaging are doing the rounds (1).  And the science historian in me is anxious to delve into the facts surrounding Franklin’s outstanding contributions to electricity and optics.  I sometimes wonder how the paladins of science, Franklin included, would react coming back to see the world as it is now- a world that they helped shape through their accomplishments.  Franklin is oft portrayed in childhood fables as the man who dared to fly a kite during a thunderstorm in 1752 as a way of proving that lightning was ‘electrified air’ (2).  Fast-forward 250 years to the present and we find that electricity plays an intimate and exploitable role in key biological processes.   

Last month’s publication of a study describing how electrical impulses could be used to improve numerical acumen in individuals affected by developmental dyscalculia and related disorders for instance was a showpiece case that exemplifies how electricity may impact cognition (3).  Cohen Kadosh and others from the University of Oxford and University College London performed transcranial direct current stimulation (TDCS) on 16 test subjects by applying weak but constant 1mA currents via scalp electrodes to their heads over 20 minute periods.  Tests designed to quantify numerical acumen affirmed the researcher’s expectations:  anodal and cathodal stimulation to the left and right parietal lobes of the brain respectively, produced marked improvements in numeracy tasks (the right parietal lobe is heavily implicated in numerical abilities) (3).  Importantly these improvements occurred irrespective of the subjects’ previous exposure to “critical educational material or culture-specific devices” (rulers, graphs, multiplication tables and the like) (3).  Remarkably the effects of electrode stimulation were visible as far out as six months beyond treatment (3).  The non-invasive nature of TDCS is its most attractive clinical feature.  In the words of the authors: “no [other] pharmacological interventions have been found that could target numerical cognition directly without holding substantial side effects for other domains such as attention” (3).

At more or less the same time that Franklin was tinkering with his kite, physicians were busy-bodying away on defining the role that electricity could play in public health.  One French opportunist at the time reportedly received twenty patients a day, ever trusting of his claim that he could heal limb paralysis through electrical stimulation (4).  Other miracle ‘curists’ capitalized on the ensuing interest over an ever-growing list of ailments- gout, rheumatism, chilblains, diarrhea, deafness and venereal disease- that they alleged could be readily tamed by administering electrical impulses to believably relevant parts of the human anatomy (4).  None of these carried the experimental rigor shown by Italian anatomist Luigi Galvani whose work in the 1780s and 90s with twitching frogs’ legs and iron hooks galvanized the idea that living things could generate their own electricity (4).   Frogs legs began to display “spontaneous, irregular and frequent movements” in the absence of any external electrical source whenever they were linked by iron rods to the spinal cord.  At its crux, life itself appeared to be electric.  This was indisputably Galvani’s most exciting realization (4), perhaps on par in its personal significance to Einstein’s “gluecklichste Gedanke” during his formulation of general relativity over a century later.

The implications of Galvani’s work were not immediately embraced by his contemporaries.  Like many a good science story others, notably Italian scientist Alessandro Volta, were leery of the stand-on-your-head verdict he adopted (4).  But of course we now know that electricity is “fundamental to biology”.  In many animals neurons fire rapid electrical signals (sometimes reaching speeds of up to tens of meters per second) along extensions called axons (4).  Small channels spanning the axon cell membrane open and allow an “inrush of sodium ions” in response to electrical signals from neurons upstream.  Work on squids and electric eels has given us many of the foundational insights into how this happens (4).  But most surprising of all for old school biologists is the recent discovery that bacteria can also conduct electricity by forming minute ‘conductive filamentous extracellular appendages’ (or nanowires) that extend well beyond the typical length of a single bacterium (5).  

Avant-garde experiments performed by biologists from the University of South California (USC), the Craig Venter Institute and the University of Western Ontario have laid the groundwork for understanding how bacterial nanowires transport electrons (5).  Depositing and dehydrating bacterial samples onto silicon substrates is not a job for the faint-hearted.  But USC’s Mohamed El-Naggar and his collaborators did just that.  And what they observed after flicking on the circuit switch was nothing short of remarkable- an electrical current, equivalent to an electron transport rate of 109 electrons per second, traveling along single nanowires (5).  Moreover bacterial mutants lacking functional genes for c-type cytochromes, long thought to be associated with nanowire conductivity, produced non-conductive appendages (5).  The choice of bacterial strain in this study was pivotal.  Shewanella onidensis MR-1 is known amongst microbiologists to be one of several in a class of bacteria that can readily generate energy using metal oxides as electron acceptors (6).  And the downstream applications proffered by the authors and others in light of these findings are as remunerative and beneficial to humanity as they come: (i) improved efficiency of microbial fuel cells for the production of electricity, (ii) disruption of pathogenic bacterial biofilms and (iii) bioremediation of environments contaminated with toxic heavy metals through the exploitation of Shewanella’s ability to reduce heavy metals (6). 

In a letter written late in life, Benjamin Franklin conveyed his excitement over the progress of science to New England church minister and emigrant John Lothropp.  He wrote:

“I have sometimes almost wished it had been my Destiny to be born two or three Centuries hence. For Inventions of Improvement are prolific, and beget more of their Kind. The present Progress is rapid. Many of great Importance, now unthought of, will before that Period be procur’d; and then I might not only enjoy their Advantages, but have my Curiosity satisfy’d in knowing what they are to be” (1). 

From the vantage point of ‘three centuries hence’ it is safe to say that the ‘volted architecture’ of biology is one that continues to astound.  And the latest ventures on numeracy and bacterial nanowires using electricity constitute a tiny part of a broader realization of Franklin’s vision for a better world.       

Further Reading

  1. The International Traveling Exhibition: Benjamin Franklin, In Search Of A Better World, part of the Franklin tercentenary celebrations, see http://www.benfranklin300.org/exhibit.htm
  2. ‘ Electrified Ben’ in Resources For Science Learning, The Franklin Institute, See http://fi.edu/franklin/scientst/electric.html
  3. Roi Cohen Kadosh, Sonja Soskic, Teresa Iuculano, Ryota Kanai, Vincent Walsh (2010) Modulating Neuronal Activity Produces Specific and Long-Lasting Changes in Numerical Competence, Current Biology, Volume 20, November 23rd 2010, doi:10.1016/j.cub.2010.10.007
  4. Len Fisher (2004) Weighing The Soul: The Evolution Of Scientific Beliefs, Weidenfeld & Nicholson, London, pp. 93-105
  5. Mohamed Y. El-Naggar,  Greg Wanger, Kar Man Leung, Thomas D. Yuzvinsky, Gordon Southam, Jun Yang, Woon Ming Lau, Kenneth H. NealsonYuri A. Gorby (2010)  Electrical transport along bacterial nanowires from Shewanella oneidensis MR-1, PNAS, Volume 107, October 19th 2010, pp. 18127-18131
  6. Jeff Akst (2010) Bacteria form electric circuits?, The Scientist, See http://www.the-scientist.com/news/display/57741/, Published 11th October, 2010
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