Taking the good with the bad: when beneficial bacteria do bad things on the side

If you have followed this blog at all, you will know my passion for highlighting the important and beneficial aspects of bacteria. You know, those too-small-to-see factories (or circuses) that we hardly know anything about; recent estimates are between 0.1 and 0.5% of the total bacterial species. But, one thing we do know so far is that most of them are not vicious killers out to destroy everything in their path, thankfully, considering we harbour about 10,000 bacterial species in or on our bodies alone. With the sequencing efforts led by the Joint Genome Institute (JGI), the number of known bacterial genes is ever increasing.

Now consider my horror when I read recently that one of my favorites, Geobacter, was found to harbour a secret (to researchers any way) arsenal to convert elemental mercury to methylmercury. Yes, the same Geobacter I praised in a recent post for its benefits to mankind also can synthesize an extremely toxic form of mercury. In a recent article published in Science, scientists led by a group at Oak Ridge National Laboratory have finally solved a long-standing mystery in the origin of natural methylmercury in the environment. For decades, scientists were unable to find the genetic basis of mercury methylation among bacteria. Apparently, the problem was finding the right type of bacterial protein that could transfer a special type of methyl group to a mercury atom. Proteins that transfer methyl groups(-CH3), called methyltransferases, are common from bacteria to humans. However, only a special type of methyltransferase could satisfy the criteria for transfer to mercury. Researchers also had narrowed down possible pathways (or cirucus rings ) the methyltransferase are found. Using genome sequences of 6 known bacteria that can produce methylmercury and 8 closely related organisms, scientists were able to use bioinformatics to identify a two-gene cluster they predicted were the missing link the mercury methylation. To test their hypothesis, researchers deleted these two genes and tested the modified strain of Desulfovibrio desulfuricans ND132 for the ability to methylate mercury. To their delight, when knocking out these two genes, which they named hgcA and hgcB, the mutant strain could no longer produce methylmercury. By adding these two genes back into the bacterium, mercury methylation was restored thus confirming these two genes are responsible. Once these genes/proteins had been verified within this bacterium, researchers were able to check other sequenced genomes for their presence. Similar proteins were identified in 52 other bacterial species including many beloved Geobacter spp.

So, here we have the metabolically robust Geobacter that are poster-bugs for bioremediation and microbial fuel cells also having a bad habit: methylating mercury.

Related articles

• Micro! Polo!: Discovering the beneficial bacteria needed to clean our messes (mhrussel.wordpress.com)
• MyTH: A new weekly series about one bacterial species. First Post: Escherichia coli (mhrussel.wordpress.com)
• The biggest ring under the little Big Top: The bacterial circus revisited (mhrussel.wordpress.com)
• MyTH: Week 4 bacteria highlight: Geobacter spp. (mhrussel.wordpress.com)
• What is in a genome? (mhrussel.wordpress.com)
• ORNL scientists solve mercury mystery, Science reports (eurekalert.org)
• Animated GIF: Extracellular electron transfer to soluble iron. Example of Geobacter respiration (mhrussel.wordpress.com)
• Another animated GIF: growing iron oxide on Geobacter pili: bacterial nanowires (mhrussel.wordpress.com)
  

  Inner and outer membrane protein components of the Geobacter oxidative metabolism complexes (right 4 protein models on bottom) with MacA (lower left) for metal reduction, periplasmic cytochrome PpcA and the outer membrane proteins, OmcF, OmcS, and OmcZ.

  

  Electrons: yellow
  iron: black
  MacA protein: dark green
  PpcA: blue
  OmcB: black
  other outer membrane cytochromes: orange and light green

  

  A bacterial nanowire. Electrons (yellow) are passed through pili (purple) to OmcS (cyan) for reduction of iron (black).

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