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Dissimilatory Bacteria - Part I: Model Species Shewanella oneidensis -Organism Introduced

Today I came across a really interesting review article with the title “Towards environmental systems biology of Shewanella” that was published in the Nature Reviews journal (August 2008). I became interested in it because the organism has some very interesting properites. Furthermore, the review article shows how interdisciplinary fields such as biology, chemistry, bioinformatics, and engineering can work together to come up with new ideas. So in the next two posts, I will describe what these little organisms are and explore a few unique potential uses.

What is Shewanella spp?

Shewanella spp is a genus of many related microbes that lives in many aquatic and sedimentary ecosystems under aerobic as well as anaerobic conditions where it can use many carbon sources as its food. Although the microbes have been in culture collections since the middle of the 20th century, it was not until 1988 that work on a particular member Shewanella oneidensis MR-1 lead to the recognition for microbes in this genus to transfer electrons to solid a great variety of metal oxides (called reduction) when there is no oxygen (anaerobic conditions). There are a few other bacterial species that can do this, however, S. oneidensis are remarkably able to perform this task to metal oxides outside the cell which is known as dissimilatory.

The versatility of alternate electron acceptors in Shewanella species includes metal oxides (iron, manganese, urianium (IV), chromioum (IV), iodate, technetium, neptunium, plutonium, selenite, telleurite, and vanadate oxides) and some nitroaromatic compounds. The ability to use exotic substances such as uranium or plutonium oxides for example makes S oneidensis especially interesting for helping to cleaning up environments contaminated with radionuclides (a process called bioremediation). The fact that these microbes can be found in so many different environments begs the question.....

How is Shewanella oneidensis able to thrive in so many different environmental conditions?

In order to survive in many different conditions, it is essential that signals from the environment (such as temperature, moisture, availability of oxygen and nutrients) be transmitted to the inside the cell where decisions are made. This is done through specific receptors on the surface of the cell, transmitters of information on the inside of the cell and regulators throughout the cell and inside the nucleus. Usually one receptor can detect a specific type signal only (e.g.: pH, temperature, presence of oxygen etc.).

A high degree of flexibility in living environments suggests that S. oneidensis should have a high number of sensor-transmitter mechanisms. Indeed, through whole-genome sequencing and cross-species comparison, scientists have found that the S. oneidensis MR1 genome has 211 one-component systems, and 47 two-component systems. These one-component and two-component systems are different types of receptors-transmitter systems. One-component systems are receptors that can also transmit a signal inside the cell. Two-component systems have to work together to achieve the same result, but because of the combinatory nature are more flexible. To give some perspective on one particular example, while the garden-variety E. coli has one chemotaxis gene and five chemoreceptors (receptors with a particular shape or motif), S. oneidensis MR-1 has 3 sets of chemotaxis genes and 27 receptors.

A lot of work has been going on since 1988 with this species in trying to figure out how the organism works, and how to apply this to some practical problems. The review article mainly focused more on system-wide comparisons between Shewanella and other bacteria concerning transcriptional regulation, metabolic networks, and makes the point that a combination of in silico experiments and wet lab experiments are currently laying the groundwork for understanding this diverse genus.

Next time, we will explore how Shewanella can be used for practical purposes.

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