| Shewanella oneidensis | |
|---|---|
| Scientific classification | |
| Kingdom: | Bacteria |
| Phylum: | Proteobacteria |
| Class: | Gamma Proteobacteria |
| Order: | Alteromonadales |
| Family: | Shewanellaceae |
| Genus: | Shewanella |
| Binomial name | |
| Shewanella oneidensis |
|
Shewanella oneidensis is a bacterium which can reduce poisonous heavy metal and can live in both environments with or without oxygen. This proteobacterium was first isolated from Lake Oneida, NY in 1988, which is where the name is derived from.[1] This species is also sometimes referred to as Shewanella oneidensis MR-1, indicating "metal reducing", a special feature of this particular organism. Shewanella oneidensis is a facultative bacterium, capable of surviving and proliferating in both aerobic and anaerobic conditions. The special interest in S. oneidensis MR-1 revolves around its behavior in anaerobic an environment contaminated by heavy metals such as iron, lead, and perhaps even uranium. Cellular respiration for these bacteria is not restricted to heavy metals though; the bacteria can also target sulfates, nitrates and chromates when grown anaerobically.
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S. oneidensis MR-1 belongs to a class of bacteria known as "Dissimilatory Metal Reducing Bacteria (DMRB)" because of their ability to couple metal reduction with their metabolism. The means of reducing the metals is of particular controversy, as current research using Scanning Electron Microscopy and Transmission Electron Microscopy has revealed abnormal structural protrusions resembling bacterial filaments that are thought to be involved in the metal reduction. This process of producing an external filament is completely absent from conventional bacterial respiration and is the center of many current studies.
The mechanics of this bacterium's resistance and using of heavy metal ions is deeply related to its metabolism pathway web. Putative multidrug efflux transporters, detoxification proteins, extracytoplasmic sigma factors and PAS-domain regulators are shown to have higher expression activity in presence of heavy metal. Cytochrome c class protein SO3300 also have an elevated transcription.[2] For example, when reducing U(VI), special cytochromes such as MtrC and OmcA are used to forming UO2 nanoparticles and associate it with biopolymers.[3]
Co-cultures of Shewanella and Synechococcus have been used to produce long chain hydrocarbons directly from carbon dioxide, water, and sunlight.[4]
Pellicle is a variety of biofilm which is formed between the air and the liquid in which bacteria grow.[5] In a biofilm, bacterial cells interact with each other to protect their community and co-operate metabolically (Microbial communities).[6] In Shewanella oneidensis, pellicle formation is typical and also related to the process of reducing heaving metal; so pellicle formation is extensively researched in this species. Pellicle is usually formed in three steps: cells attaching to the triple surface of culture device, air and liquid, then developing an one-layered biofilm from the initial cells, and subsequently maturing to a complicated three-dimensional structure.[7] In a developed pellicle, there are a number of substances between the cells (extracellular polymeric substances) which help maintain the pellicle matrix. The process of pellicle formation involves a number of significant microbial activities and related substances. For the extracellular polymeric substances, many proteins and other bio-macromolecules are required.
Interestingly, many metal cations are also required in the process. EDTA control and extensive cation presence/absence tests show that Ca(II), Mn(II), Cu(II) and Zn(II) are all essential in this process, probably functioning as a part of a coenzyme or prosthetic group. Mg(II) have partial effect, while Fe(II) and Fe(III) are not only un-needed but even inhibitory to some point. As for the cellular structures, flagella are considered to be contributing to the formation of pellicle. This is easy to understand since the biofilm needs bacterial cells to move in a certain manner, while flagella is the organelle which have locomotive function.[8] However, mutant strains lacking flagella can still form pellicle, only with a much slower progress speed.
As a facultative anaerobe with branching electron transport pathway, Shewanella oneidensis is considered a model organism in microbiology. In 2002, the complete genome sequence was published, it has a 4.9Mb circular chromosome that is predicted to encode 4,758 protein open reading frames. It also has a 161kb plasmid with 173 open reading frames.[9] A re-annotation was made in 2003.[10] The genome is accessible on the Internet, such as on NCBI (refer to external links).[11][12]