Bacterial Compasses

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I’m happy and proud to tell you that Lab Rat was kind enough to write today’s blogpost. She brings you a fascinating story about little magnetic particles found in some bacteria, that may help them find their way like compasses do. Normally she writes great posts on bacteria on her own blog, which recently moved to FoS. Her fascination and love for microbes really shine through in her writing, so go check it out !

Magnetosomes are little membrane bound particles which are found in certain bacteria. These particles contain magnetite crystals which, as well as making them sound like very small X-men, allow the bacteria to line up in the direction of the earth’s geomagnetic field, essentially acting like a compass. Why this is helpfully is not entirely certain, but it may give help to give the bacteria a sense of direction while searching for a suitable environment in which to live.

These particles tend to line up on one side of the bacteria to form a long chain of individually membrane wrapped particles, as shown in the figure below:

One of the potentially most interesting things about these magnetic particles is that they are held in organelle type structures, surrounded by a membrane. Organelles are little sub-cellular structures that are usually found in eukaryotes, ( i.e the nucleus, mitochondria, Golgi apparatus etc.) Finding these inside bacteria is interesting, as it shows that bacteria can contain organelle-type structures within them, giving them intracellular organisation despite their small size.

To take a look at which genes might be controlling this very precise arrangement of the magnetosomes (which is essential for their magnetic behaviour) mutations were made of genes involved in the actin cytoskeleton; a network of fibres which runs right through the bacterial cell and helps to hold everything in place. This system of taking out a gene to see what stops working when it’s not there is a common one in microbiology and seems to be one of the main techniques for working with yeast. The figure below shows firstly the wild type cell (A) with the magnetosomes in yellow and secondly (B) the bacteria with the mamK gene deleted. Without mamK the magnetosomes loose their organised positions and can be found all over the place. Figure C shows the same cell as in B, but with the mamK gene re-introduced on a small plasmid (circular loop of DNA) which has been artificially brought into the bacteria.

The MamK gene codes for a protein which is a bacterial homologue of the fibrous eukaryotic protein actin called MreB. MreB forms filaments and is involved in other cellular processes such as determining cell shape, cell polarity (i.e which end of the cell is the front) and chromosome segregation during mitosis. Although these results make it tempting to suggest that these fibres line up the magnetosomes it may be more complicated (although equally likely it may not). The mutational analysis only shows that these fibres are vital for magnetosome organisation, that may be because their helping to order some other system (maybe of fibres maybe of membranes) that holds the magnetosomes in place. This method of using actin-like filaments to hold these organelles in place is very similar to the way eukaryotic organelles are held in place, by filamentous actin and myosin proteins.

The reference paper doesn’t mention it, but as it’s getting close to iGEM season I can’t help but wonder what implications these little magnetic bacteria could have in synthetic biology. A bacterial compass would be fun to produce, but is never going to be cheaper or more accurate than a small piece of metal stroked along a magnet. Likewise using bacteria rather than iron filings to create the patterns around a magnet might not have any immediately obvious potential applications, but could create some beautiful pictures, especially if coloured purple!

Komeili, A. (2006). Magnetosomes Are Cell Membrane Invaginations Organized by the Actin-Like Protein MamK Science, 311 (5758), 242-245 DOI: 10.1126/science.1123231

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