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Bacteria that live on the Atkins Diet

The views expressed are those of the author and are not necessarily those of Scientific American.

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Bacteria have adapted to live in many niches; from the environmental bacteria that live in the soil and the seas, to the highly specialised intracellular bacteria that rely exclusively on their surrounding host for nutrients. While all bacteria face challenges in adapting their environment to suit them, intracellular bacteria face a particularly interesting challenge, as they must adapt to the conditions inside a living creature without killing their host.

All living organisms require a source of carbon, nitrogen and oxgyen in order to survive. Most organisms, particular the multicellular ones, prefer to use glucose or glycogen as a source of carbon, but some bacteria have found alternative ways of surviving. The bacteria Legionella pneumophila, which can live inside human or amoeba cells and is the cause of Legionnaires disease, gets its energy sources from the amino acids that make up proteins. Essentially, it lives on the Atkins diet, consuming high levels of protein and much lower levels of carbs.

The amino acid serine

The chemical structure of the amino acid Serine - which the bacteria uses as a source of both carbon and nitrogen. From wikimedia commons.

There are amino-acids floating around inside eukaryote host cells, but they tend to be at low concentrations. In order to get more of the amino-acids it requires the Legionella actively encourages the eukaryotic host cell to start breaking down its own proteins. The breakdown of proteins is a normal process that happens within your cells everyday as the making and breaking of proteins is vital for producing chemical signals and modifying metabolic pathways. The bacteria latch onto this system and start running it for their own ends.

To break down proteins a human (or amoeba) cell sticks a little label onto them, a small chemical molecule called ubiquitin (which I will just notate as U for now). A protein that needs to be removed will be marked with these little U tags, which are a signal for other controlling proteins to manhandle the doomed molecule over a big molecule called the proteosome, which breaks it down.


The process of ubiquitination. The U-labelled protein (maroon) is broken down into component parts by the proteosome (green). Picture (c) me.

The legionella gets into the cell wrapped in a little protective coat of outer cell membrane. Once inside this vacuole, it starts making itself at home, secreting around 300 proteins into its surrounding host in order to make itself a proper niche to live in. Included amongst these proteins is one called AnkB, which anchors itself into the vacuole membrane, facing out into the host cell. The AnkB itself doesn’t appear to add U-tags to the proteins, but it seems to strongly encourage the existing host tagging machinery to go overboard with the U-tags.

These tagged proteins are then degraded by the host-cell machinery providing the perfect environment for the Legionella to live in. If the gene for AnkB is knocked out, the bacteria suffers severe growth defects, which can be partially recovered by carefully  feeding the bacteria with a mixture of amino-acids allowing it to grow again. In mice models, bacteria with the AnkB gene knocked out were incapable of causing the bacterial pulmonary disease altogether.

Furthermore, this is not a species specific response either. Using the host-degradation machinery to provide the bacterial Atkins diet is common in infected organisms from humans and mice right down to amoeba. Finding a single gene of such importance may prove a major help for vaccine and drug development in the future.

University of Louisville Press Release.

Price CT, Al-Quadan T, Santic M, Rosenshine I, & Abu Kwaik Y (2011). Host Proteasomal Degradation Generates Amino Acids Essential for Intracellular Bacterial Growth. Science (New York, N.Y.) PMID: 22096100

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. pylori 4:13 am 11/24/2011

    I should add that your diagram of proteasomal degradation is slightly inaccurate. While in this process proteins are tagged with ubiquitin, it specifically needs to be a polyubiquitin chain of at least 4 Ub monomers (and not to separate parts of the protein molecule as in your diagram). If I was to be really picky I’d also mention that the green molecule that’s supposed to represent the proteasome is misleading, it seems to give the impression that it engulfs the targeted protein (much like a macrophage) but this is not the case. A more accurate, but still rudimentary, image would be a cylinder where the protein unfolds and then the polypeptide chain enters the cylinder (the proteasome) at one end, and then on the other end the amino acids leave as monomers. I say this only to give a more accurate depiction of the process, and not to misinform the public by ‘dumbing’ it down too much.

    Link to this
  2. 2. S.E. Gould in reply to S.E. Gould 4:54 am 11/24/2011

    Thanks for the comment! I wasn’t too bothered with the accuracy of the poly-U tags, I just wanted to give the impression of the protein being clearly ‘marked’ for degradation. I did have issues with the proteasome – I didn’t *really* want to show it as an engulfing crusher, but that was the best way I could think to portray the molecule getting broken up. Believe me, I did have a go at making it actually look like a cylindrical proteasome molecule but my MS-Paint skills are simply not up to the task and it just ended up looking too confusing and blobby.

    I am constantly on the lookout for well explained understandable illustrations, but right now I don’t have the money to hire an illustrator, and I often don’t have to time to spend hours constructing a perfect picture. It’s not a case of deciding whether or not to ‘dumb-down’ the process, but working out how much of an accurate impression my non-existent MS Paint skills will allow. With this illustration all I wanted to get across was “Protein tagged -> Protein destroyed” and although this may leave the public slightly misinformed as to the biochemical details of proteasomes I think it’s better to have an understandable post with slightly ‘off’ diagrams than a confusing post with no pictural aids.

    Having said that, if you know, or want to make, a good creative-commons-licence picture of protein degradation, please do get in touch and I would be happy to feature it on the blog! Perhaps with a slightly more involved description of how the process works at a biochemical level.

    Link to this

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