Monday, February 26, 2018

BacterioFiles 329 - Special Secreting Structure Studied

Type 6 Secretion System protein
By User:Astrojan, CC-BY 4.0
This episode: New type of secretion system discovered that bacteria use to stab amoeba predators to escape their digestion!

Download Episode (7.8 MB, 8.5 minutes)

Show notes:
Microbe of the episode: Avastrovirus 1

News item

Journal Paper:
Böck D, Medeiros JM, Tsao H-F, Penz T, Weiss GL, Aistleitner K, Horn M, Pilhofer M. 2017. In situ architecture, function, and evolution of a contractile injection system. Science 357:713–717.

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    Episode outline:

    • Background: Microbes target enemies/prey
      • Not just random toxin release
      • Ep 116 tit-for-tat
    • Various types of secretion system, ~6
      • Some just put outside
      • Others actually inject into other cell
    • Many in pathogens to target potential victims/hosts
      • Others competitors/rivals, derived from phage tails
      • Like hypodermic needles
    • What’s new: Now, scientists publishing in Science have done a detailed study on a particular system in a bacterial symbiont of amoebas!
    • Amoebophilus asiaticus, obligate intracellular symbiont
      • No secretion systems obvious in genome
      • But genes for something similar to phage tail, possibly interesting
        • Similar to kinds released into environment instead of held in cell
    • Methods: Under electron scope, cryotomography, showed tail-like structures
      • But held in cell, not released
      • Found in bundles of individual structures, 2-34
      • Meets all criteria to be classified as T6SS
    • What function?
      • Amoeba takes up bacteria in compartments
      • They escape and replicate
      • Then leave host after ~6 days
      • Secretion structures observed while outside host, but not after escaping into cytosol
      • Bacteria that couldn’t make them didn’t get in well
      • So seem to help them get in and escape phagosome compartments
      • Not surprising; often helpful for that
    • Why so different genetically?
      • Compared involved genes to other systems
      • Tail structures released into environment, and secretion systems held in membrane
      • This seems to have come from former but now acts like latter
    • Summary: Amoeba-invading bacterium makes secretion system that acts like others already known but has different evolutionary history
    • Applications and implications: Discovering new systems like this important
      • Pathogens use a lot, can understand or target them better
      • Useful microbes could use to fight pathogens or other pests
        • Kill amoebas that cause disease, for example
    • What do I think: Life kinda like ultimate handyman
      • Take any structure and make something new
      • Even if equivalent already exists with same function
      • Use what is there, can't use what isn't there
    • Life is very adaptable


    1. Is this tail-like structure a variation of a pseudopodia?

      1. Not really; pseudopodia are more eukaryotic structures, like in amoebas that can reshape their cell membrane as needed. These structures here are tail-like not because they are actually like tails for the cells (that'd be more like flagella probably), but they are structurally similar to the tails of viruses that infect bacteria. These viruses, bacteriophages, often have a head like a little ball with the genetic material inside, and then a tail sticking out that attaches to the bacteria it infects and injects the genetic material into it to infect it.

    2. How do the other secretion systems differ from system 6 that the Amoebophilus asiaticus uses in this episode?

      1. I think structurally they are broadly similar, but it's possible to look at the genes that are the blueprints for making them and understand something about how they each came to be what they are now. The A. asiaticus system apparently took an unusual route of evolution to get where it is. Or maybe it's not unusual but we just haven't found other cases of it yet.

    3. Hello Dr. Noar,

      Bacterium that have the tail-like structure housed in the cell seems to serve like a hypodermic needle to inject to pierce the amoeba as an escape mechanism. Are these tail-like structures found in all of the same species of bacteria or are some bacteria on the same species lacking this structure? If some of the same species don't have this component is it possibly the effects of a genetic mutation within the bacteria for evolutionary purposes?

      Thank you,
      Zoe Wafford

      1. Hi Zoe,

        There are a lot of different species that have this kind of secretion system that is similar to phage tails, actually! However this Amoebophilus is the first found to have this particular subtype. Many pathogens are known to have the more widely studied version, called Type VI Secretion System, such as Francisella tularensis, Burkholderia pseudomallei, and Vibrio cholerae, which use the system to target eukaryotic cells (like ours). Other microbes target other bacteria, like in Pseudomonas aeruginosa and Agrobacterium tumefaciens.

        Actually, of the large group of bacteria in the phylum Proteobacteria, about 25% of their genomes have genes for this kind of secretion system. So it's actually very common, but some species may have lost it by chance mutations, or gained it by transfer of genes from a different species. Evolutionarily speaking, if the secretion system doesn't happen to be useful in a microbe's environment, there's no detriment to the microbes that happen to lose it by random mutations (and depending on whether it's actually deleted from the genome, it might be an advantage because then they don't need to spend the resources copying the genes).

        Thanks for listening,