Monday, March 19, 2018

BacterioFiles 332 - Moth Minors Missing Microbes

Manduca sexta caterpillar,
tobacco hornworm
This episode: Unlike most animals, caterpillars don't seem to have a resident gut microbe to help them in various ways!

Thanks to Tobin Hammer for his contribution!
Download Episode (14 MB, 15.3 minutes)

Show notes:
Microbe of the episode: Borrelia graingeri

News item

Journal Paper:
Hammer TJ, Janzen DH, Hallwachs W, Jaffe SP, Fierer N. 2017. Caterpillars lack a resident gut microbiome. Proc Natl Acad Sci 114:9641–9646.

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    Episode outline:
    • Background: Microbiota: basically another organ
      • Very important for health/life 
    • Also important for insects: statement 2
      • That was Tobin Hammer, first author on today’s study and graduate student at University of Colorado Boulder.
      • See eps 80, 131, 250, 251 for more about insect microbes 
    • What’s new: But now, Tobin Hammer and colleagues Daniel Janzen, Winnie Hallwachs, Sam Jaffe, and Noah Fierer, publishing in Proceedings of the National Academy of Sciences, report on a surprising exception to this common setup! 
    • Statement 1 
    • Caterpillars are intermediate stage
      • In between egg and butterfly or moth
      • Mainly for feeding and growing quickly 
    • Methods: Looked at many different kinds of caterpillars 
    • Had to collect samples from caterpillar gut
      • Funny story: statement 5 
    • Then sequenced 16S rRNA gene from samples
      • Much lower amounts of bacterial DNA than other insects
      • Regardless of caterpillar type
      • Most of what was there was from chloroplasts (from diet) or mitochondria (from own cells)
      • And of what was there, seemed unstable and like passing through from diet, not resident
      • Number decreased a lot from food to feces, no visible growth 
    • Then looked at one particular species: Manduca sexta, or tobacco hornworm
      • Moth species with large green caterpillar form with little horn on back end
      • Model lab species
      • Often found eating large amounts of tomato in garden
        • Stare at plants until it becomes visible 
    • Collected individuals from wild
      • Treated with antibiotics that reduced bacterial numbers even more than already low
      • Didn’t affect body weight or development time or anything 
    • Summary: So to summarize, here’s Tobin: statement 3 
    • Applications and implications: This finding is very interesting, potentially useful: statement 4 
    • Clarifications if necessary: Measurement by molecular bio/sequencing techniques imprecise
      • Specific to bacteria 
    • What do I think: Much more research to be done on this kind of subject: statement 6 
    • Caterpillar gut is pretty harsh place
      • very alkaline, similar to strong ammonia solution in pH
      • Antimicrobial compounds produced by caterpillar
      • Fast content turnover 
    • Still other microbe symbionts in other areas of body, just not gut
      • Ep 166 metamorphosis microbes 
    • Not many things can live on leaves without microbial help
      • Caterpillars seem to break it down adequately on own, not super-great but good enough 
    • Simpler in some ways, don’t have to spend effort mediating immune tolerance
      • Resist pathogens and everything
      • No symbiosis transactions to deal with, just produce what you need on your own
      • Self-reliance, DIY approach
      • May depend on genes taken from microbes though, don’t need the whole microbes anymore 
    • Not necessarily a species thing, maybe just a stage
      • Microbes important in stages other than larval 
    • But if some organisms don’t need microbial symbionts, that’s fine
      • All part of amazing diversity of life 
    Author Transcript:
    So we wanted to know what role or roles gut microbes play in caterpillar feeding and development. 

    So we've known for a long time that microbes are absolutely vital to the ability of things like aphids, termites, leaf-cutter ants, and a bunch of other insects: their ability to feed on plant material. And it's the same with us humans, we rely heavily on microbes in our gut to digest food and protect us from pathogens.

    we found that caterpillars are doing something totally different: instead of fostering the growth of beneficial microbes, they appear to be able to process leaves and grow on their own, and they actively exclude microbes from colonizing their guts. 

    We think this is an important finding first because it contradicts a common assumption that all animals rely on microbes. It might also be useful information for controlling caterpillar pests, as it really matters whether their digestion is or is not mediated by microbes. 

    One of the less glamorous parts of the study was actually collecting the samples, most of which were caterpillar feces. I had discovered earlier that the feces needs to be preserved while fresh, because if you wait too long after the caterpillar defecates, the fecal material will have started to decompose, and then you have microbes growing that weren't originally abundant in the insect's gut. So basically what that meant is, my two undergraduate assistants and I spent many many hours, literally watching and waiting for caterpillars to poop, to get that fresh sample. 

    Anyways, after the publication, I'm now really interested in what makes caterpillars different from those other animals that we know rely on microbes, things like aphids, termites, leaf-cutter ants, etc. And I'm also interested in how many other types of animal there are out there that are similar in this regard. There has been some great work showing parallel results in some ants, and in stick insects, and in a number of other groups, so I think it's a more common lifestyle than we appreciate.


    1. Do you think this could change the way we look at and further study our microbiota, for the human body?

      1. Sure it could! Understanding of our microbiota has grown immensely in the past few years, but there's still a long way to go. Having a variety of different model systems to study can make this easier: by comparing them, we can better learn what roles microbes play and when they can be more or less useful.

    2. Do any organisms that are not insects, but have multiple stages in life, such as frogs, may also have changing microbes similar to what is thought for the caterpillar?

      1. That is an excellent question. The metamorphosis that frogs undergo isn't quite as drastic as that of some insects; the transition from caterpillar to butterfly, for example, involves a complete rearrangement of tissues, while other animals may increase in size and number of limbs and a few other changes.

        But the change in lifestyle can definitely affect (and be affected by) the microbiota. Frogs go from a vegetarian diet in the tadpole stage to eating insects, and though it doesn't disappear and reappear entirely, their gut changes shape to accommodate this switch, and the microbe community changes as well: