The Termite Gut and its Symbiotic Microbes

00:00:07.25 I'm Jared Leadbetter
00:00:09.08 and I'm at the California Institute of Technology.
00:00:12.07 And, for some 24 years now,
00:00:14.14 my research has sought to clarify the relationship
00:00:16.26 between termites and their hindgut microbes.
00:00:19.21 Now, of particular interest to me
00:00:22.22 is the metabolism of hydrogen
00:00:24.20 that is generated during this fermentation of wood,
00:00:27.11 but today I want to give you a broad overview
00:00:30.17 on microbial diversity
00:00:33.08 and on some of the essentials
00:00:35.22 of termite hindgut microbiology.
00:00:38.26 So, I want to talk about
00:00:41.07 biological diversity in general,
00:00:43.14 because many who are interested in biology
00:00:45.17 are missing, actually, some of the
00:00:48.21 full diversity that we see in the microbial world.
00:00:52.01 And, then I also want to comment about
00:00:54.15 how some of this diversity is surprisingly abundant
00:00:56.28 in certain areas of the world,
00:00:58.23 and that we must understand
00:01:01.11 what that diversity does and how it functions.
00:01:03.03 So, this will bring me to termites
00:01:05.17 and the symbiosis they form with their hindgut microbes,
00:01:08.14 and I want to introduce you to several major groups
00:01:11.10 of termite hindgut microbes,
00:01:13.17 the cellulose decomposing protozoa,
00:01:17.01 methane producing archaea,
00:01:18.27 and a group of abundant and unusual bacterial
00:01:21.27 called spirochetes.
00:01:24.12 I'm going to then touch on
00:01:26.26 how we can study different termites
00:01:29.06 and learn about different events that may have occurred
00:01:31.19 during this symbiosis in the past,
00:01:33.22 and then I'll give you some conclusions.
00:01:39.02 I wonder how many people who are watching this
00:01:41.11 have grown up thinking about three kingdoms.
00:01:45.10 Up until the 1960s,
00:01:47.01 I think most people grew up thinking
00:01:50.17 that there were animals, plants, and fungi,
00:01:52.24 and that maybe, depending on their education,
00:01:55.27 even up through current years,
00:01:59.22 most enthusiasts of biology
00:02:02.11 understood there to be between three and five kingdoms.
00:02:05.07 Maybe you had the bacteria as a fourth
00:02:08.29 and the protists as a fifth,
00:02:13.04 But, starting in the 1960s we had a revolution
00:02:16.25 in the study of the relationships between different organisms,
00:02:19.10 and started to realize that many things that we were seeing,
00:02:22.06 and also not seeing,
00:02:24.13 are very, very different from these three major groups.
00:02:27.12 So, for example, if you look at a key gene
00:02:30.10 that is present in all known organisms,
00:02:33.00 you can make comparisons between this gene
00:02:36.13 and from that infer
00:02:38.24 how those organisms are related to each other.
00:02:41.16 The thing I want to point out on this slide
00:02:44.13 is that you have the fungi
00:02:46.02 and the animals
00:02:48.02 and the plants,
00:02:50.16 and those are just three twigs on a branch
00:02:53.27 that has many other twigs.
00:02:56.03 Really?
00:02:58.10 If those three twigs and the length of those lines
00:03:01.11 denote evolutionary relationships,
00:03:04.27 then there are more than three kingdoms.
00:03:07.25 There are easily a half-dozen.
00:03:10.20 The other thing I want to point out here
00:03:13.04 is that if you think about "plant metabolism"
00:03:16.11 there are some organisms on this tree
00:03:19.14 which also carry out photosynthesis,
00:03:23.13 let's say the kelp or the red seaweed,
00:03:28.09 but those branches are actually
00:03:31.11 very different from the plants.
00:03:32.28 They are as distantly related to plants
00:03:35.08 as you and I are from plants,
00:03:37.15 and I think that's very important.
00:03:39.12 Also, when we talk about single-celled eukaryotes, like protozoa,
00:03:43.11 we realize, oh,
00:03:46.17 Paramecium and the protozoan Babesia
00:03:49.08 are actually two very different organisms,
00:03:51.21 again, as distantly related to each other
00:03:54.14 as we are from let's say the yeast
00:03:57.01 that you use to make beer and bread.
00:04:03.11 So, the number of kingdoms
00:04:08.16 or major divisions of life
00:04:11.01 already gets more complex than those three that we know.
00:04:15.13 The truth is actually much more complex than that,
00:04:17.22 because this is now just a snippet
00:04:20.27 of a branch of a much more complex tree.
00:04:23.19 You'll see that I have just blown up
00:04:26.14 this section of a much larger tree.
00:04:29.27 One of the things I want to point out on this slide
00:04:32.18 is that there are many, many branches on this,
00:04:34.22 easily 100
00:04:37.18 which are more distant from each other
00:04:40.22 than the distance between corn and animals,
00:04:44.12 and what that suggests, then,
00:04:46.13 is that we have a lot to learn about
00:04:51.24 the differences between these different groups.
00:04:53.28 So, for instance, everything
00:04:56.12 that is lying outside of this circle is a microbe,
00:05:00.02 the single-celled organisms
00:05:01.19 which are smaller than you can see with your naked eye.
00:05:04.11 So, for as much as we can appreciate biological diversity
00:05:06.19 that you can see,
00:05:08.17 the true diversity of life
00:05:10.22 is beyond the resolution of the human eye
00:05:12.12 and we have to use other methods to really
00:05:14.02 understand how it works.
00:05:16.14 The second thing I want to point out
00:05:18.23 is how different the way of living is
00:05:22.15 for corn, or a plant,
00:05:24.20 and ourselves,
00:05:27.05 or from a yeast that's used to make bread and beer.
00:05:29.28 If the length of those lines,
00:05:31.25 which is comparatively short,
00:05:33.28 and the differences of these organisms is so great,
00:05:38.06 imagine the possible differences
00:05:41.13 in the ways that these organisms live.
00:05:44.08 So, we are potentially really missing out,
00:05:46.28 not just on the diversity
00:05:49.17 in terms of how things are related,
00:05:51.16 but also missing out on the diversity
00:05:53.25 of what organisms are actually doing in the environment.
00:05:56.10 And so, if we're to understand the environment,
00:05:58.16 we really have to learn more about
00:06:00.22 what these other organisms are doing.
00:06:04.27 Let's come back to this tree.
00:06:07.06 Let's come back to this organism kelp.
00:06:09.23 I want to illustrate another point:
00:06:11.25 it's not just that there are many, many organisms out there
00:06:14.25 which are different from the organisms we're most familiar with,
00:06:17.04 but in some environments those organisms are present
00:06:21.15 and very, very abundant.
00:06:23.14 Take the kelp -
00:06:25.29 you can find kelp forests off the coast of California,
00:06:29.02 and those kelp are performing
00:06:31.21 what we might call "plant metabolism".
00:06:33.25 So, they are the primary producers in those environments,
00:06:36.06 but, keep in mind, they're not plants.
00:06:39.03 So, the story of these coastal ecosystems
00:06:43.03 is in a large part driven
00:06:45.12 by an organism that's very different from a plant,
00:06:47.20 and so if we want to understand those coastal ecosystems
00:06:50.10 part of the story is understanding
00:06:53.02 the biology of kelp
00:06:54.20 and understanding in what ways they are similar
00:06:57.03 and different to the terrestrial,
00:06:59.05 or land plants that we study.
00:07:03.24 Now, I want to shift from the oceans
00:07:06.02 to my own research, which is study on
00:07:08.25 termites and their hindgut microbes,
00:07:11.00 and I want to point out that the termite hindgut
00:07:14.02 is an environment.
00:07:16.02 It happens to be an environment that lives
00:07:18.17 in a small insect,
00:07:20.15 but we can compare and contrast that environment
00:07:22.16 with, for instance,
00:07:24.24 a very rich marine environment like the Sargasso Sea.
00:07:28.03 There are certain reasons why
00:07:30.16 you might want to study a small environment
00:07:31.14 like in a termite.
00:07:33.00 The Sargasso Sea is a wonderful and amazing place,
00:07:34.28 and very important to study, but it's a thousand kilometers across,
00:07:38.07 and there's only one of the them.
00:07:41.03 The termite hindgut is only about one cubic millimeter in volume,
00:07:46.01 and yet it contains hundreds and hundreds
00:07:48.13 of microbes that you find no where else in nature.
00:07:51.08 Just if you were to take the top millimeter
00:07:53.14 of the Sargasso Sea,
00:07:55.22 the volume of that across those thousands of kilometers
00:07:59.19 is 19 orders of magnitude greater
00:08:01.27 than the volume of that one termite.
00:08:03.21 So, we can actually bring a termite into the laboratory
00:08:05.28 and be able to study an entire environment.
00:08:09.25 The hindgut... it's tiny yet complex...
00:08:12.18 many hundreds of species
00:08:14.20 and some of those species are yet unstudied,
00:08:17.13 so it is still bewildering complex,
00:08:20.11 and it's well-bounded.
00:08:22.12 We know that the gut lining
00:08:23.00 and the outside of the termite
00:08:24.27 are where you might define the boundaries of that system.
00:08:27.26 The Sargasso Sea is wonderful,
00:08:29.25 but there's only one of them,
00:08:31.29 and its boundaries are a little bit user-defined.
00:08:34.03 We think that it's, you know,
00:08:35.23 the currents that are intersecting here and there
00:08:37.25 are what bound that region,
00:08:39.13 whereas in the insect it's very well-bounded.
00:08:42.00 And, of course, the termite is available
00:08:44.09 in large number of replicates,
00:08:46.04 so we can, in a laboratory,
00:08:48.22 have that one environment that's tiny and well-bounded
00:08:51.10 replicated in termite, after termite, after termite.
00:08:54.13 So, we can start to do some comparative studies
00:08:56.08 and perturbation studies
00:08:58.07 which are just not possible with a large environment
00:09:00.25 like the Sargasso Sea,
00:09:02.13 for which there's only one.
00:09:07.06 So, I study a very particular termite
00:09:09.14 that we find in ponderosa pine
00:09:11.18 that's fallen in the Angeles National Forest
00:09:14.02 of southern California,
00:09:15.19 and here is one of these ponderosa pines
00:09:18.10 and two of my former students,
00:09:20.25 who have peeled off some of the bark from this log,
00:09:22.28 which has been on the ground for probably five or ten years,
00:09:25.15 and if you look a little bit more closely
00:09:27.24 you can see that just on the underside of that bark
00:09:29.18 are a number of termites
00:09:31.19 of different, what we call, morphological castes.
00:09:34.02 These ones with the dark mandibles are actually soldiers.
00:09:36.29 Rather than eating wood,
00:09:38.21 they have big mandibles that they can use to attack,
00:09:41.01 for instance, another termite or an invading ant.
00:09:45.22 So, this is termite that we study, for the most part,
00:09:49.01 in my laboratory.
00:09:50.18 It's the Dampwood Termite, Zootermopsis nevadensis,
00:09:54.17 and it's about a centimeter in length.
00:09:56.09 It's one of the larger termites that you'll find on Earth.
00:09:58.27 Now, this is what we call a worker,
00:10:02.24 and from another specimen I've extracted the hindgut tract,
00:10:07.29 and that is shown here.
00:10:10.18 And, what you'll observe is that
00:10:13.17 there is a long tubular region,
00:10:15.18 which is somewhat analogous to our small intestine,
00:10:18.24 and then you have this hindgut paunch,
00:10:22.10 which is somewhat analogous
00:10:25.07 to our large intestine,
00:10:27.02 and it's in this paunch
00:10:29.22 that really you find a lot of things that excite microbiologists.
00:10:33.04 You find a density of microbes
00:10:35.08 you find nowhere else in nature,
00:10:37.12 and they represent all three domains of life.
00:10:40.04 Earlier on that slide,
00:10:42.06 I'd shown you that the tree of life,
00:10:44.16 where you actually have three major subgroups,
00:10:47.05 and those are the archaea,
00:10:49.00 the bacteria,
00:10:50.19 and the eukaryotes.
00:10:52.09 You find single-celled relatives of all three of those groups,
00:10:56.06 comprising hundreds of species,
00:10:58.07 in this hindgut paunch.
00:11:00.19 So, before I tell you about termite gut microbes,
00:11:03.04 I want to tell you a little bit more about termites.
00:11:05.17 There are about 3000 species of termites on Earth.
00:11:10.24 Termites are related to cockroaches
00:11:13.24 and to the mantids, like the praying mantis,
00:11:15.19 and they're actually, although insects,
00:11:18.05 quite distantly related to ants, wasps, and bees,
00:11:22.02 which are also social.
00:11:24.06 So, this is an example of where
00:11:27.06 society, or sociality in insects,
00:11:29.12 has arisen in two different groups of insects
00:11:31.08 which are very different from each other.
00:11:33.19 So, termites, and those 3,000 species of them,
00:11:37.07 can be subgrouped into several different families,
00:11:41.11 and their closest relative in the insect world
00:11:45.04 is what we call the Wood Roach,
00:11:47.04 which is a non-social but wood-feeing insect
00:11:49.22 that you find in the Carolinas
00:11:52.19 and the Pacific Northwest
00:11:54.22 and in parts of China.
00:11:56.26 And, many of the microbiology...
00:12:00.02 the features of the microbiology in the Wood Roach
00:12:02.22 are actually shared with the termites,
00:12:06.13 and so it's thought that many of the
00:12:09.07 microbial processes that arose
00:12:11.28 arose in the last common ancestor of this roach and termites.
00:12:16.08 Now, the termite that I study, Zootermopsis,
00:12:19.07 belongs to this one groups in the Termopsidae.
00:12:21.22 So, over time, we can start to ask,
00:12:24.12 are any of the patterns that we see here
00:12:27.18 present in some of these other groups?
00:12:31.09 Now, if you extract the gut from Zootermopsis,
00:12:34.03 or another termite,
00:12:36.08 and you take a cross-section through that gut,
00:12:38.20 what you'll see is that the insect tissue itself
00:12:40.25 is actually very, very thin.
00:12:43.05 It's only about 10 microns in diameter,
00:12:46.05 so 1% of a millimeter in thickness.
00:12:49.14 The bulk volume of that is comprised by the gut contents,
00:12:53.10 and what you see here, these larger objects,
00:12:56.05 are single-celled eukaryotes called protozoa.
00:12:59.20 Now, historically, until the mid-1990s,
00:13:03.01 this region, this environment
00:13:05.17 was thought to be completely anoxic,
00:13:07.23 so, devoid of oxygen.
00:13:09.20 Many of the microbes that you find
00:13:11.18 in the core regions of this gut are poisoned by oxygen.
00:13:15.12 Really?
00:13:16.21 Here you have this insect,
00:13:18.08 which is living in the aerobic world
00:13:20.09 and wandering around on its six legs
00:13:23.00 in a piece of wood or on a piece of wood,
00:13:27.15 containing microbes which are poisoned by oxygen.
00:13:31.09 The story is actually even more complex,
00:13:33.13 because it turns out,
00:13:35.13 through studies that were performed
00:13:37.05 by Andreas Brune and John Breznak in the mid-1990s,
00:13:40.19 the oxygen actually diffuses across the insect gut wall
00:13:45.03 and then is consumed by biological processes
00:13:48.07 in the periphery of that hindgut,
00:13:50.28 and it's those biological processes in the periphery
00:13:54.07 which lead to the lack of oxygen in the core,
00:13:56.20 which protects some of those oxygen-sensitive microorganisms.
00:14:00.29 So, not only do you find organisms
00:14:03.03 which are unique to the termite gut habitat,
00:14:06.01 that you find nowhere else on Earth,
00:14:07.28 but many of these are poisoned by oxygen
00:14:10.04 and are very sensitive to desiccation.
00:14:12.29 So, their life outside the termite is very, very limited.
00:14:18.20 So, what you have
00:14:22.01 are microbes that are very dependent on their host
00:14:24.14 and, because of their processes,
00:14:26.10 which allow the host to derive nutrition from wood,
00:14:29.10 makes the host very dependent on their microbes.
00:14:33.25 And, when a termite emerges from its egg,
00:14:37.01 it doesn't have these microbes in its gut.
00:14:39.01 It's fed those microbes by other of its littermates
00:14:43.02 or by its parents,
00:14:45.06 and if those microbes don't take hold
00:14:48.04 that termite will fail,
00:14:51.21 and if that termite fails
00:14:54.25 those microbes will also fail.
00:14:57.15 So, when we look at one termite
00:14:59.14 that's walking around on a piece of wood today,
00:15:02.05 we are looking at over 100 million years
00:15:04.26 of having this microbial community
00:15:07.16 passed from one termite to the next termite to the next termite,
00:15:11.14 generation to generation to generation.
00:15:14.04 It's quite a remarkable story
00:15:17.04 of a journey that's been taken
00:15:19.04 between many, many organisms together.
00:15:22.26 So, if we go and look at a little bit of a higher magnification
00:15:26.08 of what's inside the gut,
00:15:29.13 this is what we call a DIC image
00:15:34.04 using a microscope
00:15:36.02 of some of the larger protozoal cells.
00:15:37.29 These are about 60 microns in length,
00:15:40.23 so roughly 20 of these laid end to end to end
00:15:43.27 would be a millimeter of length,
00:15:46.05 and some of these are the primary agents
00:15:48.08 of wood degradation in this termite.
00:15:50.02 You also see some smaller cells
00:15:52.26 which are also protozoa.
00:15:55.13 So, these are single-celled eukaryotes,
00:15:58.26 of which there are about a dozen
00:16:00.27 in this one termite
00:16:02.23 that you find nowhere else in nature,
00:16:04.14 and their closest relatives are in other termites.
00:16:08.04 There are some interesting associations
00:16:10.03 that you find between these protozoa and certain bacteria.
00:16:13.23 For instance, if you look at the surface of one of these,
00:16:16.15 at higher magnification,
00:16:18.11 you'll see that those protozoa
00:16:19.28 are covered with long lines of grooves,
00:16:23.03 and in those grooves you see these little black objects?
00:16:26.15 Those are bacteria.
00:16:29.09 So, the surface of that single cell of eukaryote
00:16:32.23 is arrayed with a very regular group
00:16:36.16 of a very specific bacterium
00:16:39.19 and, without knowing much more about,
00:16:42.07 the notion you might have is that
00:16:44.19 there is something that that protozoan
00:16:47.01 is getting from that bacterium and vice versa.
00:16:50.22 So, there are associations between the microbes
00:16:52.29 that live in the gut,
00:16:54.23 and there are associations with those microbes and their host.
00:16:58.04 So, there are many levels of biological interactions
00:17:00.06 which are occurring in this environment.
00:17:03.03 Another example of a protozoan that has a bacterial association
00:17:10.16 is this organism, which is called Streblomastix strix.
00:17:13.22 The eukaryote, the single-celled protozoan,
00:17:17.10 is actually very, very slender
00:17:19.04 and running through the center of this.
00:17:21.06 That protozoan is covered with a blanket, or a coat,
00:17:25.06 of long, thin bacterial cells
00:17:28.07 that are creating those ridges that you see.
00:17:31.05 We know very little about the interaction
00:17:33.06 between this protozoan and the bacteria
00:17:35.09 and what they're doing for each other,
00:17:37.22 but clearly it's a very specific and interesting interaction.
00:17:42.06 There are many cases of protozoa
00:17:44.27 and the bacteria that occur in these environments,
00:17:47.11 of which we have much more to learn in the future.
00:17:52.04 Now, there's an opening line to a book
00:17:54.26 by the biophysicist Howard Berg.
00:17:57.20 His book is called "Random Walks in Biology",
00:18:00.23 and the opening lines
00:18:03.11 are that biology is wet and dynamic.
00:18:06.10 What does that mean?
00:18:08.01 I've just showed you these still pictures,
00:18:10.02 but I think that the still pictures
00:18:12.12 don't do this environment justice.
00:18:14.25 Really, when you're looking at this environment live,
00:18:17.24 this is now some of that gut fluid
00:18:19.23 which has been diluted.
00:18:21.19 It's even more densely packed than this inside the termite,
00:18:24.02 but if you dilute that fluid and put it on a microscope slide
00:18:26.22 you see some of these protozoa
00:18:29.15 and coursing unamongst them,
00:18:31.15 lots of bacteria which are moving so quickly
00:18:33.22 you can barely focus on them.
00:18:36.12 I can just look at this forever,
00:18:38.26 and you can look at it using different types of microscopy
00:18:43.02 to show different details on some of these cells.
00:18:46.02 The point here is that
00:18:48.11 when I go to work every morning
00:18:50.04 I go to work in what I call a miniature Alice in Wonderland.
00:18:52.23 That it is, just from a naturalist's standpoint,
00:18:55.13 a very wonderful and diverse place
00:18:58.00 that begs lots of questions.
00:19:01.27 So, what is the interaction
00:19:03.29 that termites have with their gut microbes?
00:19:07.10 I want to give you an overview
00:19:09.08 of some of the major things
00:19:11.15 that we've learned over about the last 100 years
00:19:14.00 on the association between
00:19:16.19 the insect and its gut microbes.
00:19:19.00 Now, microbes have a huge challenge in life.
00:19:21.26 The challenge is, how do you eat something
00:19:24.02 larger than your head?
00:19:26.06 If you are the size of
00:19:29.18 one thousandths of a millimeter,
00:19:32.02 how do you gain access to nutrients
00:19:34.19 in a 2x4 or in a big log?
00:19:37.12 So, you have a really wonderful association
00:19:40.01 with an insect that has jaws and grinding mandibles,
00:19:43.12 which are very, very hard,
00:19:45.01 that can then take a large block of wood
00:19:47.09 and grind it into really small particles,
00:19:50.07 and then bring them into the gut
00:19:52.03 in a very controlled and wonderful environment
00:19:54.08 in which these microbes can thrive.
00:19:56.22 Now, in the hindgut, these protozoa that I showed you
00:20:01.03 have enzymes of their own,
00:20:03.02 and enzymes that they recruit from the insect,
00:20:05.14 to start breaking down the polysaccharides in wood...
00:20:08.18 the cellulose and another component
00:20:10.23 which we call xylan or the hemicellulose...
00:20:14.18 and these protozoa perform a very unusual fermentation.
00:20:17.22 It's a fermentation that differs
00:20:20.00 from the one that you use to make sauerkraut
00:20:22.02 or that you use to make beer and wine.
00:20:25.00 What those protozoa do
00:20:27.02 is they break down the hexoses in cellulose,
00:20:29.28 primarily to acetate,
00:20:32.00 so, neutralized vinegar,
00:20:35.05 and that acetate builds up in the hindgut of the termite
00:20:37.22 and is absorbed by the insect.
00:20:39.29 So, the insect is actually absorbing the acetate
00:20:43.21 and using it as its biofuel.
00:20:46.01 It is the source of carbon for the insect
00:20:48.19 and the source of energy for the insect.
00:20:53.00 Now, those protozoa
00:20:55.07 also produce hydrogen gas,
00:20:57.21 so think the 1930s,
00:20:59.27 this classic picture of the Hindenburg blimp
00:21:01.26 over New Jersey, blowing up in fire.
00:21:04.10 It was filled with hydrogen.
00:21:06.01 There's a lot of energy in hydrogen.
00:21:07.21 It's not just combustible;
00:21:09.24 it's an energy source that can be used by different microorganisms.
00:21:13.04 So, in the termite,
00:21:15.10 and in many environments which are non-marine
00:21:17.16 and devoid of oxygen,
00:21:19.17 hydrogen and CO2 is converted into methane
00:21:23.08 by a group of organisms called methanogenic archaea,
00:21:26.29 and this methane is emitted by the insect.
00:21:30.00 It is sort of lost calories.
00:21:31.24 So, as we know, we can burn methane,
00:21:33.24 we use it as a fuel,
00:21:35.24 and that methane which is emitted by the insect, then,
00:21:38.22 is a fuel, a potential energy source,
00:21:40.21 which is lost from the system.
00:21:43.07 So, we can use a form of microscopy
00:21:46.07 to observe these methanogenic archaea in the termite.
00:21:50.07 Several years ago, I was trying to find out,
00:21:54.07 where are those archaea present in the system?
00:21:56.09 And what I learned is that
00:21:59.06 they are colonizing the gut wall of many...
00:22:03.21 inside the gut wall of many termites.
00:22:05.18 So, if you dissect out the gut,
00:22:07.29 you cut open the gut, you wash away all the contents,
00:22:11.05 and you sort of open that up
00:22:13.00 and look at the internal surface of it,
00:22:15.09 you can look for a type of fluorescence
00:22:19.08 called F420 fluorescence.
00:22:21.20 These organisms that make methane contain a vitamin,
00:22:24.04 and when you shine UV light on that vitamin
00:22:26.13 the vitamin turns blue.
00:22:28.16 And so, with a proper microscope,
00:22:30.29 you can start to see a number of different cell types,
00:22:33.18 which are blue,
00:22:35.18 that live on the inside of this gut wall.
00:22:38.12 And, in this image you see that there are
00:22:40.13 three different morphologies of organisms.
00:22:42.10 Now, I'm somebody who likes food,
00:22:43.28 so I like to say that this one long one
00:22:45.29 looks like long, blue spaghetti,
00:22:47.25 the curves rods look like basmati rice,
00:22:49.28 and some of these straight rods look like regular rice.
00:22:53.06 Now, termites emit up to 4% of global methane every year.
00:22:57.03 So, by studying these organisms in their environment,
00:23:00.00 and also culturing them and bringing them into the lab,
00:23:02.18 we can put a face on the process,
00:23:06.00 at a single microbial cell level,
00:23:08.19 for actually a very significant source of global methane...
00:23:12.17 not the most significant source of global methane,
00:23:15.25 but a small but significant source.
00:23:19.28 I like to show this slide,
00:23:23.01 of a paper mache cow,
00:23:25.01 because I knew that when I first got to Caltech
00:23:27.00 I was having some impact on undergraduate life there,
00:23:30.02 because when I talked about termites
00:23:32.22 and processes that occur in a cow
00:23:34.22 the next Caltech Ditch Day,
00:23:36.26 that occurs every spring,
00:23:39.06 the students had made this large paper mache cow
00:23:41.17 and filled it with chocolate pudding,
00:23:44.08 Easter grass, oatmeal,
00:23:47.06 and Easter eggs that were filled with clues
00:23:49.12 on where the students should go to their next puzzle.
00:23:53.21 And, you can see here,
00:23:55.23 the students trying to find those eggs.
00:23:57.28 The point I want to make here is that
00:24:01.11 cows lose about 20% of their electrons in their food
00:24:05.05 as methane.
00:24:07.06 It's a huge waste of energy.
00:24:09.09 And, although termites contain methanogens,
00:24:11.19 and emit methane,
00:24:13.25 it's only a very small amount of this hydrogen and CO2
00:24:16.25 that is lost to the system as methane.
00:24:19.11 On a global scale, it's significant,
00:24:21.22 but, actually, on a global scale,
00:24:23.22 the methane emission by termites
00:24:25.09 would be much more significant
00:24:28.03 if this hydrogen and CO2 was not being consumed
00:24:30.21 by a different group of organisms
00:24:33.14 which we call CO2-reducing homoacetogens.
00:24:36.17 So, many termites contain microbes
00:24:38.28 that completely push these methane organisms
00:24:41.19 out of the picture,
00:24:43.22 or push 90% of them out of the picture.
00:24:47.11 So, many termites will take...
00:24:49.20 have microbes that convert hydrogen and CO2 into acetate,
00:24:52.19 and this acetate then goes into that pool in the gut
00:24:56.05 and is absorbed by the insect.
00:24:58.10 So, up to a third to a fifth
00:25:01.01 of the acetate which is used as the biofuel
00:25:03.22 by these insects
00:25:05.27 is derived from carbon dioxide and hydrogen
00:25:09.02 by way of the activity of those protozoa,
00:25:11.11 and by way of the activity
00:25:14.00 of these organisms here.
00:25:16.02 So, I have long been interested
00:25:18.00 in the interaction between organisms competing
00:25:20.24 for this hydrogen and CO2
00:25:22.26 that make acetate and that make methane
00:25:25.26 from those...
00:25:28.02 to understand how they compete with each other,
00:25:30.01 how has this process come to pass in termites,
00:25:33.25 why doesn't it occur in the cow rumen,
00:25:35.29 and how do these hydrogen consumers
00:25:38.06 interact with the organisms which are producing the hydrogen,
00:25:40.18 the protozoa.
00:25:44.16 So, CO2-reductive acetogenesis
00:25:47.20 is a bacterial activity.
00:25:51.27 The process involves
00:25:54.13 the fixation of two molecules of carbon dioxide...
00:25:58.11 one, two...
00:26:00.26 with four molecules of hydrogen...
00:26:03.20 one, two, three, four...
00:26:08.09 and in the process, those two carbons are joined
00:26:11.07 and reduced to form the acetate,
00:26:13.15 and this metabolism
00:26:15.28 actually yields energy for the bacteria
00:26:17.24 which are performing it,
00:26:19.22 in addition to yielding the acetate
00:26:21.10 which can be used by the insect.
00:26:22.10 So, it's a mutually beneficial metabolism
00:26:26.08 that takes hydrogen produced during this fermentation,
00:26:28.18 turns it into additional fuel for the insect,
00:26:30.25 meanwhile supporting the activity of the bacteria that perform it.
00:26:35.22 But, for years,
00:26:37.19 we did not have a very good understanding
00:26:40.03 about what bacteria in the termite
00:26:42.19 are actually catalyzing this process.
00:26:44.09 We had some ideas, but, over the years,
00:26:47.06 we've been trying to learn more.
00:26:49.10 Now, if you look in the hindguts of termites
00:26:51.23 you'll even see, on some of these protozoa,
00:26:55.05 that there are very abundant spiral-shaped organisms,
00:26:57.24 which can be attached to the protozoa
00:26:59.28 and that can also be seen
00:27:03.10 living and swimming amongst the protozoa,
00:27:06.03 and in most termites these organisms we call spirochetes
00:27:09.10 are some of the more abundant bacteria
00:27:11.11 that you'll see swimming in and amongst these protozoa.
00:27:14.28 If you go to another portion of the gut,
00:27:17.09 maybe you'll see that there are even more
00:27:19.14 of these spiral-shaped organisms.
00:27:21.13 So, starting in the 1990s,
00:27:23.17 scientists at Michigan State and in Germany
00:27:27.02 discovered that these spirochetes
00:27:29.19 are actually very closely related to Treponema pallidum.
00:27:33.28 That's one of the most famous organisms in microbiology.
00:27:37.03 It's what causes syphilis.
00:27:39.11 Actually, all these bacteria in the termite
00:27:42.16 are species that belong to the same genus
00:27:46.13 as the agent of syphilis,
00:27:48.13 and yet you always find these organisms
00:27:51.12 present in happy and healthy termites.
00:27:53.28 But we didn't know what they did because, like syphilis,
00:27:57.13 they had never been cultured in vitro.
00:27:59.28 First observed in the 1860s,
00:28:01.21 over a century went by
00:28:03.26 before we had actually learned about what any of these do,
00:28:06.07 and I would still argue that we are still in our infancy
00:28:09.04 of understanding what the full breadth of the different roles
00:28:11.25 of the hundred or more species of spirochetes
00:28:14.12 that you can see in an individual termite hindgut.
00:28:18.21 But, a number of years ago,
00:28:20.27 I really endeavored for a very long period of time
00:28:23.04 to try to coax one or two of these species
00:28:26.11 into laboratory culture so that we could ask what they do,
00:28:29.12 and I had an idea:
00:28:31.19 maybe some of these are these acetogens
00:28:33.12 that can take hydrogen and CO2
00:28:35.07 and make acetate.
00:28:37.01 The problem with that is that activity
00:28:39.05 was not known to occur in any spirochete,
00:28:42.08 and none of these spirochetes from the termite
00:28:44.07 had been cultured.
00:28:46.01 So, it's an idea, but what you really need to do
00:28:47.24 is get one of these into the laboratory and ask it,
00:28:50.22 are you capable of doing that?
00:28:52.08 And, if not, what do you do?
00:28:55.20 So, what is a spirochete?
00:28:57.25 This is a political cartoon from the early 70s,
00:29:00.29 and will sort of shoot right over the heads
00:29:04.02 of almost all of us,
00:29:05.27 but I still include it because it's a little bit
00:29:08.21 of American history that Richard Nixon's first vice president,
00:29:11.23 Spiro T. Agnew,
00:29:14.15 sort of had to leave office for some misdealings that he had,
00:29:19.16 and this was before even the Watergate scandal blew up.
00:29:22.25 So, when I say spirochete,
00:29:24.26 I'm not talking about a spirochete,
00:29:27.21 but a different organism,
00:29:30.15 and these are bacteria
00:29:33.17 that have a very unusual body plan.
00:29:35.28 So, many bacteria can swim
00:29:38.04 and they have flagella that extend into the extracellular milieu
00:29:42.10 and act like propellers,
00:29:44.25 but spirochetes have flagella
00:29:46.17 that extend not out of the cell,
00:29:49.14 but out past the first membrane,
00:29:52.21 but lie in between the inner and the outer membrane
00:29:55.03 of the bacterial cell,
00:29:58.27 and actually will wrap around the cell, okay?
00:30:02.12 If you look at a cross-section, you can see what I mean.
00:30:05.24 These are the flagella that lie
00:30:08.06 in between the inner and the outer membrane,
00:30:10.04 and when those flagella turn
00:30:12.05 the entire cell becomes a propeller,
00:30:14.12 as opposed to being attached to the propeller,
00:30:17.01 and spirochetes are known to be able to move
00:30:19.11 into very thick, viscous environments,
00:30:22.02 and are sort of the world record holders in the microbial world
00:30:25.07 for being able to wiggle into really thick and tight places.
00:30:29.12 And, all the organisms that have this body plan
00:30:31.10 are also related to each other,
00:30:33.09 so it's both a related group
00:30:35.19 by genetics and by their body plan.
00:30:40.01 So, these are microscopic images
00:30:42.06 of the first termite gut spirochete that was isolated.
00:30:45.29 We call this organism Treponema primitia,
00:30:49.01 and the first thing we wanted to ask
00:30:51.21 was whether it was really a spirochete,
00:30:54.06 and what I want to point out here is that
00:30:56.09 if you look at it with a whole-cell negative stain
00:30:58.05 by transmission electron microscopy,
00:31:00.05 or a thin section,
00:31:02.02 it has these hallmark flagella
00:31:05.18 that are lying in between the inner and the outer membrane.
00:31:08.23 Now, the second thing we learned about Treponema primitia
00:31:11.28 is that it is actually a hydrogen+CO2 acetogen.
00:31:15.11 Hydrogen stimulated its growth
00:31:18.01 and it consumed hydrogen and made acetate
00:31:21.18 in the expected four hydrogen to one acetate stoichiometry.
00:31:25.26 You could also grow this organism
00:31:27.27 under radioactive carbon dioxide,
00:31:29.29 and when you do that
00:31:32.07 it generates radioactive acetate,
00:31:34.21 so it's fixing CO2 into organic carbon,
00:31:39.02 and when you ask,
00:31:41.19 are both carbon positions of acetate labeled?
00:31:44.14 They were.
00:31:46.10 And lastly, there are enzymes associated with this pathway,
00:31:49.17 and this organism exhibits them all.
00:31:52.03 So, it is a bonafide hydrogen/CO2 acetogen
00:31:55.12 and, although a close relative
00:31:57.07 of the organism that causes syphilis,
00:31:59.08 this organism actually plays a key role
00:32:01.17 in the fermentation of food in the termite,
00:32:04.03 and in taking some nutritional value of that wood
00:32:07.04 and passing it back on to the termite.
00:32:10.06 Now, we have been studying this organism
00:32:12.22 for almost 20 years now,
00:32:14.21 and one of the things that we've done
00:32:16.28 is really looked at its genes for this pathway,
00:32:19.10 and used our study of these genes, in red,
00:32:23.15 to do comparative studies in other termites
00:32:26.07 and also in this particular termite and ask,
00:32:29.29 can we learn if this is the only acetogen,
00:32:33.18 or if there are other species,
00:32:35.13 and who are those other species?
00:32:37.25 So, we can take an approach, now,
00:32:39.28 where we can take a look at the diversity
00:32:42.16 of these genes for this pathway in this one termite,
00:32:45.23 but also in members of these two other major subgroups
00:32:48.29 of the termite line of descent,
00:32:51.11 and also in the Wood Roach.
00:32:53.11 And so, we've been learning a lot
00:32:55.16 about the diversity of organisms
00:32:57.18 that can carry out that metabolism
00:32:59.24 in a diversity of different species
00:33:02.25 and actually major subgroupings of insects that eat wood.
00:33:07.25 Now, we've also been able
00:33:11.29 to isolate a second spirochete from Zootermopsis.
00:33:15.04 This one we call Treponema azotonutricium.
00:33:18.28 Now, this organism
00:33:21.06 plays a very different role in the symbiosis
00:33:23.08 with the termite and its hindgut microbes.
00:33:26.01 So, if you think about it,
00:33:28.02 wood is not only tough to eat,
00:33:30.23 it's not a very good source of protein.
00:33:33.10 You know, at best, it's like a potato, right?
00:33:36.29 It's got a lot of polysaccharide,
00:33:38.23 a lot of carbs,
00:33:40.25 but not a lot of nitrogen.
00:33:43.00 So, you can be degrading that wood
00:33:45.06 and providing the calories to the host,
00:33:48.01 but that's only one of the hosts major problems in life.
00:33:50.26 The other problem is to make protein,
00:33:54.04 and so if you ask what's going to
00:33:56.08 limit the ability of this insect that's eating a block of wood,
00:33:58.27 or your home,
00:34:00.25 what's going to limit its proliferation,
00:34:03.15 part of the story is on protein.
00:34:06.10 So, it turns out that some 35 years ago
00:34:10.06 John Breznak, at Michigan State University,
00:34:13.10 discovered that termites contain microbes
00:34:15.25 that can take atmospheric nitrogen,
00:34:17.25 which is bathing all of us in the atmosphere all around us,
00:34:21.01 and can take that and turn it into protein,
00:34:24.09 that can then be fed to the insect.
00:34:27.18 And, this particular spirochete,
00:34:29.26 when we got it into culture in the laboratory,
00:34:32.19 we could show can do that same activity.
00:34:35.28 So, this is one of the organisms that we say
00:34:38.17 can exhibit diazotrophic growth.
00:34:40.18 It can grow with N2 gas
00:34:43.11 as its sole source of eventual protein,
00:34:46.08 and it shows several activities
00:34:48.17 which are associated with that activity,
00:34:51.04 and therefore it's playing a role
00:34:53.13 in taking a very abundant but unusable source of nitrogen
00:34:56.01 around the insect
00:34:57.25 and actually feeding the insect protein-level nitrogen.
00:35:01.29 I'll mention too that this particular organism
00:35:04.29 is unlike the first.
00:35:06.24 It's not an organism that can consume hydrogen
00:35:10.20 and fix CO2 into acetate.
00:35:13.07 It actually degrades sugars
00:35:15.14 and produces hydrogen that it can feed to the other spirochete.
00:35:18.19 So, it plays different roles in this symbiosis.
00:35:23.05 So, I've talked to you about some protozoa,
00:35:28.13 about some methanogenic archaea,
00:35:32.20 and about just some of the many, many bacteria
00:35:35.09 that you can find in a termite.
00:35:38.02 There are many other stories I could tell you,
00:35:40.05 but I want to leave off with my talk
00:35:41.27 by just pointing out that
00:35:46.06 we've talked about these three major groups in this environment.
00:35:49.15 That environment is dominated
00:35:51.11 by members of diverse groups
00:35:53.11 which are very different from the host itself.
00:35:55.28 So, just like the kelp forests,
00:35:57.23 these are environments
00:35:59.28 which are dominated by genetic groups
00:36:02.01 and groups performing physiologies
00:36:03.26 which are very different from sort of the paradigms of biology,
00:36:07.01 and that we learn a lot by studying them.
00:36:09.27 So, I think with that,
00:36:11.15 I'll close this introductory talk
00:36:14.08 on termite gut microbiology.
00:36:16.15 There are many, many general aspects
00:36:18.14 that we could discuss,
00:36:20.00 and of course can go into many other aspects
00:36:22.08 in great detail.
00:36:23.20 And so, I'd like to point out that
00:36:25.28 there have been many research groups and scientists
00:36:28.07 that have been working on termites for well over a century,
00:36:30.25 and I've tried to encapsulate some of their findings,
00:36:33.00 as well as the findings of my own laboratory,
00:36:35.21 into the presentation that I have given you today.
00:36:38.16 Thank you very much.

This Talk

Speaker: Jared Leadbetter

Audience:

Student
Educators of Adv. Undergrad / Grad
Researcher

Recorded: July 2014

Talk Overview

Leadbetter begins his seminar by comparing the biological diversity in the gut of the termite to the diversity found in the Sargasso Sea. The hindgut of the dampwood termite Zootermopsis nevadensis has one of the highest densities of microbes found on earth and includes bacteria, archaea and eukaryotes of all shapes and sizes. Protozoa in the termite gut breakdown the polysaccharides in wood to produce acetate; a food source for the termite. The breakdown of wood also produces H₂ and CO₂. Archaea in the gut convert the H₂ and CO₂ to methane, while bacteria compete to convert the H₂ and CO₂ to more acetate thus reducing methane production. Leadbetter and his colleagues were the first to identify and successfully culture acetogenic spirochetes from the termite gut. They have since found gut bacteria capable of fixing atmospheric nitrogen and producing protein. Using genetics, Leadbetter is now studying the diversity and evolution of termites and their gut bacteria.

Speaker Bio

Jared Leadbetter

Jared Leadbetter

Jared Leadbetter was an undergraduate biology student at Goucher College when he attended a summer course on microbial diversity at the Marine Biological Laboratory in Woods Hole, Massachusetts. It was here that he first became fascinated with the amazing environment of the termite gut. Leadbetter went on to study termite gut microbes for his PhD… Continue Reading

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