I'm one of those dreadful animal-centric zoologically inclined biologists. Plants? What are those? Fungi? They're related to metazoans somehow. Lichens? Not even on the radar. The first step in fixing a problem, though, is recognizing that you have one. So I confess to you, O Readers, that my name is PZ, and I am a metazoaphile. But I can get better.
My path to opening up to wider horizons is to focus on what I find most interesting about animals, and that is that they are networks of cells driven by networks of genes that generate patterned responses of expression by cell signaling, or communication. See? I'm already a little weird. Show me a baby bunny, and I don't just see a cute little furry pal with an adorable twitchy nose, I see an organized and coherent array of differentiated tissues that arose by a temporal sequence of cell-cell interactions, and I just wanna open him up and play with his widdle epithelial sheets and dismantle his pwetty ducts and struts and fibers and fluids, oochy coo. And ultimately, I want to take apart each cell and ask why it has its particular assortment of genes switched off and on, and how its state affects its neighbors and the whole of the organism.
Which means, lately, that I've acquired a growing interest in bacteria. If I were 30 years younger, I could probably be seduced into a career in microbiology.
There are a couple of reasons why an animal-centric biologist would be interested in bacteria. One is the principle of it; the mechanisms that animal cells use to build complex arrangements of tissues were all first pioneered in single-celled organisms. We have elaborated and added details to gene- and cell-level phenomena, but it's a collection of significant quantitative differences, with nothing known that is essentially new in metazoan cells. All the cool stuff was worked out by evolution in the 3-4billion years before the Cambrian, a potential that simply blossomed in the past half-billion years into big conglomerations of cells. Understanding how the building blocks of multicellularity work individually ought to be a prerequisite to understanding how the assemblages work.
But there's another reason, too, a difference in perspective. It is our conceit to regard ourselves as individuals of Homo sapiens, a body of cells clonally derived from a single human cell. It's not true. It turns out that each one of us is actually a whole population of species, linked by our evolutionary history and lumbering through the world as a team. Genus Homo is also genera Escherichi and Bacteroidetes and Firmicutes and many others.

Physiology
Let's begin with the most widely known factor: we're mostly bacterial in cell numbers, with about ten times as many bacterial cells as human cells. Most of these are nestled deep in our guts, where they are indispensible. In mammals, they help break down complex polysaccharides which we can then absorb through the wall of the digestive tract — these are compounds that would be simply lost without bacterial assistance. Even more dramatically, termite guts contain colonies of bacteria that produce enzymes to break down cellulose. Another insect, aphids, live in plant saps which have negligible protein components, and they rely on gut bacteria that can synthesize nine essential amino acids. One cool feature is that the bacteria can't complete the synthesis of leucine; the last step is carried out by aphid enzymes. The synthetic pathway is split acros two different species!
Another weird twist is that gut bacteria can affect morphology (or vice versa; physiology influences which gut bacteria thrive). Mice with a genetic predisposition to obesity were found to have a different distribution of gut bacteria; fat mice are full of Firmicutes, while lean mice are loaded with Bacteroidetes. Something in the genetics of the obese mice seems to favor the proliferation of that one species. Cause and effect is not so easily separated, though, since doing a fecal transplant and inoculating the guts of germ free mice with the bacteria from obese mice vs. lean mice has a surprising effect: the mice given obese mouse fecal enemas subsequently increased their body fat by 60%. The bacteria promoted more fat storage in the host animal.
So what, you may be thinking, it's mice. However, it turns out that obese humans tend to have reduced amounts of Bacteroidetes species in their guts than lean people, and weight loss is accompanied by an increase in Bacteroidetes. Fecal transplants are not recommended as a weight loss technique…at least not yet.
They have worked for some other problems. Crohn's disease and ulcerative colitis are diseases that involve intestinal inflammation, and they're also associated with imbalances in the species distribution of gut bacteria. Some promising treatments have involved collecting feces from healthy individuals, and using a nasogastric tube to inoculate the guts of Crohn's patients with the stuff. Ick, I know, but it seems to have worked surprisingly well in a small number of patients.
Development
Bacteria are present in the gut from a very early age, and populate the digestive epithelia. There must be interactions going on, and it appears that the bacteria are actually regulating the growth of the gut lining.
Germ-free zebrafish lines have no gut bacteria, and they also have problems. The intestinal lining arrests its development and fails to fully differentiate; the lining also grows much more slowly. They also have difficulty absorbing some nutrients. Add bacteria, though, and growth and differentiation resume. This is a case where the developmental program and the bacterial influences are interdependent, and it makes sense — they've co-evolved.
It's not just fish, either — these are conserved interactions across the vertebrates. Mice exhibit the same dependence on gut flora for development of the intestinal lining.
The very best example of a developmental dependence on bacteria, though, is in squid. The bobtail squid has a light-emitting organ that relies on colonization by a luminescent bacterium, Vibrio fischeri. The animal gleans the bacteria from the water with a special ciliated epithelium and secreted mucus that seems to be just the right flavor for Vibrio, and the bacteria migrate deep into the light-emitting organ. Once colonized, the squid dismantles the harvesting cilia and downregulates the secretion of mucus. If no bacteria of the right species are present, it maintains the cilia. If the bacteria in the organ die, resumes mucus production.

Bacterial symbionts induce light-organ morphogenesis in squid. A Adult squid (E scolopes). SEM images of epithelial fields before B and after C regression of ciliated appendage. Scale bar, 50 mm. Ciliated appendages are marked by an orange dashed line.
Evolution
If something affects development and physiology, it affects evolution, so evolutionary importance is simply rather unavoidable. However, there's also one somewhat surprising observation (to me, at least — microbiologists probably expect it): different species of related organisms can have different microbial populations, even when raised in identical conditions. Different Hydra species in the lab under controlled conditions have recognizably different populations of bacteria living on their epithelia, and Hydra of the same species collected in the wild have similar distributions of species. The properties of each Hydra species uniquely favor different distributions of bacteria, and the bacteria are also preferentially colonizing particular species of Hydra.
Hydra are wonderful experimental animals in that one can ablate stem cells for a particular tissue type, and still get an animal that develops and lives; do the same thing to a vertebrate, for instance knocking out the mesodermal lineage in the embryo, and you get an aborted blob. In Hydra, you get a tissue that survives and is colonized by bacteria…but the kinds of bacteria populating it is different from the populations in the intact animal. The animal and the bacteria are swapping molecular signals that specify favored relationships. Again, these are coevolved populations that recognize molecular properties of the host and symbiont.
This is all getting very complicated. I'm used to thinking in terms of networks of genes: there are regulatory interactions between genes in a single cell that establish cell-type specific patterns of gene activity; all express a common core of genes, but different cell types, such as a neuron vs. a cell of the digestive epithelia, will also have their own unique special-purpose genes switched on. I'm also comfortable thinking of networks of cells: cells are in constant negotiations with their neighbors, mainting a common pattern of expression within a tissue, and defining interacting edges with other tissues. Cells are continually sending out messages about their state into the system and responding to local and global signals. All this is part of the normal process of thinking developmentally.
Now, though, there's another layer: we have to think in terms of networks of species that cooperate in the development and physiology of individual multi-cellular organisms. Purity is compromised. My precious animalia — they're inconceivable without bringing bacteria into the picture.
Fraune S, Bosch TCG (2010) Why bacteria matter in animal development and evolution. Bioessays 32:571-580.

105 Comments
John Wilkins · 21 July 2010
Elio Schaechter had a little piece on this recently in which he tracked down the claim that the ratio of endogenous to commensual cells is 10:1.
John Wilkins · 21 July 2010
I mean of course the other way round...
MrG · 21 July 2010
There's been a lot of work lately on the human "microbiome" of symbiotic microoganisms ... but just recently I've been seeing reports of early work on a complementary "virome".
It seems the complex human microecology also includes an extensive set of viruses -- and they may well be a necessary component of the whole.
That makes me just stare off into space: "HOW?" How can an obligate parasite be a good member of the community? Well, for sure this is fun stuff.
MrG · 21 July 2010
Jorge · 21 July 2010
Not to mention all those beautifully useful bacteria that live within our metazoan cells and which we call mitochondria. Bacterial symbiosis goes way, waaay back to the very early stages of biological evolution.
Mike Elzinga · 21 July 2010
Mattir · 21 July 2010
There was an interesting article in Science News recently about C-section babies being colonized by relatively weird and potentially nasty hospital bacteria rather than mom-vaginal-tract bacteria and how this might be a reason for giving C-section babies a dose of mom-bacteria right after birth.
I can't get the code to work right, but here's the citation: http://www.sciencenews.org/view/generic/id/60461/title/Baby%E2%80%99s_first_bacteria_depend_on_birth_route
MrG · 21 July 2010
qetzal · 21 July 2010
ckc (not kc) · 21 July 2010
(psst - plants have differentiated tissues, too)
robert van bakel · 21 July 2010
Does this imply that todays parasites beyond bacterial in size, for instance, worm, tick, mite, might evolve into a mutually beneficial relationship?
fnxtr · 22 July 2010
fnxtr · 22 July 2010
There must be a more hygienic -- or at least less aesthetically offensive -- way to introduce Bacteriodetes into a patient?
Dana Hunter · 22 July 2010
Bob O'H · 22 July 2010
Ryan K · 22 July 2010
Very nice post, PZ! I'm continually fascinated by host/microbe interactions. Thanks for pointing to that article; I'll have to get that and read it now.
My name is Ryan and I'm an angiospermaphile who can't be bothered with "primitive" ferns or bryophytes.
Roger · 22 July 2010
"Show me a baby bunny, and I don't just see a cute little furry pal with an adorable twitchy nose, I see an organized and coherent array of differentiated tissues that arose by a temporal sequence of cell-cell interactions, and I just wanna open him up and play with his widdle epithelial sheets and dismantle his pwetty ducts and struts and fibers and fluids, oochy coo."
Sir, I must warn you that this kind of attitude will not make you popular at dinner parties. I recommend either professional help or keeping very quiet about it.
Futhermore please extend my deepest sympathies to any collections of organized and coherent array of differentiated tissues that arose by a temporal sequence of cell-cell interactions, or "pets" as I call them, in your household.
snaxalotl · 22 July 2010
could anyone elaborate? when a hydra loses a tissue type, is it resurrecting an ancient relationship with bacteria that existed when tissues were colonies of unrelated species? (rather like completing the signal to V'Ger)
harold · 22 July 2010
Frank J · 22 July 2010
fnxtr · 22 July 2010
John Kwok · 22 July 2010
This comment has been moved to The Bathroom Wall.
John Kwok · 22 July 2010
This comment has been moved to The Bathroom Wall.
harold · 22 July 2010
harold · 22 July 2010
John Kwok · 22 July 2010
This comment has been moved to The Bathroom Wall.
Random Lurker · 22 July 2010
I've always wondered how gut bacteria get in there in the first place. I suspect it's related to Adam Savage's iconic statement, "There's poo everywhere!" but I've always wondered for sure.
Ntrsvic · 22 July 2010
Why are all of Kwok's comments being moved to the bathroom wall? They seem quite reasonable to me? or is it that Kwok has rubbed so many people the wrong way?
In a similar thread, one of my projects deals with a 'plant' toxin that might only be partially made by the plant, while the toxin may actually be finalized (if you will) by a fungus that lives on the plant.
MrG · 22 July 2010
PZ and JK have history. I do not blame PZ.
MrG · 22 July 2010
RBH · 22 July 2010
While we do have some site-wide bannees, PT contributors moderate the comment threads on their own posts. So we will occasionally differ on occasion with respect to who is persona non grata in those threads and on what will trigger moderation. IIRC, I have also on occasion moved John Kwok's comments to the BW when I deemed it appropriate to do so.
That said, I strongly recommend that this thread not get derailed into a discussion of moderation rather than commensuals.
MrG · 22 July 2010
Ntrsvic · 22 July 2010
Dale Husband · 22 July 2010
Colin Meier · 22 July 2010
Would this fall into Dawkin's 'Extended Phenotype'? Under that reading, it's possible (speculatively) the microbiome has exerted an effect for its own benefit on our genome. Although I suspect I'm misremembering the concept completely...
Dale Husband · 22 July 2010
Back on topic, I imagine that nearly all the symbiotic relationships we see today started out millions of years ago as parasitic in nature. But when a parasite kills its host, it is also likely to die. But when the parasite becomes less destructive and actually gives something back to the host, then the relationship becomes more sustainable for the parasites.
Imagine a flea or tick species that actually evolved the ability to inject vaccines to protect a host, like a dog or cat, from diseases instead of causing disease. Such a parasite would be far more successful, wouldn't it?
harold · 22 July 2010
PZ Myers · 22 July 2010
This has nothing to do with his politics (although those are pretty batty), and everything to do with the fact that Kwok is insane, obsessive, and bizarrely disturbed. He will continue to be promptly deleted from any post of mine.
harold · 22 July 2010
Scott Hatfield · 22 July 2010
Ah, PZ, going into withdrawal I see.
Nice post!
'Tis Himself · 22 July 2010
BedfordStuyvesant High in New York City) regardless of the topic being discussed. John Kwok is the subject of an article at Rationalwiki. It has a detailed explanation of how Kwok worked hard to get banned from Pharyngula.MrG · 22 July 2010
JK's politics may not be an irrelevant consideration. However, I don't think he would act any different if he was a Left-winger. Some of the verbiage would merely change.
Stanton · 22 July 2010
John Kwok · 23 July 2010
This comment has been moved to The Bathroom Wall.
John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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JT · 23 July 2010
Frank J · 23 July 2010
In an admittedly bizarre attempt to bring this thread back on topic I wonder what Bobby Jindal, who supposedly has a biology degree, and might have even taken a class taught by Ken Miller, would think of this. Maybe he could test his exorcism "theory" to see if getting rid of those species that "possess" us might save us from such "evils" as "Darwinism."
John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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MrG · 23 July 2010
william e emba · 23 July 2010
John Kwok · 23 July 2010
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Dale Husband · 23 July 2010
John Kwok · 23 July 2010
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MrG · 23 July 2010
John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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Dale Husband · 23 July 2010
JT · 23 July 2010
MrG · 23 July 2010
JT · 23 July 2010
John. You have NO RIGHT to condemn PZ for allowing "jokes" of that sort why YOU YOURSELF have done far worse. One of the first posts of yours I read, and one I'll allays remember and associate with you was when you "joked" to Rilke's Granddaughter (sp?) about how you looked forward to her "assuming room temperature soon". That, by your standards, and by mine, constitutes a death threat, and says all I need to know about your integrity.
fnxtr · 23 July 2010
Oh for fuck's sake get over yourselves!
PZ doesn't like you John. Suck it up and move on.
John Kwok · 23 July 2010
This comment has been moved to The Bathroom Wall.
John Kwok · 23 July 2010
This comment has been moved to The Bathroom Wall.
MrG · 23 July 2010
Actually, I will applaud.
John Kwok · 23 July 2010
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MrG · 23 July 2010
John Kwok · 23 July 2010
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John Kwok · 23 July 2010
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Hygaboo Andersen · 23 July 2010
Hygaboo Andersen · 23 July 2010
MrG · 23 July 2010
JT · 23 July 2010
Hygaboo Andersen · 23 July 2010
MrG · 23 July 2010
JT · 23 July 2010
MrG · 23 July 2010
JT · 23 July 2010
Ntrsvic · 23 July 2010
My bad.
MrG · 23 July 2010
MrG · 23 July 2010
fnxtr · 23 July 2010
Why is it so many of these anti-reality nutjobs have a sodomy fixation? Or is this the same troll with a different rubber mask on?
MrG · 23 July 2010
I think I'm going to gradually put together a zoology of creationists. This one's the "incoherent ranter", furiously tossing out all kinds of mad claims that barely make sense and demanding answers.
Then there's the "chapter & verse", who responds to any question by citing scripture.
However, I have a strong suspicion that somebody over on talk.origins has actually done this. I'll have to poke around.
PZ Myers · 23 July 2010
Next time I have to clean up Kwok's noise, I'm also going to delete every comment replying to him.
eric · 23 July 2010
MrG · 23 July 2010
Please. Kill any of my responses now.
MrG · 23 July 2010
I would classify "Beebee" Byers as an "aimless rambler".
But you know, taxonimists are noted for quarreling over their work. I don't think "troll" is useful in the naming system because in this case that's at the family level.
Hmm, somehow I recall Roadrunner / Coyote cartoons: "Roadrunner (hot-roddicus supersonicus)".
eric · 23 July 2010
MrG · 23 July 2010
Yeah, the problem with "troll" as a classification element is that it doesn't provide a distinction among them.
I thing Beebee might be better called an "aimless babbler".
And then there's the "BS degree", archetypically a computer scientist with a program that disproves evo science (but don't ask him if he's going to submit a paper to the appropriate journals); and "off the meds", Ray Martinez being a classic example -- the kind of people one feels embarrassed to argue with because they're clearly dysfunctional.
Kevin B · 23 July 2010
John Kwok · 23 July 2010
This comment has been moved to The Bathroom Wall.
John Kwok · 23 July 2010
This comment has been moved to The Bathroom Wall.
Frank J · 23 July 2010
Dale Husband · 23 July 2010
Frank J · 23 July 2010
MrG:
Note: The article in my first link was based on an early version of NCSE's "creation-evolution continuum." The latter was later updated as ID showed itself to be a "big tent" scam, and not just a more common-descent-friendly version of OEC.
MrG · 23 July 2010
Eh, waddya expect -- explosions, crocoducks, whatever.
MrG · 23 July 2010
harold · 23 July 2010
snaxalotl · 24 July 2010
D. P. Robin · 27 July 2010
I think that Hygaboo Andersen's homophobic slur against those who accept evolutionary science (see his link in http://pandasthumb.org/archives/2010/07/no-metazoan-is.html#comment-224846 ) entitles him/her a perminant ticket to the BW.
dpr