Do Bacteria Have Bacteria?

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Drew Smith:

Do bacteria have bacteria? Yes.

We know that bacteria range in size from 0.2 micrometers to nearly one millimeter. That’s more than a thousand-fold difference, easily enough to accommodate a small bacterium inside a larger one.

Nothing forbids bacteria from invading other bacteria, and in biology, that which is not forbidden is inevitable.

We have at least one example: Like many mealybugs, Planococcus citri has a bacterial endosymbiont, in this case the β-proteobacterium Tremblaya princeps. And this endosymbiont in turn has the γ-proteobacterium Moranella endobia living inside it. See for yourself:

Fluorescent In-Situ Hybridization confirming that intrabacterial symbionts reside inside Tremblaya cells in (A) M. hirsutus and (B) P. marginatus mealybugs. Tremblaya cells are in green, and γ-proteobacterial symbionts are in red. (Scale bar: 10 μm.)
Fluorescent In-Situ Hybridization confirming that intrabacterial symbionts reside inside Tremblaya cells in (A) M. hirsutus and (B) P. marginatus mealybugs. Tremblaya cells are in green, and γ-proteobacterial symbionts are in red. (Scale bar: 10 μm.)

I don’t know of examples of free-living bacteria hosting other bacteria within them, but that reflects either my ignorance or the likelihood that we haven’t looked hard enough for them. I’m sure they are out there.

Most (not all) scientists studying the origin of eukaryotic cells believe that they are descended from Archaea.

All scientists accept that the mitochondria which live inside eukaryotic cells are descendants of invasive alpha-proteobacteria. What’s not clear is whether archeal cells became eukaryotic in nature—that is, acquired internal membranes and transport systems—before or after acquiring mitochondria. The two scenarios can be sketched out like this:


The two hypotheses on the origin of eukaryotes:

(A) Archaezoan hypothesis.

(B) Symbiotic hypothesis.

The shapes within the eukaryotic cell denote the nucleus, the endomembrane system, and the cytoskeleton. The irregular gray shape denotes a putative wall-less archaeon that could have been the host of the alpha-proteobacterial endosymbiont, whereas the oblong red shape denotes a typical archaeon with a cell wall. A: archaea; B: bacteria; E: eukaryote; LUCA: last universal common ancestor of cellular life forms; LECA: last eukaryotic common ancestor; E-arch: putative archaezoan (primitive amitochondrial eukaryote); E-mit: primitive mitochondrial eukaryote; alpha:alpha-proteobacterium, ancestor of the mitochondrion.

The Archaezoan hypothesis has been given a bit of a boost by the discovery of Lokiarcheota. This complex Archaean has genes for phagocytosis, intracellular membrane formation and intracellular transport and signaling—hallmark activities of eukaryotic cells. The Lokiarcheotan genes are clearly related to eukaryotic genes, indicating a common origin.

Bacteria-within-bacteria is not only not a crazy idea, it probably accounts for the origin of Eucarya, and thus our own species.

We don’t know how common this arrangement is—we mostly study bacteria these days by sequencing their DNA. This is great for detecting uncultivatable species (which are 99 percent of them), but doesn’t tell us whether they are free-living or are some kind of symbiont. For that, someone would have to spend a lot of time prepping environmental samples for close examination by microscopic methods, a tedious project indeed. But one well worth doing, as it may shed more light on the history of life—which is often a history of conflict turned to cooperation. That’s a story which never gets old or stale.

This post originally appeared on Quora. Click here to view.

Why is Winnie the Pooh Called a Pooh?

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iStock.com/CatLane

Since A.A. Milne published the first official Winnie the Pooh story in 1926, the character has become beloved by children across many generations. Milne’s writing clearly struck a chord, and the character’s many subsequent TV and film adaptations have endeared him to an even wider audience.

But why is Winnie called a Pooh rather than a bear? Given that most children (and grown-ups, for that matter) have a different idea of what a Pooh is, how has the name stuck?

The answer lies back in the 1920s.

In fact, when first introduced by Milne, Winnie wasn’t even Winnie. Initially, he went by the name of Edward Bear, before changing to Winnie in time for that aforementioned official 1926 debut. The "Winnie" part of the name came from a visit to the London Zoo, where Milne saw a black bear who had been named after the city of Winnipeg, Canada.

As for Pooh? Well, originally Pooh was a swan, a different character entirely.

In the book When We Were Very Young (the same book that introduced Edward Bear), Milne wrote a poem, telling how Christopher Robin would feed the swan in the mornings.

He told how Christopher Robin had given the swan the name "Pooh," explaining that “this is a very fine name for a swan, because if you call him and he doesn’t come (which is a thing swans are good at), then you can pretend that you were just saying ‘Pooh!’ to show him how little you wanted him."

Milne indeed knew what he was doing by using such a word. The names "Winnie" and "Pooh" were soon brought together, and Winnie the Pooh was born. Milne still took a little time out to explain why Winnie was a Pooh, though.

As he would write in the first chapter of the first Winnie the Pooh book, “But his arms were so stiff ... they stayed up straight in the air for more than a week, and whenever a fly came and settled on his nose he had to blow it off. And I think—but I am not sure—that that is why he is always called Pooh."

It's not the most convincing explanation, but it's a formal explanation nonetheless.

Not that the reasoning ultimately mattered too much. The name stuck, having never seen a focus group in its life. A much loved childhood character, with a vaguely funny name, would go on to superstardom. And even be honored with his own holiday, Winnie the Pooh Day, which occurs annually on January 18th.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

Why is Winnie the Pooh Called a Pooh?

iStock.com/CatLane
iStock.com/CatLane

Since A.A. Milne published the first official Winnie the Pooh story in 1926, the character has become beloved by children across many generations. Milne’s writing clearly struck a chord, and the character’s many subsequent TV and film adaptations have endeared him to an even wider audience.

But why is Winnie called a Pooh rather than a bear? Given that most children (and grown-ups, for that matter) have a different idea of what a Pooh is, how has the name stuck?

The answer lies back in the 1920s.

In fact, when first introduced by Milne, Winnie wasn’t even Winnie. Initially, he went by the name of Edward Bear, before changing to Winnie in time for that aforementioned official 1926 debut. The "Winnie" part of the name came from a visit to the London Zoo, where Milne saw a black bear who had been named after the city of Winnipeg, Canada.

As for Pooh? Well, originally Pooh was a swan, a different character entirely.

In the book When We Were Very Young (the same book that introduced Edward Bear), Milne wrote a poem, telling how Christopher Robin would feed the swan in the mornings.

He told how Christopher Robin had given the swan the name "Pooh," explaining that “this is a very fine name for a swan, because if you call him and he doesn’t come (which is a thing swans are good at), then you can pretend that you were just saying ‘Pooh!’ to show him how little you wanted him."

Milne indeed knew what he was doing by using such a word. The names "Winnie" and "Pooh" were soon brought together, and Winnie the Pooh was born. Milne still took a little time out to explain why Winnie was a Pooh, though.

As he would write in the first chapter of the first Winnie the Pooh book, “But his arms were so stiff ... they stayed up straight in the air for more than a week, and whenever a fly came and settled on his nose he had to blow it off. And I think—but I am not sure—that that is why he is always called Pooh."

It's not the most convincing explanation, but it's a formal explanation nonetheless.

Not that the reasoning ultimately mattered too much. The name stuck, having never seen a focus group in its life. A much loved childhood character, with a vaguely funny name, would go on to superstardom. And even be honored with his own holiday, Winnie the Pooh Day, which occurs annually on January 18th.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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