The Cell Within the Cell

[THEO] Okay, picture this. You're a cell, and inside you — not stuck to your surface, actually inside — is another living thing. A whole separate organism with its own genome, eating, replicating, maybe even doing you favors. That's not science fiction. That's basically how you got your mitochondria.

[DR. MARA] It's the oldest story in eukaryotic biology. Every cell with a nucleus is a chimera — a host that, somewhere around a billion and a half years ago, swallowed a bacterium and never let it go. That bacterium became the mitochondrion. A second event, later, brought in a cyanobacterium and gave us chloroplasts.

[THEO] So endosymbiosis isn't a weird edge case. It's the main event. It built the kind of cell we're made of.

[DR. MARA] Right. And the reason it's worth a whole segment is that these partnerships are still happening, all around us, at every stage of that journey — from a bacterium that just moved in last week, to one that's been there so long it's barely a separate thing anymore. If you line them up, you can almost watch the transition happen in slow motion.

[THEO] Which is the through-line today. How does a free-living microbe become a resident? And the first stop isn't even inside a cell — it's one bacterium talking to another.

[DR. MARA] This is the 2017 Bartonella work, and I'd argue it's the conceptual root. Before you can live inside something, you need a way to deliver molecules across a boundary into another cell. Bacteria already have that machinery — the type IV secretion system, the T4SS. It's a molecular syringe. Normally it's how they pass plasmids around during conjugation.

[THEO] The USB port of the bacterial world. Plug in, transfer data.

[DR. MARA] More or less. The clever part of this paper is how they measured it. They built a sensor called CRAfT — the donor bacterium makes an effector protein fused to Cre recombinase, and the recipient carries a loxP site wired to an antibiotic resistance switch. If the protein actually gets injected, Cre flips the switch, and the recipient survives on the plate. A permanent molecular receipt.

[THEO] And the numbers there are wild. Transfer of the protein happened at basically a hundred percent per donor cell. But transfer of the plasmid itself — the DNA — was only a tenth of a percent to one percent.

[DR. MARA] Which tells you the system is far better at shipping proteins than DNA. And the protein they caught it shipping was VbhT, a FicT toxin. Here's the evolutionary punchline: that toxin started life as half of a toxin-antitoxin module — a little bacterial suicide pact where a cell makes a poison and its own antidote. They showed it's become an effector the bacterium injects into other cells.

[THEO] So a thing that was an internal self-control gadget got repurposed into a weapon you fire at your neighbor.

[DR. MARA] An evolutionary intermediate. That's the word that matters. You're watching a module slide from one job to another. And that logic — repurposing existing machinery to act across a cell boundary — is exactly what endosymbiosis needs.

[THEO] Okay, so that's the delivery problem. Next turning point jumps us from "talking across cells" to actually living inside one — and this one's a nightmare to study.

[DR. MARA] The 2021 microsporidia paper. Microsporidia are fungal-ish intracellular parasites, and these particular ones feminize their host — a crustacean, Gammarus roeselii. They're passed strictly from mother to offspring, vertically, through the egg. They never make an environmental spore you could filter out and purify.

[THEO] So you can't separate the symbiont from the host to sequence it. They're physically inseparable.

[DR. MARA] So you don't separate them. You sequence everything together — host DNA and symbiont DNA in one messy pile — and you deconvolve it computationally afterward. They pulled out assemblies for three microsporidia species plus a draft genome of the host.

[THEO] And then the lovely comparative move: feminizing is also something Wolbachia does, and Wolbachia's a bacterium.

[DR. MARA] Convergent evolution. A eukaryote and a bacterium independently arrived at manipulating host sex. They nominated candidate genes with DNA-binding and membrane-fusion functions. Nothing proven mechanistically yet — these are candidates — but it says the "live inside and manipulate the host" strategy gets reinvented over and over.

[THEO] Which brings us to my favorite, because it's endosymbiosis inside the endosymbiont. Midichloria.

[DR. MARA] Midichloria mitochondrii. A bacterium that lives inside the mitochondria of ticks. Inside the organelle. The question was: how does it move through the cell? One model, the Bdellovibrio-like model, said it behaves like a predatory bacterium — invade a mitochondrion, consume it, burst out, move on.

[THEO] Lyse and leave. Smash and grab.

[DR. MARA] The 2021 study tested that with quantitative electron microscopy and network simulations, and rejected it. The bacterium traverses the mitochondrial network — the mitochondria are connected, they form a web — and it moves between organelles rather than destroying them. And that gentler behavior reproduces the power-law colonization pattern they see in tick eggs.

[THEO] This is the part I love as a physics person — they settled a biological mechanism with a distribution. The shape of the colonization curve only comes out one way.

[DR. MARA] And it matters for the arc, because a good long-term tenant doesn't burn down the house. Bdellovibrio is a predator. Midichloria is a lodger. That restraint is the signature of a partnership on its way to becoming permanent.

[THEO] Then the field pivots hard toward tools — the two 2023 entries. Akkermansia and the transposon work.

[DR. MARA] Akkermansia muciniphila lives in your gut mucus and eats mucin — the glycoprotein your gut lining secretes. The 2023 transposon screen found its MUL system, a pili-periplasmic complex, actively imports mucin into intracellular compartments they call mucinosomes.

[THEO] It's not grazing on the surface. It's internalizing its food. And they found the MUL system is what lets it outcompete other gut microbes, and it even tamps down the host's own sterol-synthesis genes.

[DR. MARA] And the method underneath both 2023 papers is transposon insertion sequencing — RB-TnSeq, randomly barcoded. You knock out every gene across a huge library at once, barcode each mutant, and read out which ones lose the competition in a real gut. It's how you go from "this organism is interesting" to "this gene does this job."

[THEO] Which is the toolkit the whole field needed. You can't engineer a symbiosis you can't interrogate gene by gene.

[DR. MARA] And that sets up the newest paper, which is almost cheeky. 2024 — they put red algal chloroplasts from Cyanidioschyzon merolae inside mammalian CHO-K1 cells.

[THEO] And the chloroplasts kept doing photosynthesis. Photosystem II electron transport, running, for at least forty-eight hours, inside an animal cell.

[DR. MARA] First confirmed photosynthesis in an animal cell. It's a tiny, gene-rich chloroplast — a 243-gene genome — and it stayed functional. Now, forty-eight hours is not a stable endosymbiosis. Nobody's claiming the animal cell adopted it.

[THEO] But it's the whole billion-year story in a dish. Take the organelle, put it in a host that never had it, and see if the lights stay on.

[DR. MARA] And they did. Which is where the arc points — from watching ancient partnerships, to measuring them, to building new ones deliberately. We've gone from reading the history to running the experiment.

[THEO] The cell within the cell, on purpose. That's where we'll leave it — back after the break.