Engineering The Unengineerable Microbe

[THEO] Okay, picture this. You've got a microbe — maybe it eats cellulose, maybe it does photosynthesis in some weird way nobody's reproduced in a lab, maybe it lives in a hot spring and makes a molecule you'd love to produce at scale. And you want to engineer it. Give it a new gene, tune a pathway. There's just one problem.

[DR. MARA] You can't get the DNA inside.

[THEO] You can't get the DNA inside! Which, when you say it out loud, sounds almost silly. We've been moving genes around since the seventies.

[DR. MARA] In a handful of organisms. That's the part people outside the field miss. Nearly everything we know about genetic engineering runs through maybe three or four workhorses — E. coli, baker's yeast, a couple of others. Everything else is what we politely call "recalcitrant." The DNA goes in, and the cell either destroys it or never takes it up in the first place.

[THEO] And this matters because the interesting biology is mostly out there, not in the workhorses. The organism that degrades plant biomass for biofuels, the one that fixes carbon in a novel way — those are the ones we can't touch. So the whole subject today is really: how do you breach the wall of a cell that doesn't want your DNA?

[DR. MARA] Let me define the obstacles, because there are two distinct ones and they get conflated. First, the physical barrier — getting a big charged molecule across a membrane, and in bacteria across a cell wall, in fungi across a thick wall too. Second, the biological defense. Many microbes carry restriction-modification systems — enzymes that recognize foreign DNA as foreign and chop it up. Your own DNA is methylated in a specific pattern, the invader's isn't, so it gets cut.

[THEO] That's the microbial immune system, basically. Self versus non-self at the molecular level.

[DR. MARA] Precisely. So any delivery method has to solve the physical problem and ideally evade or disarm the defense. The arc of this field is really people chipping at both.

[THEO] So where does the story start? Our oldest paper is 1994, and it's not even bacteria — it's a fungus, Aspergillus niger.

[DR. MARA] And it's a lovely starting point because it shows the physical problem in its rawest form. The classic way to transform a filamentous fungus was protoplasting — you strip off the cell wall entirely with lytic enzymes, leaving a naked, fragile protoplast, then coax DNA in. Tedious, finicky, hard to reproduce.

[THEO] Strip the castle walls down to nothing, basically, so you can walk in the front door — except now the castle falls over.

[DR. MARA] What this 1994 work showed is you could skip most of that. Take intact germinating conidia — the spores, just as they're waking up and starting to grow — and electroporate them.

[THEO] Electroporation — that's the one I actually love from a physics angle. You hit the cells with a short, strong electric pulse and it transiently pokes holes in the membrane. The field literally destabilizes the lipid bilayer, pores open for a moment, DNA slips through, and the membrane reseals. No enzymes required.

[DR. MARA] They got about a hundred colonies per microgram of plasmid from intact conidia. Modest, but it worked without full protoplasting. And a little enzyme pretreatment — loosening the wall rather than removing it — doubled the frequency.

[THEO] So that's the first turning point: you don't have to demolish the wall, you just have to weaken it enough and then zap. Timing matters — the germinating spore is a window.

[DR. MARA] Then the field moves to bacteria and gets much more quantitative. 2005, Choi and colleagues, Pseudomonas aeruginosa. Their contribution is almost embarrassingly practical — a ten-minute prep in a microcentrifuge to make the cells electrocompetent.

[THEO] Ten minutes! What was the old way?

[DR. MARA] For moving chromosomal DNA into P. aeruginosa, people leaned on transduction — using a phage to carry DNA — or conjugation, the bacterial mating-tube transfer. Both work, both are slow and constrained. This simple prep gave a ten-thousand-fold jump in electroporation efficiency.

[THEO] Four orders of magnitude from a better sample prep. That's the kind of thing that doesn't win prizes but quietly changes what everyone does on Monday morning.

[DR. MARA] And it reframes the problem. Suddenly electroporation isn't just viable, it displaces the older biological methods for that organism. The through-line is forming: push on the physical delivery hard enough and you can retire the workarounds.

[THEO] But not for everyone, right? Because the 2009 review is where somebody steps back and says, wait, there's no magic bullet.

[DR. MARA] Aune and Aachmann, yes. They survey the artificial delivery methods across difficult bacteria, and the honest conclusion is that no single approach dominates. You have to match the method's mechanism to the specific organism's barrier. If the problem is a tough wall, electroporation helps. If the problem is restriction-modification, you need to bypass that defense — maybe methylate your DNA in advance to mimic self, maybe use a strain with the enzymes knocked out.

[THEO] So this is the moment the field grows up a little. Stops looking for the one trick and starts thinking like, diagnose the patient first.

[DR. MARA] That's exactly the maturation. And it explicitly names restriction-modification bypass as a lever, which ties back to that self/non-self defense we set up.

[THEO] Now 2012 takes a hard left turn that I did not see coming — mitochondria.

[DR. MARA] It's a fascinating detour. The mitochondrion has its own genome, and it's been nearly impossible to transform. This work shows isolated plant and mammalian mitochondria will actively import double-stranded DNA through a channel called VDAC — the voltage-dependent anion channel in the outer membrane.

[THEO] So instead of forcing DNA through with a zap, the organelle has a doorway and it just... takes it in?

[DR. MARA] An active import route, yes. And there's a nice mechanistic wrinkle — plant mitochondria additionally require ANT, another transporter, plus terminal inverted repeats on the DNA. Mammalian import doesn't need those. Same organelle, different rules depending on lineage.

[THEO] Which rhymes with the 2009 message, doesn't it? Different system, different barrier, different key.

[DR. MARA] It does. And 2014 brings that idea back to whole bacteria from the other direction — natural transformation. Some bacteria take up DNA on their own, no electroporation needed. This review covers about eighty species that share conserved uptake machinery but wildly divergent rules for when they switch it on.

[THEO] So the hardware is the same, the software is different. And the big question is why bacteria even do this — just for food?

[DR. MARA] The evidence in that review favors genetic diversity and chromosome repair as the primary drivers, over nutrition. Which matters for us, because if you understand the regulatory trigger, you can switch competence on deliberately and let the cell import DNA for you.

[THEO] Which brings us right to 2018 and the toolbox mindset.

[DR. MARA] The non-model prokaryote engineering review — out of NREL and CU Boulder. It names the prize directly: organisms doing photosynthesis, autotrophy, cellulose degradation, all with real bioeconomy value, all still locked out of metabolic engineering. And it argues the design rules for building a genetic toolbox in these organisms just haven't been synthesized well.

[THEO] So twenty-four years from "zap a fungal spore" to "here's a design manual for the unengineerable." The through-line the whole way is: stop treating delivery as one problem.

[DR. MARA] Match the method to the barrier. Physical, enzymatic, regulatory. That's where it's heading — toolboxes, not tricks.

[THEO] Beautiful place to leave it. After the break, the mailbag — and someone's asking about tardigrades, so, buckle up.