Episode Transcript
[00:00:00] Speaker A: Foreign.
Welcome to Base by Base, the papercast that brings genomics to you wherever you are.
Today we're diving into, well, really one of the big paradoxes in infectious diseases. We all know about tuberculosis, tb. It's caused by this group of bacteria, the Mycobacterium tuberculosis complex, or mtbc, a huge global problem.
But here's the question, the one that really got us started on this deep dive. How is it possible that two pathogens sharing, I mean, over 99.9% of their DNA, can cause diseases that are just fundamentally different? Think about it. Standard human TB from M. Tuberculosis or mtb, it's usually this chronic thing, right? Long term, can go on for years. But then you look at the data in these sources and you find these other strains, genetically almost twins, that cause this incredibly rapid acute disease death within weeks, like four weeks. It's not just more severe, it's a totally different way of causing disease. So this deep dive is all about unpacking this really dramatic kind of unexpected virulence hierarchy. It really challenges the standard way we think about tb.
[00:01:12] Speaker B: It absolutely does. It makes us rethink fundamentally what virulence even means for these bacteria. And today we really want to celebrate the work that finally put numbers on this, that quantified this hierarchy systematically. Yeah, we're talking about the paper Virulence hierarchies within the Mycobacterium Tuberculosis complex. It was published in PNAS October 16, 2025. This was a major collaborative effort led by Sarah N. Danchak and Marcel Hebert, but involving a really extensive team. You've got people from McGill, Yale School of Public Health, VIO, the vaccine and Infectious Disease Organization and several others. They really brought together the right expertise to do this essential head to head comparison, comparing the human strains with the animal strains. Something that was Frank missing.
[00:01:58] Speaker A: Right. Okay, so let's quickly define who's who in this complex, especially these sometimes overlooked animal strains. So mtbc, it includes the human ones, mostly mtb, but also these animal associated lineages. And there are major zoonotic threats, meaning they jump from animals to humans. And we're focusing on two main ones here from the animal side. First, M. Bovis, that's the classic one, causes bovine tb, cattle tb, it's everywhere, a global issue. And second, this more newly recognized one, M. Orges, it was only officially named a cause of zoonotic TB in what, 2024?
[00:02:31] Speaker B: That's right, very recently.
[00:02:33] Speaker A: But it's already known as this significant pathogen in multiple hosts, especially across South Asia.
[00:02:38] Speaker B: Yeah, and that really highlights the gap these researchers were trying to fill. You know, there were hints, older studies suggesting maybe Embovis was a bit tougher than mtb. But a proper rigorous comparison, one that put MTB and Bovis and this emerging immortal side by side in the same standardized models, it just hadn't really been done properly.
[00:02:57] Speaker A: So we didn't really know why they behaved so differently.
[00:03:00] Speaker B: Exactly. We needed to get beyond just if they were different to the why, down to the molecules. Why do these near identical bugs act in such opposite ways? Yeah, so their mission was clear.
Systematic comparative analysis using both natural hosts and lab models to really nail this down.
[00:03:17] Speaker A: Okay, let's talk methodology then. Because getting at these differences, which might seem subtle genetically, but are huge phenotypically, that requires the right models.
[00:03:26] Speaker B: Precisely. You need relevance and control. So they started with whole skin calves.
[00:03:31] Speaker A: The natural host for mbovin.
[00:03:33] Speaker B: Exactly. A natural target. They infected these calves using an aerosol route which mimics natural inhalation.
Pretty high dose, about 10,000 CFU's. Colony forming units. And they watched them for 15 weeks. They looked at the obvious things like lesions, both visible ones, and microscopic details with H and E staining. And crucially, how many bacteria were actually in the tissues. The bacterial burden, 15 weeks.
[00:03:56] Speaker A: Okay. And the second model, the lab workhorse.
[00:03:59] Speaker B: Right. The C57BL6 mouse. Standard lab model. Also infected via aerosol, but a different dose size. Oh, yes, much smaller. Around 150 to 300 CFU. Still enough to be lethal for the animal strains, as it turned out.
[00:04:12] Speaker A: And what were the key readouts there? Survival must have been a big one.
[00:04:15] Speaker B: Absolutely critical. They tracked survival time, plotted those kaplan meier curves, you see. They also figured out the LD50 day, the dose needed to kill 50% of the animals. And again, detailed pathology.
[00:04:26] Speaker A: But they went beyond just, you know, counting bacteria and looking at damaged tissue. They really dug into the why behind the pathology, especially the immune response. You mentioned some advanced imaging.
[00:04:36] Speaker B: Yes, this was key. They suspected it wasn't just about more bugs, but how the host reacted. So they used single cell imaging, mass.
[00:04:44] Speaker A: Cytometry, S. Cimc Okay, S. CIMC how does that help?
[00:04:48] Speaker B: It's pretty amazing, actually. It lets you map the immune cells right there in the tissue, almost cell by cell. You can identify specific markers for neutrophils. Macrophages look at inflammatory signals like IL1 beta. So you see not just which cells are there, but where they are in relation to the infection and sort of what they're doing. Yeah. They also use standard immunohistochemistry IHC to complement this.
[00:05:10] Speaker A: So a really detailed picture of the immune battleground. And on the bacterial side, how did they hunt for the specific weapons these strains might be using?
[00:05:18] Speaker B: They looked at what the bacteria were secreting using comparative proteomics on the stuff bacteria released into their environment, the culture filtrate or secretome, looking for proteins that.
[00:05:29] Speaker A: The really virulent strains pumped out. But MTB didn't.
[00:05:33] Speaker B: Exactly. And that directly led them to the germ disruption studies. They used this technique called orbit recombineering. Think of it like a very precise genetic scissor. They used it to snip out specific genes, both well known ones like Sxa, which makes the famous ESAT6 protein. And also these lineage associated ones they found in the proteomics, like the genes for MPT70 and MPT83, to see if.
[00:05:57] Speaker A: Removing the gene actually changed the disease, made it less virulent.
[00:06:01] Speaker B: Precisely. Does removing this factor dial back that hypervirulence?
[00:06:05] Speaker A: Okay, let's get to the results, because the numbers here, they really tell a story. Starting with the calves, the natural host model.
[00:06:11] Speaker B: Right? So 15 weeks post infection, the calves that guide MTB, they basically had it under control. Very minimal visible damage, minimal pathology. And importantly, you couldn't even detect bacteria in their lymph nodes. Classic chronic contained infection.
[00:06:25] Speaker A: But M. Bovis and M. Origis, a completely different picture.
[00:06:29] Speaker B: Massive lesions you could see with the naked eye. Lots of necrotic dead tissue, really enlarged lymph nodes just packed with bacteria.
[00:06:37] Speaker A: How much more bacteria?
[00:06:38] Speaker B: Up to four logs. Higher burden. Yeah, that's 10,000 times more bacteria in some tissues compared to the MTB group, where it was often undetectable. Wow. And Am Origis in particular seemed to cause even more severe lung damage than Ambovis. The animal strains were just playing a different, much nastier game.
[00:06:56] Speaker A: Okay, and did the mouse model back this up? The survival times you mentioned?
[00:06:59] Speaker B: Dramatically. So this is maybe the starkest result.
Mice infected with that standard low dose of M. Bovis or M. Orges, median survival was just 24 to 31 days, less than a month.
[00:07:13] Speaker A: And the MTB mice, they live longer.
[00:07:15] Speaker B: Than 200 days on average.
[00:07:16] Speaker A: At that same dose over 200 days versus under 31 days. That's not subtle.
[00:07:21] Speaker B: Not at all. And the LDV calculation confirmed it. The dose needed to kill half the mice was about 10 times lower for the animal strains than for MTB. They are just far more efficient killers in this model.
[00:07:31] Speaker A: Okay, but here's where it gets really interesting, right? The first big twist. It Wasn't simply that the animal strains grew faster initially.
[00:07:38] Speaker B: That's the crucial point. When they looked around, day 21, day 28, the actual number of bacteria in the lungs of the Amorgis mice wasn't that different from the MTB mice.
[00:07:49] Speaker A: So similar bacterial load, but wildly different outcomes. How?
[00:07:52] Speaker B: It had to be the host response. The mice weren't just dying from the bacteria, they were dying from their own immune system, going into overdrive. A hyperinflammatory response.
[00:08:01] Speaker A: And the SCIMC imaging showed this clearly.
[00:08:03] Speaker B: With Amorgis, you saw this massive influx of immune cells, especially neutrophils. These cells full of mpo.
It led to rapid widespread tissue destruction, necrosis. Compare that to mtb, you get these organized granulomas trying to wall off the infection with it was just chaos. Acute destructive inflammation. That really flips the textbook idea of TB pathogenesis on its head.
[00:08:27] Speaker A: So the hunt was on for the molecular drivers of that response. The proteomics pointed to MPT70 and MPT83 being highly secreted by the animal strains, right?
[00:08:35] Speaker B: Correct. ESAT6 and its partner CFP10. They're secreted by pretty much all of them. They're core virulence factors. But MPT 70 and MPT 83 were really abundant in the secretomes of Ambophus and Amorgis, but much less so in mtb. That made them prime suspects for driving the difference.
[00:08:52] Speaker A: So then came the gene deletions to test their role. What happened when they deleted ESAT 6, the known major virulence factor?
[00:08:59] Speaker B: As you'd expect, deleting Sxa significantly reduced virulence in both Mbovis and Amorges. The mice live much longer. It confirms that the core ESX1 secretion system, which secretes ESAT6 is absolutely critical for the high virulence of the animal strains test.
[00:09:14] Speaker A: Okay, no big surprise there. But now the really interesting part. MPT70, MPT83, the proteins over secreted by the animal strains, they deleted those genes in Ambovis. What was the result?
[00:09:24] Speaker B: This was striking. Deleting MP 70, FMP 83 and Ambovis almost completely reversed the hyperviolence. Pretty much. The mice survived much much longer. Similar to MTB infection, their lung pathology was limited. And get this, in some mice, followed long term up to 52 weeks, the bacterial burden dropped by almost five logs. That's a hundred thousand fold decrease.
[00:09:46] Speaker A: Incredible. So MPT 70, MPT 83 really is a key hyperverulence factor specifically for Mbovis. It's its secret weapon.
[00:09:54] Speaker B: It seems to be a Lineage specific factor driving that extreme pathology.
[00:09:58] Speaker A: But then comes the second twist, the big mystery. What happened when they deleted the exact same genes, MT70, MBT3 and M origis?
[00:10:06] Speaker B: Yeah, this is the fascinating part. It did basically nothing.
[00:10:09] Speaker A: Nothing.
[00:10:09] Speaker B: Nothing significant. Deleting MT 70, MT 83 and M origist did not attenuate its virulence. The mice still died quickly, the bacterial burden remained high. The survival curves were almost identical to the wild type M orages.
[00:10:21] Speaker A: Even though M orages also secretes Lots of MTT 70 and MPT.
[00:10:26] Speaker B: Correct. This is probably the most profound finding. M. Aureus achieves its hypervirulence through some other mechanism, one that doesn't depend on MPT 70, MPTTA 3. Even though it makes those proteins, there's an alternative undiscovered pathway driving its lethality.
[00:10:42] Speaker A: Wow. Okay. That finding alone just redraws the map of MTBC virulence, doesn't it? We have this clear hierarchy now. Animal strains are hypervirulent compared to mtb. And that difference comes from specific factors and this destructive inflammatory response.
[00:10:57] Speaker B: Absolutely. It's not just a spectrum, it's qualitatively different types of disease potential packed into incredibly similar genomes. And this has huge implications, especially when you think about how infection happens and how immunity develops. They also looked at the route of infection, comparing that super efficient aerosol route to oral infection. Right.
[00:11:15] Speaker A: Because we often think about zoonotic TB potentially coming from contaminated food or milk involving ingestion. What happened when they gave the bacteria orally by gavage?
[00:11:24] Speaker B: Well, first they had to use a massive dose a million times higher than the aerosol dose. Like a hundred million CFU compared to a few hundred.
[00:11:30] Speaker A: A million times more.
[00:11:32] Speaker B: Yeah. And even then, what they saw was a much milder disease. It was mostly contained in the gut and the nearby lymph nodes. It wasn't lethal like the aerosol infection.
[00:11:41] Speaker A: So aerosol really is the danger zone for severe acute disease with these strains?
[00:11:47] Speaker B: It seems so, at least in this model, it's incredibly efficient at establishing that lethal lung infection.
[00:11:52] Speaker A: But then there was that other immunological finding which was almost hopeful. The idea that a prior mild infection could protect you.
[00:12:01] Speaker B: Yes, this was really remarkable. They found that mice that had first received that mild non lethal oral dose of Am origis, while they were significantly protected when later challenged with a normally lethal aerosol dose of amoragus, protected how? Their survival time jumped dramatically. Instead of dying around day 24, their median survival went up to 112 days. That prior mild exposure acted like A natural vaccination.
[00:12:26] Speaker A: That's fascinating. It suggests the immune system can learn to handle even these hypervirulent strains if it encounters them in a less threatening way first.
[00:12:35] Speaker B: Exactly. And it might involve different immune mechanisms than standard BCG vaccination triggers.
[00:12:40] Speaker A: Which leads directly into the vaccine work they did testing subunit vaccines based on these specific proteins.
[00:12:46] Speaker B: Right. They tested vaccines using just ESAT 6 or just MPT 70. And both of those single antigen vaccines provided significant protection, prolonging survival against challenge with the animal strains. It shows both the core virulence factors and these lineage specific ones are important targets for immunity.
[00:13:03] Speaker A: But the combination vaccine, H1O70, that included ESAT6 plus MPT70 and MPT83, that was the star.
[00:13:12] Speaker B: It really was. The H107E vaccine gave protection that was statistically comparable to BCG, the current gold standard vaccine. Median survival extended out to around 177, 180 days.
[00:13:22] Speaker A: Okay, so the implication there is pretty clear for vaccine development, isn't it?
[00:13:26] Speaker B: I think so. It strongly suggests we need to think differently. Given these dramatic differences in how the strains cause disease and the fact the vaccines targeting specific animal strain proteins work well, maybe we need separate vaccine strategies, One tailored for human TB and another specifically for zoonotic TB using these lineage associated antigens like MPT703.
[00:13:47] Speaker A: That makes a lot of sense. Now, this study is incredibly insightful, but like all research, it has its boundaries. What are the main limitations to keep in mind?
[00:13:55] Speaker B: Sure, it's important context. They focused on two specific animal M. Bovis and M. Origis, and two MTB strains, both from the common lineage four found often in Europe and the Americas.
[00:14:07] Speaker A: So not the full diversity of the complex.
[00:14:09] Speaker B: Right. To get the complete picture, future studies need to bring in other important members, like M. Africanum, which is prevalent in West Africa, or M. Caprae, found in goats. And maybe look at different host models too, like guinea pigs, which sometimes mimic human lung pathology a bit differently than mice.
[00:14:25] Speaker A: And critically, we still don't know what makes MRGs tick. Right, that alternative virulence mechanism.
[00:14:30] Speaker B: That's the big question left hanging. Identifying the specific factors and pathways that allow M. Orgis to be so deadly. Even without relying on MPT 7083. That's a major next step.
[00:14:41] Speaker A: Okay, so let's try and wrap this up. The take home message from this deep dive.
[00:14:45] Speaker B: I'd say it's the animal adapted TB strains, specifically Ambovis and the emerging Amoragus aren't just slightly worse than human mtb, they are hypervirulent they cause a distinct rapid destructive hyperinflammatory disease.
[00:14:58] Speaker A: And this difference relies on a mix of shared weapons like ESAT 6, but also unique lineage specific factors like MPT 70 being crucial for ambovis.
[00:15:07] Speaker B: Exactly. But with the big asterisk that M orgs hyperverilance is driven by something else entirely, something still hidden. A real molecular mystery remains.
[00:15:15] Speaker A: So here's the final thought to leave you with. We now see this clear dramatic virulence hierarchy challenging decades of thinking, and we see that prior mild exposure can actually provide strong protection against these hyperverilant strains. What does all this mean for our global TB control strategies, which have historically focused almost entirely on human MVP tv? How do we adapt? This episode was based on an Open Access article under the CCBY 4.0 license. You can find a direct link to the paper and the license in our episode description. If you enjoyed this, follow or subscribe in your podcast app and leave a five star rating. If you'd like to support our work, use the donation link in the description. Thanks for listening and join us next time as we explore more science base by base.
Sam.
