Chapter 21: Gram-Negative Bacteria Beyond Proteobacteria

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Welcome to the Dup Dive.

Today, we're skipping the textbook slog and jumping right into some of the most inventive life forms out there, the non -proteobacterial gram -negative bacteria.

Yeah, and we're starting with something really practical, actually, industrial power.

Exactly.

Because the science here, it's literally being looked at for clean fuel.

We're looking at organisms really engineered by nature, you know, to survive extremes.

And the immediate focus is temperature and, well, efficiency.

We're talking about microbes like Thermotoga.

Right, Thermotoga.

And they're being studied for something called dark fermentation.

And what does that mean, practically?

We're basically talking about converting complex carbohydrates,

think industrial food waste, like city fruits and vegetable scraps.

That's usually just thrown out.

Exactly.

Turning that into highly efficient hydrogen gas, H2.

The source material actually notes that these Thermotoga species are ridiculously efficient, almost hitting the theoretical maximum yield for hydrogen.

And that's because they secrete huge amounts of these hydrolytic enzymes that just chew up all that plant biomass.

Very effective.

And H2 as a fuel, it's compelling.

Absolutely.

Incredibly high energy density.

And the best part, when you combust it, the only byproduct is water vapor.

No greenhouse gases.

Clean energy.

And what's truly fascinating is how robust the application is.

Researchers showed they could grow Thermotoga maritima using just simple city food waste mixed with seawater from a local bay.

Just readily available stuff.

So food waste plus saltwater equals clean energy.

And here's the genius part, I think.

Thermotoga loves it hot.

It thrives at 80 degrees Celsius.

Hyperthermophilic.

Right.

And that high heat isn't a bug, it's a feature.

It acts as a built -in sterilizer.

Oh, okay.

So it keeps other things from growing.

Exactly.

It minimizes the risk of contamination from competing organisms that would otherwise try to steal that food waste.

It makes the whole process inherently cleaner, simpler, and, well, more efficient for industrial scale up.

Okay, let's really unpack this then.

Today, we're peeling back the curtain on some of the most rugged, weird, and frankly inventive forms of microbial life in this group.

We're going to look at cells that can survive radiation blasts that would kill us thousands of times over.

Organisms that have ditched their cell walls completely.

And others that literally power global nutrient cycles.

Get ready for some serious microbial ingenuity.

So we look at the evolutionary tree of bacteria.

The Philae aqua hupicae and Thermotoga are way down at the bottom.

They represent the deepest, oldest known branches.

These are your classic hyperthermophiles.

Loving the heat.

Optimally thriving above 80 degrees Celsius.

Right.

Ancient, but their lifestyles are quite different.

Okay, how so?

Well, aqua effect species are what we call chemolithoautotrophs.

Basically rock eaters.

Right.

They get energy by oxidizing inorganic stuff like hydrogen or sulfur.

And they fix their own carbon dioxide using a reversed pathway.

The reductive citric acid cycle.

Now contrast that with the Thermotoga we just mentioned.

They're chemoorganotrophs.

They eat organic carbon.

They use a functional glycolysis pathway.

Grow anaerobically.

And structurally, they're known for that loose outer sheath.

Right.

The toga.

Exactly.

The heat outer garment that gives them their name.

Okay, but here's where it gets, well, really interesting for me.

The source highlights that Thermotoga genomes are like genetic scrapbooks.

Massive evidence of horizontal gene transfer, HGT.

Significant amounts.

Yeah, like estimates suggest anywhere from 25 % up to maybe 50 % of their genes are actually archaeal in origin.

That's huge.

It really is.

They're borrowing massive functional chunks from a completely different domain of life, archaea.

Right there near the base of the bacterial tree.

It's quite remarkable.

Less bacteria, more ancient genetic library.

Okay, from that ancient foundation, let's jump to the masters of like radical survival.

The Deinococcus thermosphylum, specifically Deinococcus radiodurans.

Honing the bacterium.

Huh.

Yeah, its resilience is almost impossible to believe.

It's an extreme outlier.

The lethal dose of radiation for a human is, what, about a hundred rads?

Something like that.

D, radiodurans can handle three to five million rads.

And not just the radiation, but the extreme drying out that often comes with it.

It's the mechanism, though, that's so mind -blowing.

Radiation creates these radical oxygen species, right?

And they just shatter the genome into hundreds of pieces.

Yeah, it doesn't prevent a damage.

It just has this insane repair system.

How does it work?

Well, first, these cells are polyploid.

They have multiple backup copies of their genome, like having several blueprints.

Okay, redundancy helps.

Definitely.

Then, within maybe 12 to 24 hours, they rapidly stitch those hundreds of fragments back together.

It uses this high -speed genetic sewing machine process called extended synthesis dependent strand annealing, helped along by retomologist recombination.

Wow.

So it just finds the matching pieces and reassembles the whole thing.

Pretty much.

It's self -repair take to the absolute extreme.

Okay, moving on.

Let's dive into the phylum tenorcutes.

That means tender skin.

Ah, yes.

Home to the molecules, the mycoplasmas.

Yeah.

Famous for what they don't have.

Their cell wall.

They've completely ditched it.

Correct.

They can't synthesize the precursors needed for peptidoglycan, so no rigid wall.

This makes them highly plastic or pleomorphic.

Meaning they can change shape easily.

Yeah, they very dramatically spheres helical filaments because there's no wall holding them in a fixed shape.

But lacking that wall creates a big problem, doesn't it?

Osmotic stability.

How do they cope?

They solve it by scavenging.

They need sterols, like cholesterol, which they have to get from their host environment.

Ah, so they incorporate host cholesterol into their own membrane.

Exactly.

To stabilize their plasma membrane because they don't have that rigid outer layer.

And this reliance on the host, it contributes to their minimal nature too, right?

They're tiny.

Very tiny.

Among the smallest bacteria capable of self -reproduction, their genomes are incredibly small.

Some human pathogens have fewer than thousand genes.

So they have to be parasitic.

Heavily parasitic, yes.

They rely on the host for many essential macromolecules.

And visually, if you grew them on agar,

they have that characteristic look.

Oh yeah, the distinct fried egg appearance.

The center of the colony grows down into the agar, and the edges spread out flat along the surface.

Very recognizable.

And their reduced genome forces some brilliant alternative metabolic strategies too.

Definitely.

Consider urea plasma ureolyticum.

It's known to cause urinary tract infections.

It doesn't use a standard electron transport chain for energy.

So what does it do?

It targets urea, a common waste product in that environment.

Okay, and the trick here is how simple it is, right?

Deceptively simple.

It hydrolyzes urea into ammonia and CO2.

The ammonia then picks up a proton, becoming ammonium, NH4+.

And then?

And then it specifically exports that charged ammonium ion out of the cell using uniporters.

So by pumping out positive charge?

It effectively leaves protons behind on the outside.

This generates the electrochemical gradient, the proton motive force needed, to drive ATP synthase and make ATP.

Wow!

Turning host waste into energy using just chemistry and transport, that's clever.

Very clever.

But we also see specialized movement here.

Gliding motility, but even that varies.

Right, like mycoplasma mobile uses these steady cytoskeletal leg proteins for an even pace.

Like consistent little steps.

But M pneumonia, which causes walking pneumonia, moves more like a microscopic inchworm.

An inchworm?

How?

It uses a special attachment organelle at one end.

It kind of alternately compresses and extends, grabbing and pulling itself forward.

It gives it this characteristic, uneven sort of halting movement.

Okay, next up, we're grouping three phyla together into what's called the PVC superphylum.

Right, that's planktonmiseids, vircomicrobia, and chlamydia.

And they share some atypical features, like they often lack the standard FTSZ protein needed for typical bacterial cell division?

That's one unifying feature, yes.

They're kind of the oddballs of the bacterial world in some ways.

Let's start with planktonmiseids, and specifically the enamex bacteria.

These are fascinating.

They've evolved a highly complex internal structure, a specialized compartment called the anemoxysome.

And this is where a really critical reaction happens.

Anaerobic ammonia oxidation.

Animagus.

Exactly.

NH4 plus NO2 yields N2 gas.

And this isn't just some obscure reaction, is it?

It's globally important.

Hugely important.

This single reaction happening inside these tiny bacterial compartments is estimated to contribute up to 70 % of all nitrogen cycling in the world's oceans.

70%.

That's staggering.

A massive planet scale process driven from inside a bacterium.

And that compartment, the anemoxysome, is built for protection.

The reaction involves a highly toxic intermediate hydrazine.

Rocket fuel, basically.

Not something you want flitting around the cell.

Definitely not.

So to shield the rest of the cell, the anemoxysome is bounded by a unique single bilayer membrane.

It contains special lipids called laterane lipids.

Laterane lipids.

What do they look like?

If you could see them, they literally look like tiny molecular ladders fused together.

They pack incredibly tightly, forming an unusually dense impermeable barrier.

Okay, so it physically traps the toxic hydrazine inside that reaction chamber.

Precisely.

A brilliant structural solution to a chemical problem.

Okay, shifting gears within PVC to varicam acrobia.

The warty microbes.

Another diverse group.

You have examples like Acidamethyloselex fumeralicum.

It's an acid -tolerant methylotroph.

Meaning it eats methane.

Oxidizes methane, yes.

A potent greenhouse gas.

Yeah.

And this one can survive at incredibly low pHs, sometimes below pH 1 .0.

Really extreme.

Wow.

And then there's the microbiome celebrity.

Acromansia mucinophila, yes.

Highly studied.

It's a gut symbiont that actually consumes mucus lining the gut.

And it's been linked to positive health outcomes.

It's associated with reducing obesity -related inflammation in humans, among other things.

Still a lot of research ongoing there.

Okay, and the third member of the PVC trio,

chlamydiae.

These are the ultimate metabolic minimalists.

They are obligate intracellular parasites.

Meaning they have to live inside a host cell.

They have to grow and reproduce inside a host cell, yes.

Their genomes are tiny, stripped down.

They just can't make many essential components themselves.

Including energy.

Especially energy.

They are true energy parasites.

They rely so heavily on the host that some have adapted their ATP synthase enzyme to essentially run in reverse.

Wait, run backwards?

How does that work?

Instead of using a proton -gradient PMF to make ATP, they use ATP from the host cell to pump protons out, creating a PMF.

Why would they do that?

That generated PMF is then used to power the import of more host ATP.

They're essentially using a bit of stolen energy to steal even more finished energy packets from the host.

That's devious.

And they have that strange life cycle too, right?

Two phases.

A very unique biphasic life cycle.

It starts with the infectious form, the elementary body, or EB.

It's small, rigid, metabolically quiet.

Okay, the invasion unit.

Exactly.

The EB gets inside the host cell.

Once inside, it differentiates into the reproductive form.

The reticulate body, or RB.

This one is larger, more flexible, and metabolically active.

So the RB is the factory.

Yes.

The RBs divide rapidly by binary fission inside a membrane -bound vesicle called an inclusion.

They fill up the inclusion.

Then they mature back into the infectious EBs.

And then they burst out.

Eventually, the host cell lyzes, releasing hundreds of new EBs to infect neighboring cells.

A very effective, albeit destructive, cycle.

All right.

Let's talk about the organisms that literally terraformed our planet.

The photosynthetic phyla.

Building the atmosphere.

We need to distinguish between the two main types, right?

Oxygenic and unoxygenic photosynthesis.

Correct.

Oxygenic is what plants and cyanobacteria do.

They use water, H2O, as the electron donor.

And crucially, produce oxygen, O2, as a byproduct.

And oxygenic.

That's used by other groups, like purple and green bacteria.

They use donors other than water, maybe hydrogen sulfide, H2S, or hydrogen gas, H2.

And importantly, they do not produce oxygen.

A key adaptation for all of them is pigments, isn't it?

Chlorophylls and bacterioclorophylls, de Liel.

Absolutely crucial.

These pigments capture light energy.

Think of them like molecular antennas.

And different pigments absorb different wavelengths of light.

So having different pigments allows them to occupy different light niches.

Precisely.

Bacterioclorophylls, for example, are specialized to absorb light in the far -red and even near -infrared spectrum, often beyond 715 nm.

Wavelengths that chlorophyll doesn't really capture.

Why does that matter?

Where would they use that?

Allows them to live in deeper anoxic water zones.

Sunlight gets filtered as it passes through water, right?

Organisms higher up grab the wavelengths chlorophyll uses.

Blakle allows these bacteria to scourge the light that penetrates deeper.

Light scavengers, okay.

We find these in the chlorobi, the green sulfur bacteria.

Yes.

They are obligate anaerobes, photolithoautotrophs.

They maximize their light harvesting using these amazing structures called chlorosomes.

Chlorosomes?

What are they like?

Imagine the large ellipsoidal vesicles, almost like sacs, attached to the inner surface of the plasma membrane.

They are absolutely packed with densely aggregated blakle pigments, acting like giant antenna complexes.

Extremely efficient light harvesting.

Very cool.

Then we have the chloroflexi, the green non -sulfur bacteria.

A bit different.

These are often filamentous gliding organisms.

Metabolically more flexible than chlorobi.

And they have a really unusual way of fixing CO2.

Not the Kelvin Cycle?

Nope.

They use a complex pathway called the 3 -hydroxypropionate biocycle.

A very different route to making cellular carbon.

Okay, but the real photosynthetic superstars, globally speaking, have to be the cyanobacteria.

Without a doubt.

The undisputed global fixers.

Just two unicellular genera, Prochlorococcus and Cnichococcus, together account for at least one -third of all global carbon fixation.

One -third of the planet's CO2 uptake from just two types of microbes.

That's incredible.

And to give you perspective, Prochlorococcus is thought to be the most abundant photosynthetic organism on Earth.

Period.

Its sheer numbers drive global ocean chemistry.

How do they do it structurally?

They have internal membrane systems, thylakoids, similar to plant chloroplasts where the light reactions happen.

They use light harvesting antenna complexes called phycobilosomes, which contain accessory pigments.

And they package their carbon -fixing enzyme, rubisco, into protein microcompartments called carboxysomes.

Carboxysomes.

Like, concentrating the CO2.

Exactly.

It enhances the efficiency of carbon fixation.

They also perform something called chromatic adaptation.

Changing color.

Essentially, yes.

They can adjust the ratio of their phycobilin pigments, like blue phycocyanin versus red phycorhithrin, depending on the color or wavelength of light available at different water depths, fine -tuning their light absorption.

Smart adaptation.

But maybe the most profound adaptation, especially for nutrient cycling, is the heterocysts, right?

Oh, absolutely profound.

When fixed nitrogen, like ammonia or nitrate, becomes scarce in the environment, certain cells within a cyanobacterial filament will differentiate into these specialized cells called heterocysts.

What makes them different?

They develop thick cell walls to limit oxygen diffusion.

And crucially,

they intentionally dismantle their photosystem 2 complex, the part of photosynthesis that actually produces oxygen.

Why get rid of oxygen production?

Because the enzyme needed for nitrogen fixation, nitrogenase, is extremely sensitive to oxygen.

It gets inactivated by it.

So the heterocyst creates an internal oxygen -free anoxic environment specifically to protect nitrogenase.

Oh, they trade photosynthesis for nitrogen fixation.

In that specific cell, yes.

The heterocyst focuses solely on fixing atmospheric nitrogen gas into, into ammonia.

It then shares this fixed nitrogen with its neighboring vegetative cells in the filament.

And what does it get in return?

The neighboring cells, which are still photosynthesizing, provide the heterocyst with carbohydrates for energy, a beautiful example of cellular cooperation and differentiation.

Okay, time to shift gears slightly and look at specialized motility, starting with the Spear Sheddies.

Instantly recognizable shapes.

Oh, yeah.

Long, slender, flexible, and helical.

They look like microscopic corkscrews or drill bits.

And they move like one, too.

How does that work?

It's not typical flagella, is it?

It's unique.

They have flagella, but they are located internally in the periplasmic space, lying between the inner membrane and the outer membrane sheath.

They're called periplasmic flagella or endoflagella.

So they're inside the cell's outer layer.

Exactly.

When these internal flagella rotate, they cause the entire flexible cell body to twist and undulate.

This generates that characteristic corkscrew motion, which is really effective for moving through viscous environments, like mucus or tissues.

And this group includes some pretty notorious pathogens.

Unfortunately, yes.

Agents of major human diseases, like treponema pallidum, which causes syphilis, and Borrelia burgdorferi, the agent of Lyme disease.

The Force actually notes T -pallidum is particularly stealthy.

How so?

Immune evasion.

Yeah, it exposes very few proteins on its outer surface membrane, making it harder for the host immune system to recognize and target it.

Clever, but nasty.

Okay, next group.

Bacteroidetes.

Hugely important in the gut, right?

Absolutely crucial players in the human gut microbiome.

They are obligate anaerobes and often make up something like 30 % of the bacteria found in fecal matter.

They are essential gut remodelers.

Remodelers?

What are they doing?

They are masters at degrading complex plant fibers and polysaccharides.

Humans can't digest themselves, things like cellulose and pectins.

They break these down into short -chain fatty acids.

Which the host can then absorb.

Exactly.

They provide a significant source of nutrition to the host by breaking down otherwise indigestible material.

They also have impressive genetic flexibility, I read, using invertible DNA elements.

Right.

These are short DNA sequences, often containing promoters, that can literally flip themselves around 180 degrees.

Like a genetic switch.

Precisely.

It acts like a switchboard, allowing them to rapidly turn different sets of genes on or off, particularly genes for cell surface proteins.

This helps them adapt quickly to changing conditions in the gut or even evade the host immune system by changing their outer appearance.

Wow.

And they also glide.

Some exhibit gliding motility, yes.

It involves specific motor proteins, called globulut proteins, embedded in the membrane.

They use the proton motive force to propel large adhesion proteins, like PRB, along helical tracks on the cell surface.

Like tiny tanks treading along the surface.

Kind of, yeah.

It's thought to be ideal for moving across surfaces and accessing insoluble substrates, like cellulose and chitin, that they need to digest.

Okay.

Finally, let's touch on the fusobacteria.

Spindle -shaped.

Yes.

Often spindle -shaped or fusiform.

They're anaerobes, found commonly in the mouth and also the gut.

Their metabolism is a bit different.

They primarily ferment amino acids and proteins rather than sugars.

And that produces?

Often yields butyric acid, which contributes to, well, rancid odors.

Pleasant.

But they have significance in health, too.

They do.

They're often found acting as bridge bacteria in oral biofilms, physically linking early and late colonizing species together, helping structure the biofilm community.

Okay.

But there's a more concerning link, too, right, with cancer?

Yes.

Crucially, fusobacterium species, particularly F.

nucleatum,

have been found in unusually high abundance within oral and colorectal tumors.

Inside the tumors themselves.

Inside the tumor microenvironment.

And studies indicate their presence isn't just coincidental.

It's associated with enhanced cancer cell proliferation.

And get this, they may even contribute to resistance to certain chemotherapeutic drugs.

How?

It seems they can actively prevent apoptosis or programmed cell death in the cancer cells they associate with,

basically helping the cancer cells survive treatment.

That is, yeah, that's a disturbing connection.

A common microbe potentially protecting cancer cells.

Hashtag tag tag outro.

So we've covered just an incredible spectrum of life today, haven't we?

From those ancient hyperthermophiles swapping genes with archaea down near the root of the bacterial tree.

To organisms like Deinococcus that can literally reassemble a shattered genome within hours after massive radiation exposure.

Just amazing resilience.

We've seen these incredible structural adaptations, laterine lipids forming impermeable barriers, specialized compartments like chlorosomes and anemoxosomes.

And heterocysts sacrificing oxygen production for nitrogen fixation.

And then the mycoplasmas taking structural minimalism to the extreme by losing the cell wall entirely.

And the metabolic strategies are just as diverse.

From rock eaters like Acrofex fixing CO2 with inorganic energy.

To the ultimate energy parasites, the chlamydiae, stealing pre -made ATP directly from their hosts.

You know, if we try to connect this to the bigger picture,

we discussed mycoplasma among the smallest known self -reproducing cells.

Surviving through this intense minimalism and dependence on a host.

Right, the epitome of reduction.

And then in the same dive, we discussed Prochlorococcus, arguably the most abundant photosynthetic organism on Earth.

Numerically dominating the oceans and single -handedly responsible for fixing maybe a third of the entire planet's carbon.

Overwhelming abundance and global impact.

So it raises this kind of fundamental question, doesn't it?

When you study the vast diversity of life, which is ultimately the more successful long -term evolutionary strategy, is it that kind of metabolic simplicity, minimalism, and reliance?

Or is it overwhelming numerical abundance and planet -scale biochemical dominance?

Something for you to consider, perhaps, as you go about your day.

Indeed.

Well, thank you for joining us for this deep dive.