Chapter 18: Mycobacteria & Actinomycetes

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Imagine a fortress.

And I don't mean a castle made of stone or, you know, reinforced steel.

I want you to picture a microscopic fortress made entirely of wax.

It sounds kind of fragile when you put it that way.

Wax usually melts, right?

Exactly.

You'd think a little heat or maybe some soap would just dissolve it.

But this fortress is so chemically impenetrable that attacks which dissolve other bacteria in seconds, they just bounce right off.

And inside that fortress lives a killer.

One that has been hunting humanity for thousands of years.

And even today, despite all our modern medicine, it remains the leading cause of death from infection worldwide.

Which is just a staggering statistic.

We get so used to hearing about, you know, flashy viruses or sudden outbreaks.

But this is the slow burn.

We're talking about the mycobacteria.

Specifically, we are doing a deep dive into Chapter 18 of Lippincott Illustrated Reviews, Microbiology.

And the mission today isn't just to memorize a list of bug names for an exam.

No.

It's to understand the engineering of a biological tank.

That's the perfect way to frame it.

We need to unpack why these things are so incredibly hard to kill.

We're going to look at the waxy fortress itself.

The structure of the bug.

Right.

Then we'll tackle the heavyweight champion, mycobacterium tuberculosis.

We'll move on to the ancient scourge of leprosy.

And then finally, we'll wrap up with these weird cousins called the Actenomyces that seem to be having a bit of an identity crisis.

Are they bacteria?

Are they fungi?

We will get there.

But for the medical students or biology buffs listening, this chapter is so high yield.

If you don't get the biology here, specifically that cell wall, the pathology of TB just won't make sense.

And neither will the treatment.

The whole reason you have to take pills for six months, which just sounds insane.

It all starts with the chemistry of that wall.

So let's start right there.

The architecture.

If you look at a standard bacterium, like say E.

coli or staph, you've got peptidoglycan

membranes.

Pretty standard stuff.

What makes a mycobacterium so different?

It's all about the fat content.

The cell wall of a mycobacterium is roughly 60 % lipid.

60%.

That's basically a floating ball of grease with some DNA inside.

Essentially, yeah.

And the star of the show here is a component called mycolic acids.

These aren't your standard dietary fats.

These are unique,

massive, long chain beta hydroxylated fatty acids.

They can have anywhere from 75 to 90 carbon atoms.

They complex with polysaccharides and peptides to create this thick, waxy shell.

So if I'm an antibiotic trying to get in or a white blood cell trying to digest this thing, I'm just hitting this heavy duty waterproof raincoat.

An industrial raincoat.

And that hydrophobicity, its ability to repel water, is its superpower.

It protects them from dehydration, which means they can survive in dry droplets for a long time.

But it also creates a massive headache for the lab.

A huge headache.

Because you can't stain them, right?

The Gram stain is the bread and butter of microbiology, purple or pink.

It is.

But if you pour crystal violet on a wax wall, it just slides right off.

You can't Gram stain them.

So we have to use the nuclear option, the acid fast stain.

I always loved this term acid fast.

It sounds like an intermittent fasting plan or something.

Not quite.

Fast here means hold fast, or to hold on tight.

The process is pretty intense.

You treat the slide with a red dye called carbulfusin.

But because of the wax, you actually have to use heat to force the dye through the wall.

So you cook it in.

You cook it in.

And once it's in, it's trapped.

Then you try to wash it out with a really harsh mixture of acidified organic solvents, usually acid alcohol.

And most bacteria would just lose the color.

Instantly.

If this were E.

coli, the acid would strip the color right out.

But the mycobacteria, they hold fast to that red dye.

They resist decolorization.

So when you look under the microscope, and I'm looking at figure 18 .2 in the text here, You see these slender bright red rods just popping out against a blue background.

It's actually really striking.

It is.

And if you look closely at that figure, you might notice the bacteria are clumping together in these serpentine string -like shapes.

Yeah, I see that.

That's called cording.

It's actually a sign of virulent M.

tuberculosis.

If you see cords, you know you're dealing with the bad stuff.

Now there's a downside to wearing a heavy suit of armor, isn't there?

It must make you slow.

Glacial.

This is the other defining feature.

Most bacteria, like E.

coli, can divide every 20 minutes.

You start a culture in the morning, you have a result by the afternoon.

Mycobacteria.

Their generation time is 8 to 24 hours.

Wow.

So while E.

coli has conquered the petri dish and thrown a party, the TB bug is just, what, finishing its first division?

Pretty much.

And clinically, this is a nightmare.

It takes two to eight weeks to grow M.

tuberculosis on special media.

The text has, figure 18 .9, the Loewenstein -Jensen slant.

It looks like scrambled eggs in a tube.

You can't tell a patient who's coughing their lungs out, hey, just sit tight for a month or two, we'll let you know.

You can't.

Yeah.

Which brings us to the main event, the white plague,

mycobacterium tuberculosis.

The text throws out a stat that, I mean, it honestly stops me in my tracks every time I read it.

One third of the world's population is infected.

It's hard to even wrap your head around that.

Now, to be clear, for most of those people, that's a latent infection.

The bacteria are asleep.

They aren't sick.

But there are still about 10 million active cases a year.

And nearly two million deaths.

And there's this massive deadly synergy with HIV.

Right.

That's the fuel on the fire.

It really is.

In some regions, 50 % of HIV patients are co -infected.

The second year immune system is compromised.

TB sees its window of opportunity.

Let's talk about that window.

How does the invasion happen?

And this is strictly airborne, right?

Exclusively.

You don't get it from shaking hands or sharing a fork.

You inhale it.

When a patient with active pulmonary TB coughs, they spray out these aerosol droplet nuclei.

And because of that waxy wall we talked about.

They don't dry out and die.

They can hang suspended in the air for 30 minutes or more.

That is just terrifying.

You walk into an empty room and the ghost of a cough is still floating there.

So I inhale this droplet, it goes deep into my lungs, lands in the alveoli.

And the body's security system responds immediately.

Alveolar macrophages, the cleanup crew, they come in and they gobble them up.

Normally this is game over for the bacteria.

The macrophage eats the bug, fuses it with a lysosome full of acid, and digests it.

But TB has a trick.

It produces these things called sulfolipids that inhibit that fusion.

It basically hacks the macrophages operating system and says, nope, do not release the acid.

So the phagosome never merges with the lysosome.

Never.

It turns the prison cell into a luxury condo.

It replicates inside the very cell that was sent to kill it.

And this concept, intracellular survival, is the absolute core of the problem.

I want to walk through the progression here because figure 18 .4 maps this out so well.

We have the macrophage failing to kill the bug.

What happens next?

The macrophage realizes it's in trouble and it signals for help.

It releases cytokines.

More macrophages rush in from the blood and they surround the infected cells.

This formation is the beginning of the tubercle.

And this leads to one of the grossest but also most descriptive terms in pathology, casus necrosis.

Yeah.

Casus literally means cheese -like.

Inside that tubercle, the macrophages in the center start to die.

But because of the enzymes and the wax from the bacteria, the tissue doesn't liquefy.

It turns into this soft, solid, cheesy mass.

Just delightful imagery.

It's the hallmark of TB.

But the body isn't done.

It realizes, okay, I can't kill this thing, so I'm going to wall it off.

It builds a collar of fibroblasts and lymphocytes around that cheesy center.

This walled -off fortress is the granuloma.

So this is the standoff.

For most people, 95 % of them, the wall holds.

The lesion arrests maybe calcifies.

You aren't sick.

You aren't contagious.

That's latent TB.

The bug is still in there, though.

It's just dormant.

It's waiting.

And for the unlucky 5 % are for people whose immune system crashes years later, like with AIDS or just old age, the wall fails.

Right.

The center liquefies.

The tubercle ruptures into a bronchial.

Now you have a cavity full of multiplying bacteria spilling into the airways.

That is active secondary TB.

The patient starts coughing up blood, spreading it to others.

And if it spills into the blood...

Then you get miliary TB.

It's named after millet seeds, because that's what it looks like on autopsy.

Thousands of tiny granulomas seeded into the kidneys, the liver, the brain.

It's catastrophic.

Before we move on, the text also mentions the gon complex.

Is that just another name for the granuloma?

It's the granuloma plus the involved lymph node.

So if you see a calcified spot in the lung and a calcified lymph node on a chest x -ray figure 18 .5 shows this really well.

That's a gon complex.

It's like the scar of a past battle.

Okay.

So we have a bug that hides inside our own cells.

This has to make diagnosis tricky.

You mentioned earlier that antibodies aren't the answer.

No, not at all.

The body makes them, but they're useless.

They can't reach the bacteria inside the macrophage.

So protection depends entirely on cell -mediated immunity, specifically your CD4 plus T cells.

And that's the whole basis of the skin test everyone's had at some point, the Man2 test.

Right.

We inject a purified protein derivative, PPD, under the skin.

We aren't asking, do you have antibodies?

We're asking, do your T cells remember this protein?

It's a recall test for your immune infantry.

Precisely.

And because T cells take time to migrate to the skin, that's why it's a delayed hypersensitivity reaction.

You have to wait 48 to 72 hours to read it.

And reading it isn't just looking for a red spot, which is a common mistake.

A crucial point.

You have to ignore the redness.

You are feeling for induration, a hard swelling.

If you can feel a hard bump larger than 10 millimeters, that's generally considered positive.

But here's the snag.

I have friends from Europe or India who always test positive, but they don't have active TB.

That's the BCG vaccine.

Many countries use an attenuated strain of bovine TB, that's M.

bovis, as a vaccine.

If you've had it, your T cells will attack that PCD and you get a positive skin test every time.

Which sounds like a diagnostic nightmare.

You don't want to treat someone for six months because of a vaccine they got 20 years ago.

Exactly.

And that's why the IGRA test, the interferon gamma release assay, is taking over.

It's a blood test.

It looks for T cell responses to antigens that are specific to M tuberculosis and not found in the BCG vaccine.

So no false positives.

No false positives.

Much cleaner.

But speed is also an issue, right?

If cultures take wicks, how do we know now?

We still do the acid -fast butymsmear because it's quick, but it can miss a lot of cases.

The real game changer is not nucleic acid amplification tests,

PCR basically.

We can detect the bacterial DNA in just a few hours.

Okay, so let's say the PCR is positive.

We have a diagnosis.

Now we have to treat it.

And this is where patients usually just balk.

Doc, you want me to take four different pills for how long?

Six months.

Minimum.

Why?

If I have strep throat, I take penicillin for 10 days and I'm good to go.

Why does TB take half a year?

It goes right back to the biology we started with.

First,

antibiotics mostly kill dividing cells.

TB grows so slowly that it's often just sitting there, metabolically quiet.

It's hard to poison something that isn't really eating.

And second, they're hiding out in those vascular cheesy necrotic centers where drugs struggle to penetrate.

You have to just keep the drug pressure on for a very long time to catch them all.

And if you stop early, say at three months because you're feeling better.

The bacteria come roaring back.

And worse, they are now often resistant.

This is how we get MDRTB multidrug resistant and even XDRTB, which is extensively drug resistant.

That's almost impossible to treat.

Almost.

So we have to hit them with everything we've got up front.

The text uses the mnemonic RPE,

rifampin, isoniazid, paraxenamide, and ethambrol.

Use all four.

Isoniazid and rifampin are the heavy hitters.

And to make sure people actually take these meds, there's a strategy called DOT.

Directly Observe Therapy.

Sounds a little Orwellian.

It can feel that way.

Yeah.

A healthcare worker literally watches you swallow the pills.

But if you look at the data in figure 18 .11, when DOT is used, treatment completion rates skyrocket and resistance drops.

It works.

I guess when the stakes are that high, you do what you have to do.

Okay, let's shift gears.

Same family, different horror story.

Leprosy, Hansen's disease.

Caused by mycobacterium leprae.

It's related to TB, but it's even slower.

And weirdly, you cannot grow it in artificial culture.

We used to have to grow it in mouse foot pads.

And famously, armadillos.

Yes.

The nine -banded armadillo is a natural reservoir in the southern U .S.

They have a low body temperature, which the bacteria just love.

Note to self, do not cuddle the armadillos.

Now the text describes leprosy as a spectrum.

Two people can have the same infection, but look completely different.

Why is that?

It's a fascinating lesson in immunology.

It all depends on whether your T cells show up to the fight.

On one end, you have tuberculoid leprosy.

The patient has a strong immune response.

Okay.

They contain the infection.

You see these blotchy anesthetic skin lesions.

And anesthetic just means they're numb because the inflammation damages the nerves.

But because the immune system is winning, there are very few bacteria found.

It's Posse bacillary.

And on the other end,

the nightmare scenario.

That's lepromatous leprosy.

Here the cell -mediated immunity is weak or absent.

The bacteria just multiply, uncheck billions of them.

This is multi -bacillary.

You get those classic disfiguring nodules, the lion -like facial features, and extensive tissue destruction.

So it's the same bug, just a completely different host response.

Exactly.

Figure 18 .3 in the book illustrates this perfectly.

If your T cells work, you get a patch of numb skin.

If they don't, you could lose your fingers.

Wow.

The chapter also briefly mentions atypical mycobacteria.

Yeah, the runyan groups.

These are environmental ones found in soil and water.

M.

kinsasi can mimic TBA.

MnOm causes fish tank granuloma.

And importantly, there is eminocobacterium avium intracellular.

And that's a big problem for AIDS patients.

A huge problem.

It causes disseminated disease and is notoriously drug -resistant.

All right, let's bring it home with the final group in chapter 18, the actinomycetes.

These guys just seem confused.

They really do.

They're true bacteria.

They're prokaryotes.

You treat them with antibiotics.

But if you look at them under the microscope, they look exactly like fungi.

They form these long branching filaments.

We have two main characters to distinguish here, actinomyces and nocardia.

How do we tell them apart?

The easiest way is context and error.

Actinomyces israeli is an anaerobe.

It hates oxygen.

It's actually part of your normal mouth flora.

So the call is coming from inside the house.

Yes.

It usually causes infection after some kind of trauma, a tooth extraction or a broken jaw.

Bacteria get deep into the tissue where there's no oxygen and they grow.

It leads to these draining abscesses, often in the jaw area.

Classically called lumpy jaw.

Exactly.

And there's a very specific diagnostic clue in the pus.

The sulfur granules.

The sulfur granules.

If you look at figure 18 .16, you can see these little yellow particles.

Now be careful.

They aren't actually sulfur.

They're just micro colonies of the bacteria.

But if you see lumpy jaw and yellow granules, you have to think actinomyces.

And you treat it with penicillin.

Right.

Now flip the coin.

Nocardia.

Nocardia is the opposite.

It's an aerobe.

It loved oxygen.

It lives in the soil, not your mouth.

So you inhale it.

It causes pneumonia that can mimic TB.

Or it can spread to the brain, usually in immunocompromised people.

Does it have the sulfur granules?

No.

But it has its own trick.

Nocardia is weakly acid fast.

It has some mycolic acid, not as much as TB, but just enough to hold the red dye if you use a weak acid wash.

And actinomyces is not acid fast at all.

Not at all.

And for treatment.

Penicillin again.

No.

Nocardia just laughs at penicillin.

If you use TMP SMX or trimethoprim sulfamethasoxazole.

Which brings us to the ultimate mnemonic for this part of the chapter.

SNAP.

Yes.

It's a lifesaver for students.

Sulfonamides for Nocardia.

Candomyces gets penicillin.

SNAP.

I love it.

Okay.

We have covered a massive amount of ground from waxy armor to armadillos.

Let's distill this down.

What are the top takeaways our listener needs to lock in?

All right.

Here's the synthesis.

One, the wax fortress.

Mycobacteria are defined by mycolic acids.

This makes them acid fast.

Two,

TB is a macrophage disease.

It survives inside the phagosome.

The granuloma is the body's attempt to jail the bug.

Three,

diagnosis.

PPD measures T cell memory.

But watch out for BCG false positives.

IGRA is more specific.

Four, TB treatment is a marathon.

RIPE drugs for six months.

Right.

Five, leprosy.

It's a spectrum based on your immunity.

Strong immunity equals tuberculoid.

Weak immunity equals lepromatous.

And finally six, actinomycetes.

Remember SNAP.

Sulfonamides for nocardia.

Actinomycetes gets penicillin.

That's perfect.

You know, looking at this chapter, it's fascinating to compare M tuberculosis to something like,

say, Ebola.

Ebola is this terrifying fast killer.

But TB, TB plays the long game.

And that's exactly why it's so successful, evolutionarily speaking.

An organism that kills its host in three days burns itself out.

TB keeps the host alive, keeps them walking around, coughing, going to work.

It uses the host to travel.

Exactly.

In terms of evolution, slow and steady really does win the race.

It is the ultimate survivalist.

A chilling thought to end on.

The wax fortress is not to be underestimated.

Thanks so much for helping us break down chapter 18.

Always a pleasure.

And to our listener, thanks for diving deep with us.

Keep those synapses firing and we'll see you in the next deep dive.