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Welcome back to The Deep Dive.
So imagine for a second you're an architect.
You're building a fortress.
Okay.
The first thing you build, the most important thing, is the walls,
right?
Thick, strong walls.
That's just basic engineering.
And it's basic microbiology.
Almost every bacterium we ever talk about is defined by its wall, its shape, its protection, how it stains.
Until today.
Uh -huh.
Until you meet today's subject.
Exactly.
Today we are talking about the anarchists of the microbial world.
We are deep diving into chapter 16 of Lippincott Illustrated Reviews, microbiology.
We're talking about mycoplasma.
The bacteria that basically looked at the blueprints saw the line that said, Peppa dog glycan cell wall.
It just went, nah, I'm good.
And that one decision to just drop the wall, it changes everything, doesn't it?
Absolutely everything.
How they look, how they act, and for you, our listeners, how you treat them.
It really is the case of the missing wall.
So that's our mission for this deep dive.
We're going to break down chapter 16, last minute lecture style.
We'll cover their unique structure, why penicillin is useless.
Then we'll hit the famous walking pneumonia.
And finally,
the more complicated genital mycoplasmas.
So if you have an exam coming up, or you just want to get this straight once and for all, stick around.
Let's start with that fundamental feature.
The no wall concept.
The book defines them as the smallest free living prokaryotes.
And when they say smallest, they're not kidding.
No, we're talking like 0 .2 to 0 .3 microns.
That's so small, they can actually slip through the filters we use to sterilize liquids.
Filters that stop other bacteria.
Right, they just squeeze right through.
Because they're squishy.
That's a great way to put it.
Yeah, they're squishy.
Most bacteria have that rigid cup to dog glycan wall.
It's like armor.
Mycoplasma doesn't have it.
So the book uses the word cleomorphic.
Which is a perfect exam word.
It just means many shapes.
They're not a rod, not a sphere.
They're just kind of formless.
Which is why people thought they were viruses for a while, right?
Exactly.
Because they pass through the filters.
But they're not viruses.
They can replicate on their own.
But how?
I mean, without a wall, how do you keep all your insides from just spilling out?
Ah, they have a trick.
This is one of the tool -less parts.
Since they can't build a wall, they reinforce their membrane.
And they're the only prokaryotes that do this.
They use sterols.
Cholesterol, the stuff we're always trying to avoid.
That's very same.
But here's the kicker.
They can't make it themselves.
So they have to get it from somewhere.
They steal it.
They're total parasites.
They require an external source of cholesterol to grow, which means they have to get it from their host.
So they literally build their skin out of our fat.
In a way, yes.
It's incredibly efficient.
Okay, let's connect this to the clinic, because the book makes a big deal of this.
No wall means no gram stain.
Yes.
This is a huge trip up for students.
You order a gram stain on a sample, and if it's mycoplasma, you see nothing.
Because the stain has nothing to stick to.
Right.
No peptidoglycan wall, no crystal violet retention.
They're just invisible on that test.
And the implications for treatment are even bigger.
This is probably the most high yield point in this section.
Oh, absolutely.
You think about our first line antibiotics,
penicillins, cephalosporins.
Beta -lactams.
Right.
And what do they all attack?
The cell wall, cell wall synthesis.
So it's like trying to knock down a wall that isn't even there.
It's completely useless.
You can give a patient a massive dose of penicillin, and the mycoplasma won't even notice.
It is naturally 100 % resistant.
Okay, so that's rule number one for the cheat sheet.
Do not use beta -lactams for mycoplasma.
Ever.
So if we can't stain them, how do we find them?
The book mentions this classic fried egg appearance.
I love this description.
If you manage to grow them on special agar, which has to be enriched with cholesterol, by the way, the colonies look just like a tiny fried egg sunny side up.
That's a great visual.
It is, but, and this is a big but,
you almost never do this for a clinical diagnosis.
Why not?
They're fastidious growers, which is the nice way of saying they are incredibly slow and picky.
It can take weeks to see a colony.
So if a patient is sick now, you can't tell them to come back in three weeks.
Exactly.
So fried egg is a great test question, but it's not a practical diagnostic tool.
Got it.
Okay, let's move on to the main pathogen in this family.
Part two of the chapter.
Mycoplasma pneumonia.
The famous walking pneumonia.
I feel like everyone says they have this at some point.
Oh, it's just walking pneumonia.
What does that actually mean?
It's really about comparing it to typical pneumonia.
Typical pneumonia, like from strep pneumo, hits you like a truck.
High fever, shaking chills, you're coughing up nasty stuff.
You are not walking anywhere.
You're in bed.
You're in bed.
But with mycoplasma pneumonia,
it's insidious.
It just sort of creeps up on you.
So like a headache, low grade fever, you just feel run down.
Right, malaise.
And then after a few days, this cough starts, a really dry hacking non -productive cough that just won't quit.
And because you're not on death's door, you keep going about your life.
You keep going to class, you go to work, you stay ambulatory, hence walking pneumonia.
Which of course makes you a great vector for spreading it.
A perfect vector.
You're spreading it through respiratory droplets and all these close contact settings.
That's why the book points out places like military barracks, college dorms, prisons.
And it hits a really specific age group, doesn't it?
It does.
It's the number one cause of pneumonia in older children and young adults.
The age is five to 20.
So a 19 -year -old college student with a nagging, dry cough.
Your alarm bells for mycoplasma should be going off.
Okay.
So how does this tiny wall -less blob cause all this trouble in the lungs?
What's the mechanism?
It's all about attachment.
It has this special protein on its surface called the P1 adhesion.
Think of it like a little grappling hook.
That's a perfect analogy.
It uses that hook to grab onto the cilia in our respiratory tract.
The cilia, those are the little hairs that sweep everything up and out.
The mucociliary escalator.
Exactly.
And mycoplasma binds to them and just paralyzes them.
It stops them from beating.
It's called ciliostasis.
So the escalator grinds to a halt.
The escalator's broken, mucus pulls up, bacteria multiply, and then it starts releasing nasty stuff like hydrogen peroxide.
So it's actively damaging the cells.
Oh yeah.
And the book mentions a specific exotoxin, the CARDS toxin, that acts a lot like pertussis toxin.
It kills the epithelial cells and they start to slough off.
And that's what's causing the inflammation and that horrible cough.
That's it.
Your body's trying to clear all that debris without any help from the cilia.
So when you get a chest x -ray on these patients, you see something kind of strange.
The classic clinical radiologic dissociation.
This is so key.
The patient might walk in, they're talking, they seem a bit tired.
But the x -ray looks awful.
The x -ray, which is figure 16 .3 in the text,
shows this patchy, diffuse, branch -out pneumonia.
It looks way worse than the patient feels.
That mismatch is a huge clue.
Now let's talk about one of the weirdest parts of this infection.
The immune response can go a bit haywire.
You're talking about cold agglutinins.
Yes.
What is that?
So in about two -thirds of patients, the body makes these IgM antibodies to fight the infection.
But for some reason, these antibodies also recognize an antigen on our own red blood cells.
So it's an autoimmune reaction.
Friendly fire.
But it's conditional.
These IgM antibodies only bind to red blood cells at cold temperatures, like below 4 degrees Celsius.
So in your fingers and toes in the winter.
Exactly.
Your fingertips, toes, the tip of your nose.
When those areas get cold, the antibodies cause the red blood cells to clump together, agglutinate.
Which would block blood flow.
It can.
It can cause Raynaud's phenomenon, where your fingers turn white or blue.
And it can also lead to a mild hemolytic anemia as the body clears out these clump cells.
Wow.
That is such a specific high -yield fact.
If you see cold agglutinins on an exam, you can be 99 % sure the answer is mycoplasma pneumonia.
Oh, okay.
So diagnosis.
Gram stain is useless.
Culture is too slow.
What do we actually do?
Today, it's all about molecular tests.
PCR is the gold standard because it's fast and specific.
Serology, looking for those IgM antibodies with an ELISA test, is also still used a lot.
And treatment.
We know penicillin is out.
What's in?
When you drugs it, don't target the cell wall.
So protein synthesis inhibitors or DNA inhibitors.
Macrolides like azithromycin.
That's the go -to.
The Z -Pak.
Or tetracyclines, like doxycycline.
For adults, you can also use fluoroquinolones like levofloxacin.
Okay, that seems straightforward enough.
Let's shift gears now.
We're leaving the lungs.
Part three of the chapter covers the genital mycoplasmas.
Right.
And this is where it gets a little more confusing for clinicians.
We're talking about three main organisms.
Mycoplasma hominis,
ureoplasma urelinicum, and mycoplasma genitalium.
And the book says right away that just finding them doesn't mean they're causing a problem.
Exactly.
M.
hominis and ureoplasma are often just commensals.
They can hang out in the urogenital tract of perfectly healthy people without causing any harm.
So it's hard to prove they're the cause of an infection.
Right.
You have to look at the whole clinical picture.
But when they do cause disease, it can be pretty serious.
Okay.
Let's start with mycoplasma hominis.
What's the association there?
Think post -partum.
It's a known cause of fever after childbirth or abortion.
It's also linked to pelvic inflammatory disease, or PID.
And the text has a really important note about its drug resistance.
This is a classic trap.
We just said macrolides are great for mycoplasma, right?
Yeah, azithromycin.
Well, M.
hominis is naturally resistant to erythromycin, a common macrolide.
It breaks the rule.
It does.
So for a post -partum fever you think is M.
hominis, you'd use doxycycline or maybe clindamycin instead.
Good to know.
Okay.
Next up, ureoplasma ureolyticum.
The name gives it away.
It loves urea.
It has a urease enzyme.
This one is a big cause of non -gonococcal urethritis, NGU, in men.
Meaning urethritis that isn't caused by gonorrhea.
And in women, it's linked to endometritis and pregnancy complications, like premature labor.
So that's definitely not just a commensal in that case.
And what about the last one, mycoplasma genitalium?
This one is now recognized as a true STI.
It's not considered normal flora.
If you find it, it's a pathogen.
And it causes urethritis and PID, just like the others.
But the treatment is becoming a nightmare.
A huge headache.
The book notes that it's often resistant to doxycycline.
Wait, doxycycline was our backup for M.
hominis.
I know.
So you've got this one species that resist macroids and another that's resisting
tetracyclines.
You really have to know which bug you're fighting.
You can't just use a one -size -fits -all approach.
Not anymore.
For M.
genitalium, the first choice is often a big dose of azithromycin.
But resistance to that is growing, too.
We're running out of easy options.
It feels like these organisms are just designed to be difficult.
They are survivors.
Strooping down to the basics was a brilliant evolutionary move.
Okay, let's wrap this up with a quick recap.
The deep dive cheat sheet for Chapter 16.
What are the absolute must -knows?
All right.
Number one, structure.
No cell wall.
That means no gram stain and no effect from penicillins.
And they have cholesterol in their membrane.
Number two, M.
pneumonia.
That's your walking pneumonia in young adults.
5 to 20.
Gradual onset, dry cough, and the x -ray looks much worse than the patient feels.
Number three, pathogenesis.
The P1 adhesion stops the cilia from working.
You can get those cold agglutinins, the IgM antibodies that clump red blood cells in the cold.
Number four, genital bugs.
Urea plasma causes urethritis.
M.
hominis is postpartum fever.
And finally, number five, tricky treatments.
The general rule is macrolides or tetracyclines.
But remember the exceptions.
M.
hominis is resistant to erythromycin.
M.
genitalium is often resistant to doxycycline.
That's a perfect summary.
It's just amazing how this one feature, the missing wall, dictates everything about them.
It's risky, but it worked.
It's the ultimate case of less is more.
Fewer targets for our drugs, you know, it forces us to be smarter.
It definitely does.
Which kind of leaves us with a final thought to chew on.
Go for it.
As we see more and more resistance in M.
genitalium and we know we can't target a cell wall, what's left?
We're in this arms race against an organism that's already a minimalist.
What do you attack when there's almost nothing left to break?
That is a very good and slightly terrifying question for the future of treatment.
And a good place to end this deep dive into chapter 16.
Hopefully we've helped make your understanding of mycoplasma a little more well structured,
even if they aren't.
Nice.
A huge thanks from the last minute lecture team for studying with us.
We'll catch you on the next deep dive.
Good luck out there.