Chapter 2: Normal Flora & the Human Microbiome

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

You know, I had a bit of an existential crisis this morning while I was drinking my coffee.

Oh yeah.

Yeah.

I was sitting there, kitchen was dead silent, nobody else home.

I felt totally alone.

But then I looked down at my hand holding the mug and I remembered what we were covering today.

And I realized I wasn't alone at all.

In fact, I was completely outnumbered.

You certainly were.

If we are going strictly by cell count, you are basically a walking, talking ecosystem.

That is both a really cool perspective and also slightly terrifying.

It's a lot to process.

Welcome back to The Deep Dive, everyone.

Today we are stripping away the macro world, the politics, the news, the noise, and we are looking at the microscopic tenants that live on us and, you know, in us.

We are tackling normal flora.

Which is, I mean, it's a fundamental concept if you want to understand human health.

We're basing this deep dive on chapter two of Lippincott Illustrated

Reviews, microbiology.

And for the students listening, whether you're prepping for step one or just trying to get through your micro course, this is absolutely high yield territory.

Right.

The mission today isn't just to memorize a laundry list of Latin names because that's boring and nobody retains it.

We want to understand the ecosystem.

Who lives where?

Why do they live there?

And, well, most importantly, what happens when the tenants start crashing the apartment?

Or when they move into a room they weren't invited to.

In clinical medicine, understanding the human microbiome is all about prediction.

If you know what bacteria normally live in the colon, you know exactly what to treat for if the colon gets perforated.

And we call these organisms normal flora,

or what was the other term?

Commensals.

Commensals.

Yeah.

I looked that up actually.

It's Latin.

Essentially meaning organisms that die in together.

That's a polite way of putting it.

They are eating at our table.

And usually they are polite guests.

In a healthy person, they're non -pathogenic.

They hang out, they eat, they reproduce, but they don't cause disease.

But there are strict rules about where the dinner party happens, right?

I see there are some VIP sections of the body where no guests are allowed.

Yeah.

Absolutely.

This is the baseline rule for this whole deep dive.

If it's open to the outside world, skin, mouth, gut, vagina, it's crowded.

But the internal organs must be sterile.

The spleen, pancreas, liver, bladder, the central nervous system, and the blood itself.

Those should be ghost towns.

So if I find bacteria in someone's blood culture, that's never normal flora.

Never.

If you find bacteria in the blood, that's not a dinner party.

That's an invasion.

That is sepsis.

Okay.

Before we start the grand tour of the body, how do we even know this stuff?

Because for a long time we were just, what, taking swabs and smearing them on petri dishes?

That was the gold standard for decades.

Culturing.

You take a sample, put it on nutrient agar, and see what grows.

But there was a massive blind spot in that method.

The picky eaters.

Exactly.

We call them fastidious organisms, or sometimes non -culturable.

A huge percentage of the bacteria living on us simply refused to grow in a lab environment.

So we just couldn't see them?

We couldn't.

They need specific conditions or specific neighbors that a petri dish can't provide.

So for years, we thought the population was much, much smaller than it actually is.

So how do we fix this?

How do we see the invisible ones now?

The game changer was DNA sequencing.

Specifically, looking for something called the 16S ribosomal RNA gene.

Okay, hold on.

You can't just drop 16S ribosomal RNA and not explain it.

That sounds like Star Trek techno babble.

Fair enough.

Think of it like a universal barcode for bacteria.

Every single bacterium has this gene, but the stripes on the barcode are slightly different for each species.

So now we don't need to grow them.

You just scan the environment for the barcode?

We just scan for the barcodes.

Exactly.

We take a swab, run it through the sequencer, and it prints out a receipt of everyone who was at the party.

And that receipt showed us.

Why?

What?

That the party was way bigger than we thought.

Massive.

We're talking hundreds of species that we never knew existed because we couldn't grow them.

I assume my receipt looks different than yours.

Oh, completely.

It's not static.

It changes based on your diet, your age, where you live, your physiology.

But, and this is why we study it, there are dominant players that tend to show up in just about everyone.

All right, let's get into it.

Let's start the tour from the outside in.

The skin.

If you're following along in the book, we're looking at figure 2 .1.

The skin is our first line of defense.

It's tough, it's dry, it's salty.

It's actually not a very friendly place for most bacteria.

But things do live there.

Oh, yes.

But we need to distinguish between tourists and residents.

You have transient flora.

These are bugs you pick up from a doorknob or a handshake.

They sit on the surface, but if you wash your hands, they're gone.

And the residents.

The residents live in the, let's say, penthouse suites, the deeper layers of the epidermis, the hair follicles, the sweat glands.

You can scrub your hands like a surgeon for five minutes, and you'll remove the surface layer.

But those residents will regenerate and repopulate the surface within hours.

So who are the main characters here?

The king of the skin.

For aerobic bacteria, the ones that need oxygen, the heavyweight champion is Staphylococcus epidermidis.

It accounts for, I think, about 90 % of the skin aerobes.

Wow.

You also see Staphylococcus aureus, especially in the nose and moist areas like the armpits.

But you mentioned hair follicles.

Those are deep.

Is there oxygen down there?

Very little.

So down there, you find the anaerobes.

And the big name is propionabacterium acnes.

That name sounds hauntingly familiar to my teenage self.

It's exactly what you think.

These bacteria live in the sebaceous glands.

They break down oils, produce fatty acids, and cause inflammation.

That is the pathophysiology of acne.

Okay.

So S epidermidis is on everyone's skin.

It's normal.

So why do we freak out about it in a hospital?

This is where context is everything.

On your arm, S epidermidis is harmless, but it has a superpower.

It loves plastic.

It produces this slime layer that lets it stick to artificial surfaces.

Like an IV catheter.

Exactly.

Say you insert an IV line.

If you drag a little bit of that S epidermidis from the skin into the vein with the needle, it finds that plastic tube adheres to it, builds a biofilm, and starts pumping bacteria directly into the bloodstream.

So a harmless skin bug becomes a blood infection just because we gave it a highway.

Correct.

It's a crime of opportunity.

There's another grim example in figure 2 .1.

It shows a photograph of an arm with some really nasty scarring and abscesses.

The caption calls it skin popping.

Yeah, that looked painful.

What is that exactly?

It's a term used in IV drug use.

When someone can't find a vein anymore, maybe their veins are collapsed, they might inject the drug straight under the skin subcutaneously.

Well, think about it.

You are taking a needle, covered in normal skin flora, and pushing it deep into the soft tissue.

But you're also injecting a drug that might restrict blood flow.

You're basically creating a closed low oxygen pocket filled with skin bacteria.

Which is a paradise for those anaerobes we talked about.

You got it.

It's perfect for the bugs that hate oxygen.

They multiply, eat the tissue, and cause these nasty necrotizing soft tissue infections.

It's a classic case of displacement taking good bugs and forcing them into a bad neighborhood.

Let's move up the map.

The eyes.

Figure 2 .2.

I always sort of assumed the eyes were sterile because, well, tears seem like they'd wash everything away.

You're half right.

Tears are a powerful defense.

They wash the eye mechanically, but they also contain an enzyme called lysozyme.

Lysozyme.

Lysozyme actually breaks down bacterial cell walls.

It's chemical warfare.

So the population is low.

Very low.

But the conjunctiva, that thin membrane over the white of the eye, isn't totally sterile.

It usually has some S.

epidermidis, maybe some corina bacteria.

But generally, the defenses keep the numbers way down.

Okay.

But just below the eyes, we have the mouth.

And I feel like the mouth is a completely different story.

It just feels like a jungle in there.

It is a jungle.

It's warm, it's wet, and we constantly feed it sugar.

It is heavily colonized.

But if you only remember one name for your exams, remember streptococcus mutans.

S.

mutans.

The dentist's enemy.

The primary cause of dental carries cavities.

Here's how it works.

S.

mutans lives in the plaque on your teeth.

When you eat sugar, the bacteria metabolize it and excrete lactic acid.

And acid dissolves.

Well, it dissolves teeth.

Basically.

It demineralizes the enamel and you get a hole.

But the danger of S.

mutans goes way beyond a root canal.

There's a really serious connection between the mouth and the heart.

This always freaks me out.

Connect the dots for us.

How does a dirty tooth stop a heart?

It's all about access.

Let's say a patient has a damaged heart valve, maybe from rheumatic fever as a kid or a congenital defect.

That valve has a rough surface.

Now that patient goes to the dentist for a deep cleaning.

Or maybe they just floss too hard.

Their gums bleed.

And the S.

mutans gets into the blood.

Yes.

Transient bacteremia.

Usually our immune system cleans that up in minutes.

But if that bacteria finds that damaged heart valve, it can stick to it.

It builds a vegetation, a big lump of bacteria and clot.

That is infective endocarditis.

Which can destroy the valve.

It can destroy the valve or it can break off and cause a stroke.

It's incredibly serious.

That's why we often give antibiotics before dental work to patients with those kinds of heart conditions.

Wild.

Okay, moving down.

We swallow the food and the bacteria and it hits the stomach.

The stomach is the bouncer.

It has a pH of around two or three.

It's an acid bath.

Most bacteria that you swallow die instantly.

So the bacterial count in the stomach is actually really low.

But some must survive and they move into the intestines.

Yeah.

And this is where the numbers get kind of insane.

You get astronomical.

As you move from the small intestine to the large intestine, the environment becomes less acidic, more stagnant.

By the time you reach the colon, you are looking at 10 to the 11th bacteria per gram of fecal matter.

10 to the 11th.

That's 100 billion.

100 billion per gram.

It is mostly bacteria.

But here is the biggest myth in microbiology.

If I asked a random person to name a gut bacteria, what do they say?

E.

coli.

It's always E.

coli.

Right.

And E.

coli is important.

Don't get me wrong.

But in terms of sheer numbers,

it's a tiny minority.

It makes up less than 0 .1 % of the total population.

Wait, really?

So who's actually running the show down there?

Anaerobes, bacteria that die in oxygen.

They thrive in the colon.

The dominant species by far is bacteroids, specifically bacteroids fragilis.

They outnumber E.

coli something like a thousand to one.

Okay.

So why does everyone talk about E.

coli and not bacteroids?

Because E.

coli is easy to grow in the lab.

It's the one we saw first, but medically you have to respect the bacteroids.

Why?

What does it do when it gets out of line?

Think about trauma.

A gunshot wound to the belly, a ruptured appendix.

If the colon wall breaks, you aren't just spilling poop into the sterile abdomen.

You are spilling billions of bacteroids.

And since they are anaeros and the gullies all closed up.

They love it.

Bacteroids fragilis is the leading cause of intra -abdominal abscesses.

It causes peritonitis.

It's harmless inside the tube, but it is deadly if it gets outside.

I'm sensing a theme here.

Location, location, location.

That is the whole theme.

Let's finish our tour with the urogenital tract.

Figure 2 .5 in the book.

We're focusing on the adult vagina here.

The adult vagina is a fascinating ecosystem because it is heavily guarded by one specific group, lactobacillus.

Like in yogurt.

Very similar.

And they do the same thing they do in milk.

They produce acid.

They ferment sugars and maintain the vaginal pH at a very acidic level, usually below 4 .5.

And that acid is a shield.

Yes, absolutely.

Most pathogens, like E.

coli or the bug that causes gonorrhea, they hate acid.

They can't grow well in it.

So the lactobacillus acts as a chemical gatekeeper.

But this system is fragile.

I feel like we mess this up with medication all the time.

We do.

It's the classic antibiotic irony.

You get a sinus infection.

Your doctor prescribes a broad spectrum antibiotic.

It kills the bacteria in your sinuses.

Great.

But it also travels through your blood and wipes out the lactobacillus in the vagina.

And once the gatekeeper is dead.

The pH rises.

It becomes neutral.

And now the opportunists move in.

You might get an overgrowth of yeast, like candied albicans causing a yeast infection.

Or you get an overgrowth of other anaerobic bacteria like Gardnerella vaginalis.

Which is bacterial vaginosis?

Exactly.

It's not an infection you catch from someone.

It's a dysbiosis, an internal collapse of the normal order.

Quick practical question on urine.

We said the bladder is sterile.

So why is a urine test so hard to get right?

Because of the exit strategy.

The urine is sterile in the bladder.

But it has to pass through the urethra to get out.

And the tip of the urethra is contaminated with skin, flora staph, strep, all that stuff.

So the first bit of urine washes those bugs right into the cup.

Exactly.

And then the lab thinks you have an infection when you don't.

That's why we always ask for a midstream catch.

You pee a little into the toilet to flush out the contaminants.

Then you catch the sterile urine in the cup.

Pro tip.

Always pee a little first.

Got it.

Okay, we've talked about the dangers.

But we evolved with these bugs.

We tolerate them.

There has to be a benefit.

Why are we friends with benefits?

It's a strategic alliance.

Lippincott outlines four main benefits.

The first one is simple real estate.

We call it colonization resistance.

I like the parking spot analogy for this one.

It works perfectly.

Imagine your gut lining is a parking lot.

If every single spot is taken by a harmless bacteroids or E.

coli, and a pathogenic salmonella shows up from a bad chicken sandwich,

where is it coming to park?

Nowhere.

It just keeps circling until it gets flushed out.

Exactly.

By occupying the space and eating the available nutrients, the normal flora starves out the invaders.

Oh wait, if they're eating our nutrients, aren't they stealing from us?

A little bit.

But they pay rent.

And that's the second benefit.

Nutrition.

Gut bacteria synthesize vitamin K.

Which we need for blood clotting.

Asperately.

Humans cannot make vitamin K on our own.

We get some from leafy greens, but we rely on our gut bacteria to keep our levels high enough to prevent bleeding disorders.

So they stop us from bleeding out.

That's a pretty good rent payment.

What else?

Immune training.

This is huge.

When a baby is born, its immune system is naive.

It doesn't know what to attack.

The early colonization of the gut teaches the immune system.

It says, this is normal, tolerate this, but attack that.

And without that training.

The immune system can become hypersensitive, leading to things like allergies or autoimmune issues later in life.

And the fourth benefit.

Chemical warfare.

We mentioned the acid in the vagina, but some gut bacteria produce bacteriocins.

These are proteins that literally punch holes in other bacteria.

They are actively murdering our enemies to protect their turf.

Brutal.

I love it.

But we have to look at the dark side again.

We touched on displacement, but the chapter lists four main ways good bugs go bad.

Right.

Let's just recap these because this is what kills patients.

Number one was displacement.

That's the catheter infection or aspiration pneumonia.

Normal bug, wrong place.

Number two was pathogen advantage.

And this brings us back to antibiotics.

Specifically to Clostridium difficile or C.

diff.

This is the nightmare scenario of the parking lot analogy.

You give strong antibiotics, you nuke the parking lot, all the normal flora die.

But C.

diff doesn't.

C.

diff has a trick.

It forms spores.

It goes into this hibernation shell that resists the antibiotic.

Once the drug is gone and the parking lot is empty,

C.

diff wakes up.

It looks around, sees all this free real estate and it takes over everything.

It releases toxins.

Horrible toxins.

They destroy the lining of the gut, creating a pseudo membrane, a layer of dead tissue.

It causes severe life threatening diarrhea.

Number three was a weird one.

Carcinogens.

Yeah.

This one is sobering.

We tend to think of the liver as the organ that processes chemicals, but the gut bacteria are chemically active too.

They can metabolize things we eat.

The example in the text is cyclamate, an artificial sweetener.

Which is banned in the US.

Right.

It is now.

Yeah.

And the reason is interesting.

The sweetener itself isn't a carcinogen, but the gut bacteria turn it into cyclohexamin, which causes bladder cancer.

It makes you wonder what else are we eating that is safe until our bacteria get a hold of it.

That's a disturbing thought.

And finally, number four, the immunocompromised host.

This brings it all back to the host.

If your immune system crashes from AIDS, chemotherapy, transplant drugs, the truce ends.

The normal flora sense the weakness.

Bacteria that were harmless suddenly start invading the blood or the tissues.

It proves that normal flora is only normal if the host is healthy.

Okay.

We have covered a massive amount of ground from the skin to the gut, the good and the bad.

I think it's time to see if everyone listening was actually paying attention.

Let's put them to the test.

All right.

We're going to run through the study questions from the chapter.

I'll tee up the scenario.

You at home think of the answer and then our expert here will confirm.

Ready?

I'm ready.

Scenario one.

We talked about the adult vagina.

What is the primary mechanism by which lactobacilli protect that environment?

Okay.

Think back to the yogurt comparison.

The answer is acid production.

They maintain an acidic low pH.

That acid is what inhibits the pathogens.

Scenario two.

You have a patient in the hospital.

They've been on heavy antibiotics for a week for a foot infection.

Suddenly they develop severe watery diarrhea and abdominal pain.

What is the most likely thing that happened here?

This is the classic history for Clostridium difficile colitis.

The antibiotics wiped out the normal flora, the competition allowing the drug resistant C.

diff to overgrow and release its toxins.

If you hear antibiotics followed by diarrhea, you have to think C.

diff.

Scenario three.

Simple numbers game.

If you had a bet on which part of the body has the absolute highest density of bacteria, where are you putting your money?

Skin, mouth or gut?

It's the gut.

No 100 billion per gram.

The skin is a desert compared to the jungle of the colon.

All right.

Hopefully you crush that.

Before we let you go, give us the 30 second elevator pitch.

If they forget everything else, what needs to stick?

Okay.

High yield summary.

Normal flora is essential.

It provides vitamin K.

It trains our immune system and it protects us from pathogens by competition, but it is location dependent.

Esipidermitis is good on skin, bad in blood.

E.

coli is good in the gut, bad in the bladder.

Know the location and you understand the disease.

Context is key.

I like that.

And one final thought to leave you with.

We spent this whole time talking about colonization bugs living on you, but there is a scary gray area called the carrier state.

Typhoid Mary.

Exactly.

A carrier is a person who is healthy, asymptomatic, but they are carrying a true pathogen, not just normal flora.

Someone can carry Salmonella typhi or Neisseria meningitidis in their throat or gut, feel totally fine, but shed that bacteria and infect everyone around them.

So you can be a biological weapon and not even know it.

Essentially.

So the question is always, is this bug a guest, which is normal flora, or is it a silent killer waiting to jump to a new host?

The line is a lot finer than you think.

On that delightfully paranoid note, we are going to sign off.

Thank you for letting us take you on a tour of your own personal zoo.

It was a pleasure.

Big thanks from the Last Minute Lecture Team for tuning in.

Keep learning, stay curious, and remember, even when you're alone, you're never truly alone.

See you next time.