Hello. My name is Stan Falkow. I am a professor of Microbiology, Immunology, and Medicine
at the Stanford University School of Medicine.
I want to talk to you today about the subject of how we study bacteria --
bacterial pathogens, in particular (those who cause disease)
to try to understand more about our own biology.
The picture you see here is one of
a macrophage, a phagocytic cell,
eating bacteria, and it almost looks as if it's sitting there eating a little bowl of peanuts,
but the bacteria are being taken up, and they're being killed.
And we often think of host-parasite relationships in that way,
but actually, likely the first time this happened in evolution,
the bacteria weren't too happy about it, and they began to evolve ways to resist
these phagocytic creatures (to them) and they began to become pathogens.
So, as humans, and most animals,
we're heir to a veritable sea of different microorganisms.
You see here a picture of bacteria.
And these are the smallest living organisms and the most numerous organisms
that inhabit us. They're simple things.
You can see that they sometimes have organelles of motility,
these flagella, these large cables that come out of them.
But, they're fairly simple.
They're free living, and they replicate quickly.
We also have viruses, which are more molecular entities
that require entry into cells in order to survive,
and they parasitize the cells, and they replicate, and they end up killing the cell,
and sometimes we handle this burden successfully,
and other times we become quite ill from them.
In many parts of the world, no longer in the United States very often,
people also have larger parasites.
You can see here that there is a hookworm, and there are tape worms,
there's the blood fluke, and there are even protozoa that live in the intestinal tract,
and these very often are silent, but sometimes they're killers.
And, most of you sitting there are not thinking that at this moment,
you may have insects grazing in your eyebrows.
But, many of you do.
This little creature loves to be in eyebrows, and is in the hair of many of you.
And there are other kinds of insects and so on that live in more intimate parts
as well. All of these go on to make up the normal flora,
mostly it's microbial cells that make up the flora.
And it's important to understand that you have 10 times
more microbial cells than you do human cells.
And that tends to make us look at the microbial population
with a little more respect than we did before, I think.
Most of the microorganisms that make up our flora,
we have never grown. We only know about them because we can use
genetic and molecular techniques, but we're learning more and more about them
every day. And they come in a variety of sizes, shapes --
some are rods, some are spirals, some are round cocci.
And, there are literally thousands of these microorganisms
that inhabit us, and we acquire them virtually from the moment we're born,
and they remain with us until we die, and then their last function
is to consume us, as it were.
So, the organisms that make up our flora are called commensals.
And commensal means literally we eat from the same plate.
So, they are part of us, and they share in our nutrition.
They vary, depending on who we are, and our background,
and how much stress we're under,
and what kind of diet we have, but in return they do give us some vitamins,
and that helps us.
They also are different in different parts of the body.
The organisms that you see, for example, in the upper respiratory tract --
the nose and the mouth --
are different than those that you see now in the gastrointestinal tract.
And we can often tell what part of the body organisms come from
just by their composition.
One of the important roles that this normal flora plays
is that it is the first line of defense against the incursion by foreigners.
And that includes foreign microorganisms.
It's almost as if they have squatter's rights.
So, an organism that comes in for the first time really has to be able to establish itself.
And, it has to establish itself in the face of this mammoth cauldron
of organisms that are already there.
The difference, then, between a pathogen and a commensal
is very often that pathogens have this ability to establish themselves
in a place that commensals can't.
For example, the liver and the spleen are sterile -- they have no normal flora.
Pathogens can get through the barrier of the normal flora,
and cross the epithelial barrier, and actually enter into the bloodstream,
and go to the liver and spleen.
So, pathogens have means of overcoming things that ordinarily
would inhibit normal commensals.
And pathogens do this because they have an inherent ability
to cross anatomic barriers or to breach defenses
that limit the normal flora and our commensals.
That distinction between pathogens and commensals
is very often genetic. It's in the genes of the microorganism.
So, pathogens can have genes that are different than those that are commensals,
and I'll talk about some of those later on, when we discuss some specific organisms.
Pathogens and commensals alike share a common landscape,
and it's the humans.
Since Adam and Eve, we've had microorganisms.
The Garden of Eden still had microorganisms, as far as we know.
And so, for humans to exist, they have to be able to limit
the microorganisms that they encounter.
And there are really 3 phases of this.
1 phase, the first phase, is a very quick phase.
It's automatic. It's something that we inherit,
and it works at a fundamental level.
There's another phase, which takes a while, but is induced,
so that humans have actually evolved that they recognize
when there has been a microbial incursion.
And then finally, the final phase is an immune phase,
which the host begins to make factors, usually
that will not only limit microorganisms, but actually kill them and clear them.
Now, the first phase is called the innate immune phase,
and it includes things like tears.
The skin, of course, is a barrier. It's like having a little coat of mail that we have around us.
There is acid in the stomach that we'll talk about later.
And, there are all kinds of aspects.
You have little hairs in your respiratory tract that are constantly beating upwards,
and anything foreign that tries to get through
your nose and into the lungs is captured in the mucus film
and actually beaten upwards by the cilia and taken out again.
We have literally dozens of these kinds of mechanisms
that have really become part of us. They are part of us.
The microbes are simple, as I've pointed out before,
and they really have walls, which have certain kinds of
proteins and carbohydrates on their surface.
And there are also aspects... there are those organs that let them swim.
They are studded with all kinds of different molecules.
And these molecules that microorganisms have are unique.
That is, they're not found in anything other than microbes,
and they're never found in animal cells.
And so they form the basis of how we detect microbes
who go beyond the normal barriers that we permit.
And we have on the surface of our cells
a number of different receptors. They're called Toll receptors.
And they are required to be on cells in order for us to have a good innate immune system.
And they recognize things like the molecules that make up the cell wall of bacteria.
They recognize the flagella that bacteria have.
They recognize the kind of nucleic acids that viruses have.
And whenever they exist, they trigger a response
that is local and often extreme in one sense
that we call inflammation
And the inflammatory response that we have to incursion by microbes
is part of our normal host defense system.
So, what happens, as you can see here,
is that there is a signaling from an organism through one of these Toll factors.
That, in turn, signals and calls in defenses. It's almost like the bugle call or clarion call
for defense. And this brings in phagocytic cells --
the ones that are supposed to eat bacteria.
And, when this is successful, as you see here,
the microbes come, and they absolutely have a feast...
the phagocytic cells have a feast on the microorganisms.
And, you can see that here. This is actually a picture of macrophages
eating the plague bacillus, taken in the microscope.
So, when all things are right, this occurs.
You have signaling, phagocytic cells come in, and they kill the bacteria.
The inflammatory response tends to inhibit things.
In the end, there are also cells which take up the final bits of these bacteria
and other microbes that have been killed,
and they process them so that we produce antibodies
that protect us against subsequent infection.
And this is called induced adaptive immunity.
And all of us have these -- normal people have it,
a perfectly normal immune system and an adaptive immune system,
and it works extremely well at limiting the incursion of microorganisms.
We, by and large, are disease-free.
So, the major point is that we spent an awful lot of our evolution
in developing ways to stop microbial intrusion.
And, obviously, it always hasn't been successful.
And, of course, age makes a difference, too.
Young people and old people, like me, are much more likely
to not have working innate and adaptive immune systems.
So, we are more susceptible to infection.
But, by and large, normal, healthy adults are quite resistant
to incursion by infectious agents.
Now, pathogenicity is a kind of life style.
And it involves the fact that an organism has to enter a host.
Once it's in the host, it has to establish itself and persist.
It has to replicate, and then eventually it has to leave the host and be disseminated.
And, in each of these cases, we're taking about
a property of the microorganism that is inherited or special.
Sometimes, commensals and pathogens have similar characteristics.
But, very often, the pathogen goes a little step further.
So, the first thing an organism has to do is to enter.
And humans have nine portals of entry that serve to permit pathogenic organisms
to enter, and that's because we need to eat, and we need to breathe,
and we need to make love to procreate, and each of these provides a portal of entry
for different microbes. And, you may, I know some of you have probably paused and
are counting now to try and figure out the 9, and we should do that.
What you can see is that, they can enter through the conjunctive of the eyes,
and then ears. If the skin becomes broken, they can
enter that way or even insects can do that.
And, they can come in, in any number of places -- the holes, if you will,
that communicate us with the environment.
So, once an organism enters, it has to be able to find a unique place to live,
as it were, a niche.
And very often, the organisms have, on their surface,
things that permit them to attach, but often they have to get there first,
so many organisms are motile, for example.
Wow! I almost got hit there, by this flagella,
and you can see, they swim, and it's a marvelous molecular motor
that goes and lets these creatures swim around in us,
and they often use that to swim through mucus
and through other barriers to get to the surface of cells where they want to be.
They have to stick. And you can see here cells that are sticking
... bacterial cells that are sticking to epithelial cells.
And you can see, it's an intimate kind of interaction
Indeed, it almost appears as if the organism is being caressed by the cell.
Or, vice versa, if you will.
So, this is a very close relationship between the microorganism and the host cell.
The host cell has to have a way to resist this.
An organism that's a pathogen has to have a way to overcome it, one way or the other.
And it can do it by secreting things or actually going ahead and trying to
overcome the host defense mechanisms -- it can avoid it, it can hide,
it can mimic. For example, some bacteria,
you see here, are surrounded by this carbohydrate.
It's called a capsule, and it's in essence,
the microbe surrounds itself with something for all intents and purposes
that prevents phagocytes from picking them up.
And it's almost as if you're trying to pick up a piece of soap in a shower.
The phagocyte tries to grasp the organism, but because of the capsule, it slips away.
It can't grasp it.
So, many organisms have capsules, and indeed, the organisms
that usually cause pneumonia and meningitis are characterized by the fact
that they have these slippery capsules that prevent phagocytosis.
So, that's one way.
Some organisms actually secrete different kinds of enzymes and toxins
that allow this organism to spread through the tissue
or they paralyze the immune functions. And I'm only going to talk about one organism here,
Streptococcus, the common organism that causes Strep Throat.
And it secretes a variety of different enzymes that gets rid of DNA,
it affects the ground substance that you have on cells, the hyaluronidase.
The hyaluronic acid makes up the extracellular matrix.
The bacterium dissolves it.
And, it produces a number of kinds of toxins that will kill cells.
And these things are molecules that adhere to things like
epithelial cells and literally punch holes in their membranes.
And all of this can conspire under the right circumstances,
so that someone will have an infection -- the Streptococcus goes too far.
And you see here the classic case of blood poisoning,
where the Streptococcus has made it through the epithelial barrier,
entered the lymphatic system, and is now moving through the circulation up,
and could kill this patient.
Usually, that's not the outcome, but it's the difference
between an asymptomatic infection and disease.
Some organisms literally breach the surface of epithelial cells
and enter.
And this is a picture of an organism, Salmonella,
which causes food poisoning... actually breaching the epithelial barrier
in the intestinal tract.
And, when you look at a culture of Salmonella
that's approaching epithelial cells, you see this .... almost as if the cell is excited.
And it ruffles because the Salmonella is actually inducing motility in it
and making the cell reach out and ingest it.
And you can see how the organism really, in this vicinity,
can enter the cell.
And so, why do bacteria and viruses invade cells? And all viruses invade cells.
And many bacteria do.
Well, they get a way... the don't have immune surveillance any more,
so there are no Toll receptors on the surface, although there's some inside.
It's better food inside.
It's free from competition of all those normal flora that are outside.
And sometimes, they use the cells as a way of moving from one place to another.
They are able to enter cells because they can recognize specific receptors on the cell
that are normally there, that are there to internalize molecules.
And the bacteria and viruses have learned how to attach to these
by having molecules on their own surface that makes basically the cell take them up
normally, by a normal mechanism.
So, the microorganisms that are pathogens actually know how to trick
the host cell into taking them up.
Now, once they get inside, what are they going to do?
We, our cells, have been programmed that, when they take up particulate material,
to put them in a machinery, the endocytic machinery,
which is designed to break things down and, in fact, in the case of bacteria,
to kill them. But, different pathogens, whether Salmonella,
the Tubercle bacillus, or another one,
the organism that causes Legionnaire's disease,
each of these has a distinct kind of strategy to enter the cell
and outwit the endocytic pathway so that they're not killed.
And indeed, they use this as a new kind of residence where they can replicate
in the safety of being within the cell
or sometimes, they actually will break out into the cytoplasm,
and there's a lecture in this series by Julie Theriot,
where she describes how organisms break out of vacuoles
and actually move around inside cells.
All of these are ways that the organism can get inside,
persist, and escape immune surveillance and replicate.
Now, one of the most common ways that microorganisms use to subvert
the host is to produce poisons. Toxins, that's what they're called.
Sometimes, bacterial components are poisons in themselves.
The outer surface... the cell wall of gram negative bacteria
(one kind of organism, characterized by things like E. coli, the enteric bacteria)
are actually components that are recognized by the innate immune system
and trigger inflammation.
The molecule that does that is called lipopolysaccharide, or LPS.
And it has a moiety on its surface, which is called Lipid A,
which is really the toxic part.
And, when this is recognized by the innate immune system,
as I indicated to you earlier, there's an inflammatory response.
If these microbes get into the bloodstream,
sometimes this inflammatory response goes too far.
And what happens then, as a result of this Toll-like inflammation,
is that you end up with things like meningitis,
and this is a picture from an unfortunate infant
in the pediatric intensive care unit at Stanford,
who has meningococcal meningitis.
And, the presence of this endotoxin in her bloodstream
has literally caused intravascular coagulation and a loss of blood supply.
And, very often, unfortunately, children who have this will lose their limbs,
and it's because the host has responded to a normal component of a microbial cell
in too exuberant a way.
Some toxins affect different parts of our cell biology.
This is another toxin... this is botulism in an infant.
And this is called the Floppy Baby syndrome
because botulism toxin has an effect where, basically, the muscles become flaccid.
Bacterial toxins are among the most potent poisons that are known.
From a biological standpoint, they're favorite tools of cell biologists
because they're extraordinary in their variety and their mode of action.
For example, there are those that go to the surface of the cell,
and they punch holes, and the cell basically bursts open.
There are some, for example, that affect the cytoskeleton of the cell,
and they make the cell traffic the uncanny organisms around.
The bacteria can motor around.
There are those that paralyze the signal transduction,
so that the cell doesn't know how to signal that it's been invaded
and tell the immune system, "Come rescue me!"
And there are all aspects of the normal cell biology
that we have that are detected and utilized, if you will,
by the microorganism for its own purposes.
So, pathogenic bacteria, and other microorganisms
interfere and manipulate, for their own benefit, the normal functions
of host cells, and they do so in a variety of ways.
The whole issue for a microorganism
is replication. Every microbe sitting in its mother's flagellum
learns that the first thing that it must learn to do
is to replicate.
And so, when you watch microbes replicate, they do so by simple binary fission.
And you can see that you simply become surrounded by them over a period of time.
Now, watching this and knowing that this doubling occurs every 20 minutes,
and if it went on freely, a single microorganism would literally grow to something
that was about 23 times the volume of the Earth,
if it actually was permitted to go unfettered.
That of course, doesn't happen. But it gives you some idea of why a surgeon
fears even a single microorganism falling into a surgical field.
And why your mother told you to always clean out a wound.
You don't want these dangerous kind of things in you.
But this is what microorganisms do.
They have to replicate.
So, the whole thing, from entry and all the steps that it has to go through
has this one function in mind: replication.
Make more of yourself.
And the reason that organisms make more of themselves,
it's either to persist or it's to go to a new host.
So, sooner or later, if you're a microorganism,
the host is going to die.
And, it's got to find a new susceptible host.
And organisms very often will induce their exit from the host in a variety of ways.
So, here is a sneeze
that you can see in slow motion.
And all of these little droplets carry a multitude of different organisms.
But, there are also normal features. The gastrointestinal tract
is the single largest source of microorganisms in our body.
It is often the way in which microbes are transmitted
from person to person, because everybody poops.
And, organisms recognize this, and therefore utilize this
as a means. And that's just not as trivial as it sounds.
Because the organism, having lived inside the host,
and been comforted in the warmth of the host and the warmth of cells
now goes into the cold, cruel world, and has to be able to survive there long enough
to be taken up by another susceptible host.
So, it's a cycle that has to persist.
Now, I've told you about each of the steps: entry, attachment, and persistence,
outwitting the host defenses, replicating once it's outwitted them,
and begin passed on. But organisms, in order to do this, have to have other means.
So, they have to understand where they are,
and they use molecular ways to do this.
The surface of a microorganism is very simple.
On the other hand, it can recognize things like the pH, the temperature,
the amount of oxygen, and it knows where it is
by that, and it turns on and turns off particular functions depending on that.
The other thing to keep in mind is that microorganisms, and particularly pathogens,
respond to a host's biological and social behavior.
So, microorganisms are opportunists in one sense.
Let me tell you about them.
There are many diseases that I consider diseases of human progress.
And, they actually constituted some of the more important medical crises in history:
Legionnaire's disease, toxic shock syndrome, of course HIV/AIDS,
Lyme disease, and more recently E. Coli hemorrhagic fever, and most recently bird flu.
These are all things that are involved where the humans
and their behavior played a major role in this.
Let me tell you about Legionnaire's disease.
When Legionnaire's disease first came out,
it was considered by many to be a massive hoax.
No one understood how a group of veterans meeting in 1976
could have suddenly this new disease and they thought it was the result
of a variety of conspiracies and what have you.
And some people just thought it didn't exist... it was just made up.
Now, actually, the organism that causes Legionnaire's disease
is called Legionella pneumophila.
And Legionella we now know actually likes to live in fresh water
and grow in protozoa.
That's where it's been for millennia.
So, it grows inside of amoeba, it replicates there, it makes lots of different numbers.
And it makes new numbers, it kills the amoeba,
bursts out, goes looking for another amoeba.
That's what it really does in nature.
But, we've changed over time.
We now take showers. We didn't 50 years ago.
Anyone going to a supermarket watches what happens
when the vegetables are being sprayed.
We have aerosolized everything around us.
And, by aerosolizing, we have put Legionella in small droplets,
to now be more available than it ever was before.
And we're older than we were before.
So, Legionella found its way into the respiratory tract
more and more frequently, and a human alveolar macrophage
looks pretty good, as compared to an amoeba.
They actually are probably related in some way... in the past.
And therefore, this organism that usually lived in freshwater protozoa
is now getting breathed into lungs, it's going into the alveoli
and being eaten by macrophages, and in the right host, under the right circumstances,
it replicates and causes pneumonia.
There are other things... toxic shock syndrome was an issue
in which, women were changing.
And they demanded products that were different.
And, companies responded by coming out with new kinds of tampons,
for example, that gave women more freedom.
They didn't require changing as often.
They were made of new kinds of chemical compounds.
This turned out to be an opportunity for Staphylococci
that colonize the vagina in some women.
And this new opportunity for the organism to replicate,
because that's what they want to do,
ended up causing a disease that had not been seen before
and considerable human misery.
The tampons were taken off the market, and the disease disappeared.
If we now look at more recent history,
we find out that we are really still witnessing a human-microbial work in progress.
There have been all kinds of new diseases that have emerged all over
the world, and we've had SARS, we have hemorrhagic E. coli.
We have more food poisoning than we did 50 years ago
because our methods of distributing food have changed over that period of time.
All of these things are changes in our behavior and our culture,
and these are things that microorganisms use to their advantage,
always with the idea to find new ways to replicate and become disseminated.
So, organisms that are pathogens
evolve, and they share the experience.
So, microorganisms, many of you may know, have different ways of sharing information,
either by using DNA molecules, by actually having cell to cell contact
a kind of fundamental or simple conjugation or sex that occurs
where genes flow from one organism, the donor, to another, the recipient,
and sometimes viruses are used to transfer genes.
And, what we now understand is that often a commensal can become a pathogen
because pathogenicity sometimes involves in genetic quantum jumps.
And we recognize now that often there are blocks of genes
that are transferred horizontally, from one organism to another,
which in entering an organism, become part of its genetic characteristic -- an island.
And they come in different kinds of forms.
Bacterial specialization, by and large,
is the result of the inheritance of blocks of different genes,
and, in the case, of pathogenicity, they're called pathogenicity islands.
And so, we now recognize the difference between some commensals
and some pathogens is the fact that they've inherited blocks of genes.
So, for example, many of you may know that E. coli
is a common member of our normal flora.
On the other hand, E. coli is the most common cause of urinary tract infections.
The difference between an E. coli that causes urinary tract infection
and an E. coli that inhabits the bowel
is because the E. coli which is a urinary tract specialist
has inherited a number of different blocks of genes...
different pathogenicity islands.
And it permits it to encode specialized structures that permit it to attach
to the bladder or to the kidney and to establish itself there.
And so, that's the difference between a pathogenic E. coli
and a commensal E. coli.
It is important to keep in mind that disease, which we focus on,
need not be the outcome of the host-parasite interaction.
In fact, it's usually not the outcome.
And very often, we're talking about an instance in which we have what is called
the iceberg concept of infectious diseases, in which
people who actually have infection are in a small number,
relative to the total number of people who are infected.
So, it's like an iceberg -- only a small amount is on the surface.
Those are the people who are ill.
Most of the people who encounter organisms escape without any knowledge.
And different parasites are different ways.
So, polio, we think of as being a terrible disease,
but actually the relative number of people who become clinically ill
who have poliovirus are few.
On the other hand, if you look at measles,
and so on, you end up with infections in which everyone may be infected,
but only 50% actually show signs of illness, and there are some diseases, like rabies,
where everyone who gets infected, as far as we know, will sicken and die.
The most common organisms that cause disease in humans,
among them, Mycobacterium tuberculosis, which in the developing world
probably infects a very high proportion of people.
90% of the people who are infected with the Tubercle bacillus
are asymptomatic. They are infected by the organism,
they carry the organism with them, to their grave,
but they never show symptoms.
The organism that causes typhoid fever, 80% of the people who acquire this
never show signs of illness. We know they've had it
because we find antibodies in their blood that indicate that they had it.
I'll tell you in a while about an organism, Helicobacter pylori,
which infects most of the world's population -- at least in the developing world --
yet about 80% of the people never show any signs or symptoms.
So, disease is the exception, rather than the rule.
Pathogenicity is a reflection of an ongoing evolution
between a parasite and a particular host.
If we understand this, and how the organism establishes itself,
overcomes our defenses, we learn a great deal about the pathogen --
perhaps how to treat it and how to prevent it from infecting by making vaccines --
but in the process, we also learn about ourselves.
And that is one of the great joys of working with these pathogens
because you hope that you can help cure things,
but you also are in a place where you can discover new things.
