Professor Dave here, let’s talk about microbes and us.
As we mentioned in the previous tutorial, microorganisms play a vital role in human
survival.
In fact, life as we know it would be impossible without them!
But how does that work, exactly?
How does our dependence on bacteria begin?
Let’s start from conception.
As a human fetus develops, it’s protected within the sterile womb of its mother.
As soon as it’s born, an infant is immediately exposed to microbes from both its mother and
the surrounding environment.
Protective microbes begin to grow first on the infant’s skin, then their oropharynx,
which is the back of the throat.
The gastrointestinal tract is colonized next, followed by other mucosal surfaces.
These populations of bacteria are called microbial flora, and they perform a wide variety of
crucial tasks.
They help protect us from infection by virulent microorganisms.
They stimulate the immune response.
They help us metabolize our food.
And they provide essential growth factors.
As we age, the balance of microbes that are on us and in us are influenced by factors
such as what we eat, our overall health, hormonal changes, and personal hygiene.
Drastic changes in any of those factors can disrupt the intricately balanced ecosystem
of microbes, which can in turn make us sick.
For example, taking antibiotics can wipe out all of the good and bad bacteria in the gut,
completely disrupting the equilibrium of microbial populations within.
Now let’s get a bit more specific.
When you’re exposed to a microorganism, there are three possible outcomes.
The first of these is called transient colonization.
The second is called permanent colonization.
And the third outcome is disease.
The language here can be a little bit tricky, so let’s make sure we understand the difference
between colonization and disease.
Microorganisms that colonize humans, whether transiently or permanently, do not interfere
with normal bodily functions.
They’re just present, along for the ride so to speak.
However, disease occurs when microorganisms cause damage to a human host.
A bacterium, virus, or other microorganism that can cause disease is called a pathogen.
Let’s explore this in more nuance.
For the most part, diseases are not simple.
In fact, most organisms do not cause a single, well-defined disease.
It’s far more common for a single bacterial pathogen to cause different types of disease
depending on the setting.
Staphylococcus aureus, for example, can cause food poisoning, pneumonia, or a wound infection.
In that same vein, some diseases can be caused by a wide range of organisms.
Interestingly, meningitis, which is an inflammation of the protective membranes covering the brain
and spinal cord, can be caused by bacteria, viruses, fungi, or parasites.
Additionally, there are relatively few organisms that can be classified as always being pathogenic.
Some examples of always-pathogenic microorganisms are the rabies virus, plasmodium species,
and bacillus anthracis.
For the most part, microorganisms are only able to cause disease under specific circumstances.
Let’s make another distinction.
Disease that arises from exposure to an external source is called an exogenous infection.
Examples of pathogens in this category include the influenza virus, the bacteria that causes
gonorrhea (Neisseria gonorrhoeae), or the bacteria that causes tetanus (Clostridium
tetani), all of which we will look at in more depth very soon.
By contrast, disease that arises from a person’s own microbial flora spreading to inappropriate
sites in the body is called an endogenous infection.
Incredibly, endogenous infections are to blame for the majority of human diseases.
Factors to consider are as follows.
There is the site of exposure.
There is the virulence, or the degree of pathology, of the organism.
And there is the host’s ability to respond to the organism.
All of these determine whether the complex interaction between a human host and a microorganism
will result in a long-term symbiotic relationship, a short-term colonization, or disease.
Finally, there are a few terms we should know that describe a microbe’s propensity for
causing disease.
A strict pathogen is an organism that is always associated with human disease.
Examples include Mycobacterium tuberculosis, which causes, as one might guess, tuberculosis,
Plasmodium species, which cause malaria, and the rabies virus, which causes rabies.
But most human infections are caused by opportunistic pathogens, which are typically part of the
normal microbial flora.
These organisms establish disease when they take advantage of an opportunity not normally
available, like a weakened immune system, disrupted microbial flora, or breached barrier
introduced by a wound.
Obviously there is a lot more to discuss here, but it helps to learn many of these basic
things up front before diving into the wide variety of things that can infect us, and
how they work.
To dive a little deeper, let’s focus on bacteria first, so that we can talk about
how we’ve come to make sense of them, and how we classify them.