Professor Dave here, let’s talk about how bacteria cause damage in humans.
To bacteria, the human body is like a playground, or a lush hotel, containing a variety of environmental
niches that almost seem custom-made to suit their needs.
We’ve got everything they need to grow and thrive in our nooks and crannies, from moisture
and warmth, to food and protection.
As we’ve evolved as a species, so have bacteria.
Over time, bacteria have gained or lost genetic traits that allow them to adapt and better
survive.
Gaining or losing a trait may allow them to invade a particular environment better, survive
longer in a particular niche, more effectively break down food, or, perhaps most importantly,
evade detection by the immune system.
Unfortunately, some of these adaptations can wreak havoc on our bodies.
In many cases, bacteria might gain or develop enhanced virulence factors, which allow them
to more effectively cause disease.
Virulent bacteria, by definition, grow and thrive at the expense of their host.
For instance, some bacteria might release toxins that can travel through the blood,
causing life-threatening disease.
Others might be able to directly degrade our tissues or trigger aggressive cascades within
our immune system.
In fact, in many cases, the symptoms that we experience are actually caused by an excessive
inflammatory or immune system response triggered by the infection, and not necessarily the
bacteria itself.
The degree of disease that bacteria can cause depends on a few things.
For instance, how important is the tissue or organ that’s affected?
You can imagine that an infection of the central nervous system would be extremely serious,
while an infection of your left pinky toe might not be as life threatening.
Another factor is the particular strain of bacteria, and how much there is of it, which
is called the “inoculum size”.
Some bacteria, like Shigella, which causes food poisoning, require a relatively tiny
inoculum size, like around two hundred, to cause serious gastrointestinal distress, while
others, like Salmonella, might require several orders of magnitude more inoculum, like a
million or more, to cause a serious infection.
However, the particular host factors into this equation as well.
If you are immunocompromised, for instance, it might take much less Salmonella to make
you sick.
Now that we have an idea of what bacteria are capable of, let’s talk in more detail
about how they’re able to cause destruction.
First, let’s talk entry.
Our bodies have natural defense mechanisms, such as skin, earwax, stomach acid, tears,
and mucous membranes.
Our skin prevents microorganisms from invading, our tears contain enzymes that attack bacteria,
our airways filter out harmful particles, and our mucous membranes are coated with secretions
that fight off microorganisms.
Despite our body’s best efforts, some of these microbes are able to bypass these defenses.
For instance, bacteria such as Salmonella, Vibrio, Bacillus cereus, and Shigella can
enter the body through ingestion.
This could be from a picnic lunch left out in the sun for too long, or as the result
of poor hand washing.
Other bacteria, such as Streptococcus, Mycobacterium, or Legionella enter through inhalation, perhaps
after a sick person’s sneeze, or infected aerosol particles floating through the air.
Clostridium tetani, the causative agent of tetanus, enters through trauma or a wound.
Other portals of entry include a mosquito bite, needlestick injuries, or sexual transmission.
Bacteria have a vast arsenal of mechanisms to both adhere to surfaces within the body
and colonize, which means to establish a microbial presence and multiply, once they’ve made
contact.
For instance, prokaryotic cells have short, hair-like structures called fimbriae or pili
that they use attach to various surfaces in nature.
Some bacteria have adhesins on the tips of these pili that have specifically evolved
to allow them to bind tightly to cells in your body.
For example, the pili of Neisseria gonorrhoeae bind specifically to oligosaccharide receptors
on epithelial cells.
Other bacteria express adhesion proteins in a variety of ways.
Another bacterial adaptation that promotes colonization is the formation of biofilms,
which are collectives of one or more types of microorganisms.
Within a biofilm, bacteria form sticky webs of polysaccharides that bind bacterial cells
together into a community, providing protection from antibiotics or host defenses.
Bacteria like Pseudomonas aeruginosa can sense when enough bacteria are present and trigger
biofilm formation through a process called quorum sensing.
Biofilms are particularly common on catheters, in dental plaque, or on implanted surgical
devices such as pacemakers.
Now that we’ve covered the ways that bacteria colonize and invade bodies, let’s talk about
specific ways they cause damage.
For some bacteria, natural byproducts of their growth can cause tissue destruction.
For instance, in your gut, anaerobic bacteria, those that don’t require oxygen for growth,
can produce toxins, enzymes, gas, and acid, all of which destroy the surrounding tissue.
Some examples are Staphylococci or Streptococci.
Once the process has begun, the bacteria have momentum, with these enzymes facilitating
the spread of disease.
Next, bacteria can produce harmful substances called toxins, meant to either attack other
bacteria in their vicinity or damage the host they’ve settled in.
Typically, toxins cause degradation or lysis of cells, or trigger destructive immune responses.
For some diseases, symptoms can be fully attributed to toxin production, with damage occurring
right where the infection is.
In other cases, such as with tetanus or certain staphylococci-associated infections, the toxin
may travel through the bloodstream and cause symptoms somewhere else in the body.
The components that make up the bacterial cell wall, in particular, can set off a powerful
chain reaction within the immune system.
For instance, during an infection of gram-positive bacteria, the peptidoglycan and the products
it breaks down into, can stimulate a fever or inflammation with devastating effects on
the body.
Or, lipopolysaccharide produced by gram-negative bacteria is categorized as endotoxin.
In low doses, endotoxin can activate the immune system or protective responses such as a fever.
In higher doses, endotoxin can trigger extremely high fever, shock, or skin lesions, which
can be deadly.
Exotoxin proteins, on the other hand, can be produced by either gram-positive or gram-negative
bacteria.
Proteins that fall into this category include those that cause cytolysis, which causes a
cell to burst from osmotic pressure, or receptor-binding proteins that either cause cell death, or
change their function altogether.
Exotoxins are often encoded on a plasmid or a phage.
Another category of toxins is superantigens, which activate the immune system to a life-threatening
degree, causing toxic shock syndrome.
Finally, bacteria have developed multiple mechanisms to escape our host defenses, especially
in the case of long-term infections.
They might alter their surface proteins to evade detection, like Neisseria gonorrhoeae,
physically hide within cells in the body, or inactivate our standard antibacterial defenses.
One of the most powerful virulence factors that some bacteria have are slime layers called
capsules.
These capsules can mimic the surface of a host cell, shielding the bacteria from typical
immune responses.
Other bacteria create makeshift shields within the site of infection, like Staphylococcus
aureus, which forms a barrier using coagulase.
We’ve covered a lot of ground here.
Ultimately, bacteria have a wide array of tricks up their sleeves.
Some might express one virulence mechanism, while others might express several in tandem.
Altogether, these mechanisms are ultimately what trigger disease symptoms.
Let’s learn more about bacterial infections in humans next.