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Notes Unit 23
Symbioses and Normal Human Flora
I. Keepers of Health
Normal Flora
Throughout our evolution, we have developed an intimate and complex relationship with microorganisms.
Hundreds of species of bacteria and fungi live in and on our bodies. The number of microbe cells in our bodies
is 10 times greater than the number of human cells we are made of!
The microorganisms that live with us normally are called our normal flora. Under normal circumstances, they
don’t cause disease. They only live on the internal and external surfaces of our body (internal meaning the
digestive tract; external meaning the skin). Babies are born without a normal flora, but quickly establish one
through contact with the outside world. The main parts of the body that support normal flora are: skin, nose,
mouth, intestinal tract, vagina, and urethra. Found anywhere else in the body, they are not normal flora.
NASAL CAVITY
Opportunists
Opportunists are normal flora organisms that cause disease when the proper opportunity arises. This can
happen when your immune system breaks down, or while you are undergoing antibiotic treatment. The best
example of this is Candida albicans, which is a normal resident of the vagina. Normally, the vaginal bacteria
keep the yeast cells in check. But when a person is taking antibiotics, the normal bacteria are killed off, and the
yeast take over. This is what happens in a yeast infection!
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Sites of Normal Flora
1. Skin – normal flora are present on all normal skin, but because most skin is so dry, so it produces oil
and perspiration to moisten itself. Humans have up to 10,000 microorganisms on each square
centimeter of dry skin, and up to a million on moist areas! About 90% of them can be removed by
washing, but they grow back to normal numbers within 8 hours. The purpose of bacteria on the skin is
to keep the skin healthy by occupying the space that pathogens would need to become established.
Several species of organisms occupy our skin: Staphylococcus sp., Corynebacterium sp., and yeast.
Also inhabiting our skin are microscopic mites.
2. Nasal Cavity and Nasopharynx – these are the passages that deliver air to the lungs. The average adult
inhales about 10,000 microorganisms each day. Because this is a warm and moist environment, there
are many microbes living there. Weakening of this system can lead to severe infections – and
weakening is caused by smoking or by certain diseases, such as cystic fibrosis. Several species live
here: Staphylococcus sp., Lactobacillis sp., Streptococcus sp., and Haemophilus sp., to name a few.
3. The Mouth – the mouth is the ideal environment for microbes, because it is warm, moist, and has plenty
of nutrients. The human mouth is densely populated with more than 80 different species of microbes.
One species living here, Streptococcus mutans, is responsible for tooth decay. This is usually eliminated
by adequate brushing.
4. The Intestinal Tract – see diagram above for this part – everything from the esophagus down. The
main part of this tract that has normal flora is the large intestine, or the colon. Cool fact:
microorganisms make up 1/3 of the weight of feces! People eliminate between 1011 and 1014 bacteria in
their feces every day! In the colon, many anaerobic (without oxygen) bacteria exist. There are too
many species to list here. These bacteria aid in digestion and help produce vitamins we need.
5. The Vagina – the vagina is the only site in the female reproductive tract that contains normal flora. It is
a warm, moist environment that supports many microorganisms.
6. The Urethra – this is the part on males and females where the urinary tract meets the outer skin. The
upper part of the urethra should be free of microorganisms.
Why Is Our Normal Flora Important?
Here are a few important reasons we need normal flora microorganisms:
1. These organisms produce antimicrobial compounds, which kill pathogens on contact.
2. They stimulate our immune system.
3. Provide us with supplemental sources of vitamins, such as vitamin K (important for blood clotting) and
vitamin B12 (important for production of red blood cells and nervous system functioning).
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II.
Carriers of Disease
So, knowing that we have many normal flora bacteria that help fight off the pathogens we come into contact
with, how does an infection occur? Well, sometimes pathogens can overcome the normal flora defenses, and
these are called virulence factors (disease-causing capabilities). To be successful, a pathogen must be able to
do some of these 7 things below:
1. Maintaining a Reservoir – every pathogen must maintain at least one reservoir (place to “hang out”
between hosts). It all its reservoirs are eliminated, it too ceases to exist. The most common reservoirs
for human pathogens are other humans, animals, and the environment.
a. Human Reservoirs – pathogens can come from other people who are sick, or from people who
are healthy who are carriers of a pathogen. A carrier is a person who harbors the pathogen, but
it doesn’t make them sick.
b. Animal Reservoirs – common animal reservoirs include cats, dogs, skunks, bats, etc. A human
disease caused by a pathogen with an animal reservoir is called a zoonosis.
c. Environmental Reservoirs – this includes things such as soil, water, etc.
2. Getting To and Entering A Host – a critical phase of any disease process is transmission – when a
pathogen leaves its reservoir and enters the body of a host. Most pathogens are transmitted a particular
way, and they enter the body at a particular site called the portal of entry.
a. Portals of Entry – the major portals of entry for a human are the lungs, skin (especially when
broken), placenta, vagina, penis, rectum, mouth.
b. Modes of Transmission – pathogens can be transmitted through respiratory droplets (sneezing,
coughing), from touching fomites (inanimate objects that have germs on them), through direct
body contact, through the fecal-oral route (when microbes is passed from the infected feces of
one organism to the mouth of another organism), through a vector (an agent that transmits
pathogens, such as fleas, ticks, mosquitos, etc), airborne transmission, and through parenteral
transmission (pathogen is deposited directly into the blood stream, tissue below the skin, or
mucous membranes).
3. Adhering To Body Surface – when a pathogen comes to rest on a body surface, it’s immediately
confronted by defenses that could dislodge it. Successful pathogens aren’t swept away because they
attach tightly to surfaces by means of adhesions (or proteins that bind to specific molecular receptors on
the skin).
4. Invading the Body – once a pathogen has successfully attached to the body surface, it can be either
invasive or noninvasive. An invasive pathogen is one that penetrates into the host cells on its way to
deeper tissues. A noninvasive pathogen is one that infects the surfaces of a host’s body.
5. Evading the Body’s Defenses – once pathogens arrive in a body, the host’s immune system kicks in,
and the pathogen must evade this system in order to be successful. They can do this in several ways:
a. Capsules – a pathogen can form a slippery mucoid capsule around its cell so the body’s cells
cannot attach to and kill it.
b. Surface Proteins – some pathogens can produce proteins on its surface that interfere with
contact between it and the body’s cells.
c. Antigenic Variation – all pathogens have antigens, or molecules that provoke an immune
response, on the surface of its cells. The body forms memory cells that remember an antigen
once it has come into contact with it, so if the body ever sees it again, it gets wiped out quickly.
Well, pathogens can change their antigens so the body doesn’t recognize them again!
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6. Multiplying in the Host – now that the pathogen has evaded the body’s defenses, it is time to multiply
and grow in its host. It does this in two ways: production of toxins, and by causing indirect damage by
stimulating the body’s defenses.
a. Toxins – toxins are poisonous products that damage human tissue. There are two types of toxins
produced:
i. Exotoxins – these are toxins that are released outside of the bacterial cell by Gram
positive organisms. They act in many different ways, but mostly by disrupting some
function of the host cell.
ii. Endotoxins – found in Gram negative bacteria, the endotoxin is actually part of the outer
membrane that only works on a host when the bacteria dies and its cell breaks down.
iii. Other – there are many other types of toxins produced by bacteria, such as hemolysins
(lyse red blood cells), leukocidins (lyse white blood cells), etc.
b. Damage Caused By Host Responses – sometimes the body’s defenses against a pathogen
disrupt normal body functions in ways that produce disease. In the case of pneumonia, it is the
accumulation of dead host immune cells in the lungs that cause the disease.
7. Leaving the Body – the final link in any chain of infection occurs when the pathogen leaves the body of
one host in order to reach another. The anatomical route through which a pathogen usually leaves the
body of its host is called its portal of exit. Example: for a respiratory pathogen, such as the cold virus,
the portal of exit is the same as the portal of entry: the nose or mouth. For most gastrointestinal
pathogens, the portal of entry is the mouth, and the portal of exit is the anus.
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