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Cornell Notes - Pathogens
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Pathogens are microorganisms that cause infectious disease. Bacteria and
viruses are the main pathogens, but parasites and fungus can also cause
disease.
Bacteria Basics - They are Alive!
Bacteria are the simplest of creatures that are considered alive. Bacteria are
everywhere. They are very simple cells that fall under the heading prokaryotic.
That word means they do not have an organized nucleus. They come in many
shapes, and their whole purpose in life is to replicate.
Bacteria don’t have an organized nucleus, but they do have DNA. It is grouped in
an area called the nucleoid. They have cell membranes like other cells and even
a protective cell wall. Their cell wall is not like the one in a plant. It's a special
kind that bacteria have for protection. They don't have any organelles except
ribosomes. (These are all characteristics of prokaryotes.)
What Do Bacteria Look Like?
Bacteria are very, very small, microscopic organisms which are less than 10
millionths of a meter in size. There are three basic shapes. Spherical bacteria
are in the shape of little spheres or balls. They usually form chains of cells like a
row of circles. Rod shaped bacteria look like the E. coli living in the intestine.
Imagine a bunch of bacteria that look like hot dogs. They can make chains like a
set of linked sausages. Spiral shaped bacteria twist a little. Think about balloon
animals for these shapes. It's like a balloon animal in the shape of a corkscrew.
What Do Bacteria Do?
Bacteria do all sorts of things. Some help plants absorb nitrogen (N) from the
soil. Cyanobacteria photosynthesize to make huge amounts of oxygen and
support life. Some bacteria is used to make yogurt and cheese while others are
used to produce medicines! However, some cause disease by releasing poisons
or toxins inside the body. Diseases caused by bacteria include food poisoning,
cholera, typhoid, botulism and whooping cough.
Some bacteria live inside the stomachs of cows to help them break down
cellulose. Cows on their own can digest grass and plants about as well as
humans. They don't get many nutrients out of the plants and can't break down
the cellulose. With the super bacteria present, however, the cellulose can be
broken down into sugars which release all of the energy they need. Imagine if
scientists could develop bacteria to live inside of humans that would work that
way. Humans could then eat grass and leaves all day long for energy.
Are Viruses Alive?
Viruses are the smallest of the small. Some scientists argue that viruses are not
even living things. It's easier to list what viruses CAN’T do as opposed to what
they can do. What viruses can't do:
(1) They can't reproduce on their own. They need to infect or invade a host cell.
That host cell will do all the work to duplicate the virus.
(2) They don't respond to anything. They either function or they are destroyed.
(3) They don't really have any working parts. While there some advanced
viruses that seem fancy, viruses don't have any of the parts normally associated
with cells. They have no nuclei, mitochondria, or ribosomes. Some viruses do not
even have cytoplasm!
DO
Viruses have a few basic parts, however. The most important part is a small
piece of DNA or RNA (never both). That strand of nucleic acid is considered the
core of the virus. The second big part is a protein coat to protect the nucleic
acid. That coat is called the capsid. The capsid protects the core but also helps
the virus infect new cells. Some viruses have another coat or shell called
the envelope. The envelope is made of lipids and proteins in the way a regular
cell membrane is structured. The envelope can help a virus get into systems
unnoticed and help them invade new host cells. Potentially deadly diseases
caused by viruses include ebola, rabies, HIV, smallpox, influenza, Dengue fever,
and Rotavirus.
Types of Viruses
There are three basic viral shapes.
1) First there are helical virions. They are set up like a tube. The protein coat
winds up like a garden hose around the core.
2) Next comes the polyhedral shape. This shape group includes the classic
virus shape that looks like a dodecahedron. A dodecahedron is a geometric
shape with twelve (12) sides. These viruses have many facets and a seemingly
hard shell of capsomeres (pieces of a capsid). There is a variation of the
polyhedral called globular. Globular shapes are basically polyhedral virions inside
of a spherical (like a ball) envelope.
3) Last is the complex virus shape with the geometric head and long legs.
Smaller than Viruses?
There are things out there even smaller than viruses. The two that scientists
have discovered are called prions and viroids. A prion is (as far as is known)
just a protein. Prions are proteins that can invade cells and somehow direct their
own duplication, making more of the isolated proteins. Viroids are a little different
in that they are just RNA. Scientists have even discovered that they are
responsible for some diseases.
Fungus Among Us
Mushrooms
Mushrooms or puffballs are a fungus known
as Club Fungi. Bunches of strands living
underground are called hyphae (pronounced hi-fah).
Those strands are the basic fungus in action,
decomposing leaves, or rotting bark on the ground.
When it's time to reproduce, they develop a stalk
and cap which is the mushroom popping out of the
ground! The stalk and cap are only one part of the
fungus. On the bottom of that cap are a set of gills
that have little clubs with fungus spores. Never eat
a mushroom found in the wild as it may be
poisonous.
Zygotes
All molds are actually a type of fungus known as Zygote Fungi (fungi is a bunch
of fungus.) Mold in the shower or mold on bread are really different types of
fungus. Molds are multicellular. They have hyphae just like mushrooms, but they
reproduce in a different way. When it's time to make more zygote fungi, they
create a stalk and release something called zygospores (thus the name zygote).
When bread gets old and turns green or black, it’s this type of zygote fungus in
action. Eventually stalks develop and the zygotes are released.
Single Cells
Sac Fungi are simple, single celled fungi known as Yeast. It is used to make
several types of food for humans. Yeast is used to make breads rise. Bread
takes one kind of sac fungus (yeast) to make it rise. Over time, a zygote
fungus (bread mold) can eventually break down bread into a ‘moldy’ green or
black – as mentioned above!
Yeast is also used to make alcohol in a process called fermentation.
Fermentation is a process in which sugars are broken down in an environment
without oxygen. This is known as anaerobic fermentation which produces
alcohol. While yeasts are single celled, they may be found living in colonies.
They reproduce very quickly and hang out together. It takes a lot of them
(because they are so small) to get a lot of work done.
Common infections caused by fungus include ringworm, athlete’s foot and fungal
meningitis, fungal nail infections and fungal eye infections.
Super-Powered Protists
The taxonomic kingdom Protista is a collection of single-celled organisms that do
not fit into any other category. These are the ‘advanced’ cells of the microbe
world in that they are eukaryotes with special structures that may be the base of
multicellular organisms. Protists are made up of protozoa, unicellular algae,
and slime molds.
Protozoa
Protozoa are the animal portion of this group (proto = first, zoa = animals). They
are the oldest known group of heterotrophic life that consume and transform
complex food particles into energy. Although protozoans are only made up of a
single cell, these organisms manage to perform all the basic tasks of life. Some
have cilia (hairs) or flagella (tail) to help them move. A classic example of a
protist is the paramecium which has little hairs to help it move, take in food and
excrete waste - the first single celled creature to do this! Amoebas are easier to
describe. They are small-single celled organisms that ooze from place to place.
They reach out with one part of the cell, a structure called a pseudopod. They
don't really have a shape because they are constantly on the move, hunting
down food and wrapping themselves around the food to absorb it into their body
for digestion. Common diseases caused by protozoa include sleeping sickness,
spread by infected insects, and giardiasis, found in contaminated food or water.
Unicellular Algae
Algae are a diverse group of aquatic organisms that have the ability to conduct
photosynthesis. Certain algae are familiar to most people; for instance,
seaweeds (such as kelp or phytoplankton), pond scum or the algal blooms in
lakes. However, there exists a vast and varied world of algae that are not only
helpful to us, but are critical to our existence!
Slime Molds
Slime molds are not molds like a fungus but are protists that are actually
independent organisms. They are found in lawns, flower beds and gardens.
Cellular slime molds include thousands of individual cells that team up and
work together. Acellular slime molds can be as huge as a couple of feet
across, but are still only one cell. Most prevalent after extended periods of
wetness, slime molds are essentially harmless and are only considered a
nuisance.
Parasitic Protists
Not all protists go about their life eating little bits of food in a pond. Some, called
sporozoans, are nasty little parasites. These protists, like all parasites, cannot
live on their own, and they harm the host organism over time. A disease called
malaria, which is spread by mosquitoes, is caused by a sporozoan protist.