Topic 5 Notes/Study Guide

The Germ Theory

The Germ Theory of Diseases is another important theory in biology. This theory states that infectious diseases are caused by microorganisms known as pathogens.

It was not until the mid-1800sthat scientists proposed that some diseases were caused by microorganisms. Microorganisms that cause disease are called PATHOGENS. Diseases caused by pathogens are called INFECTIOUS DISEASES. French chemist LOUIS PASTEUR and German bacteriologist ROBERT KOCH were the most influential in discovering microorganisms that cause infectious diseases. They called these microorganisms germs. Because they called them germs, their conclusion was known as the germ theory of diseases.

Francisco Redi

An Italian scientist who helped disprove the idea of “spontaneous generation”. He noticed that there were worms on meat a few days after he had seen flies there. He proposed that the flies laid eggs too small to see, and that these are what the maggots arose from instead of spontaneously generating.

John Snow

Contributed to the creation of the Germ Theory and helped disprove the Miasma theory by discovering how the bacterial disease cholera was spread. The techniques that Snow used are still used in the fields of epidemiology today!

Louis Pasteur

Contributed to the creation of germ theory through his experiments and developed the first vaccine against the deadly anthrax bacteria.

Pasteur boiled broth in long necked flask to kill any microorganisms. For an entire year no organisms could be found in the flask. He then broke the long neck off the flask, allowing air and dust particles to enter the broth. In just one day, the broth grew cloudy and was full of microbes. His work convinced other scientists that all living things come from other living things.

Robert Koch

Contributed to the creation of germ theory through his experiments.

Robert Koch was a German physician and one of the founders of bacteriology. He discovered the anthrax disease cycle and the bacteria that cuases tuberculosis (TB) and cholera.

Koch famously developed a set of rules to identify microrganisms that cause infectious diseases. Thes rules are known as Koch's postulates. (See below for details.)

Pasteur's Experiment:

Pasteur was the first person to show bacteria can cause disease. His experiments proved microorganisms too small to be seen existed. Since they existed, then a disease caused by microorganisms was possible.

To confirm the existence of bacteria, Pasteur designed a flask that had a long curved neck. Although the neck was open to the air, its shape prevented microorganisms from entering the flask.

Pasteur boiled a broth inside the flask. He boiled it to kill any microorganisms that might be contained in the broth.
He observed the flask for an entire year. During that time, Pasteur did not observe any microorganisms in the broth.
After a year, he broke off the neck of the flask. This allowed dust and other particles in the air to enter the flask.
After just one day, the broth became clouded from the growth of microorganisms.

Koch's Postulates:

Koch developed a set of rules to identify microorganisms that cause infectious diseases. These rules are known as KOCH'S POSTULATES. Koch’s postulates are stated as follows:

The pathogen should be found in the body of a sick organism and should not be found in the body of a healthy organism.
The pathogen can be isolated and grown in a culture. A culture is an artificial medium that supports the growth of cells.
When the cultured pathogen is placed in a healthy organism, the organism becomes infected with the same disease that made the original organism sick.
The pathogen in the second organism can be isolated. This pathogen is identical to the pathogen isolated from the first organism.

Koch’s postulates are still used today, but there are LIMITATIONS TO HIS RULES/POSTULATES:

We now know that some infectious diseases are caused by viruses and viruses cannot be grown in a culture.
Some disease-causing agents can be found in an organism but do not cause disease. Organisms that are asymptomatic but have the pathogen inside them are called CARRIERS.
Some organisms do not acquire an infectious disease when exposed to the pathogen that causes the disease. These organisms may be immune to the disease due to a previous infection or vaccination.

Prokaryotes:

Bacteria & Archaea

PROKARYOTES: unicellular organisms that lack a nucleus; DNA is stored in the cytoplasm of these organisms.

Prokaryotes vary in their size and shape, in the way they move, and in the way they obtain and release their energy.

SIZE: Prokaryotes range in size from 1 to 5 micrometers, making them smaller than eukaryotic cells.

SHAPES: Rod shaped prokarotes are called bacilli. Spherical shaped prokaryotes are called cocci. Spiral and corkscrew shaped prokaryotes are called spirilla.

MOVEMENT: Some prokaryotes do not move at all. Others propel themselves using flagella. Others secrete slime that helps them move slowly throughout their environment.

ENERGY: Prokaryotes are living things so they must have a way of getting enrgy. Prokaryotes can obtain and release energy in a variety of ways. See the tables to the right to learn more!

Some prokaryotes are DECOMPOSERS that breakdown (decompose) dead organisms supplying the environment with raw materials.
Some prokaryotes are PRODUCERS/AUTOTROPHS and make their own energy via photosynthesis or chemosynthesis.
Some prokaryotes are CONSUMERS/HETEROTROPHS and must consume other organisms to gain energy.

Some prokaryotes are nitrogen fixers. NITROGEN FIXATION is the process by which nitrogen is converted into a usable form.

RELATIONSHIP WITH HUMANS:

Prokaryotes especially bacteria are used in the production of a wide variety of foods and other commercial products. Some bacteria can digest petroleum and remove wate prodcuts and poisins from water. Others are used to synthsize drugs and chemicals using genetic enegineering techniques.
Though prokaryotes like bacteria can be found eveywhere, many bacteria bacteria live inside the human body. Bacteria can be found on your skin, in you hair, in your mouth, in your nose, and in your digestive system! (In a typical human instestine there may be as many as 30 trillion bacteria beloning to 150 different bacteria species!)

GROWTH AND REPRODUCTION:

Most prokaryotes reporduce by a process called BINARY FISSION. During this proces the cell replicates its DNA and then divides in half to produce two identical cells. Since this process does not involve the exchanging or recombining of genetic information, it is considered asexual reproduction. This process can happen very quickly! Some prokaryotes grow and divide once every 20 minutes.

Some prokaryotes fom an ENDOSPORE - a thick internal wall that encloses the DNA portion of the cytoplasm, when their external conditions become unfavorable.

Prokaryotes can also exchange genetic information through a process called CONJUGATION. During conjugation, a hollow bridge forms between two prokaryotice cells, and genetic material in the form of a plasmid moves from once cell to another. PLASMIDS are small circular pieces of DNA that prokaryotic cells may contain in their cytoplasm. These plasmids may carry genes that allow the organism to survive in new envionments and even resist fatal antiobitoics!

Biologists divide prokaryotes into two groups: Bacteria and Arachaea. These two groups make up two of the three domains of life (the third group is Eukarya).

There are three domains of life. Bacteria and Archaea make up two of the three domains.

BACTERIA:

Bacteria is the larger of the two prokaryotic domains is bacteria.

Bacteria are found everywhere! They can be found in fresh water, salt water, land, and on or even inside other organisms!
They are typically surrounded by a cell wall that protects its internal structures and helps determine its shape. The cell wall of bacteria contains PEPTIDOGLYCAN - a polysaccaride (carbohydrate polymer).
Some bacteria, like E. coli, have a second layer -capsule - outside of their peptidoglycan cell wall that makes the cell especially resistant to damage.
Both bacteria and archaea may have FLAGELLA that they use for movement or PILLI (singular = PILUS) which allow them to anchor themselves to a surface.

Prokaryotes such as arachae and bacteria can have different types of flagella as a part of their cell structure.

Bacterial Diseases

PATHOGENS are microbes that cause disease. Although any organism can be a pathogen, nearly all prokarytotic pathogens are bacteria.

Bacteria produce dieases in two main ways: they destroy living cells or release chemical (toxins) that interupt the organisms ability to mantain homeostasis.

Some bacteria direclty destroy living cells and tissues of the organisms they infect, other cause the organisms immune system to overreact, causing it to attach the organisms own body tissues. Other bacteria release posisonous toxins that interfere with the organisms normal activities. The table to the left lists some common human diseases caused by bacteria.

Prevention and Treatement:

Many bacterial diseases can be prevented by stimulating the body's immune system with vaccines.

A VACCINE is a preparation of weakened or kiled pathogens or inactivated toxins that when injected in the body causes the body to trigger an immune response to that specific disease.

IMMUNITY is the body's ability to recognize and destroy pathogens before they cause disease.

A number of drugs can be used to attac a bacterial infection. These drugs include antibiotics. ANTIBIOTICS are drugs that block the growth and reproduction of bacteria. Antibiotics dirsupt proteins or cell processes that are specific to bacterial cells, and do not harm the host's cells.

ARCHAEA:

Under a microscope archaea look very similar to bacteria. Both are small, lack a nucleus, and have cell walls. But there are some important differences between these two types of prokaryotes!

The cell walls of arachaea do not cotain peptidoglycan.
The cell mebranes of archaea contain different lipids than the membranes of bacteria.
The DNA sequences of many archaea are more like the DNA found eukaryotes than those in bacteria.

Many archaea live in extremely harsh environments, such as places with little to no oxygen, extremely salty environments, or in places with extreme temperatures.

This thermal hot pool in Yellowstone National Park with its extremely hot temperatures is the home of some archaea.

Bacteria and Archaea are both types of Prokaryotes. And share characteristics such as being unicellular organisms, lacking a nucleus, and having cell walls. But they are also different!

Comparing the Prokaryotes:

Bacteria vs Archaea

Environment:
- Archaea - Extreme
- Bacteria - Present Everywhere
Cell Wall:
- Archaea - Lacks Peptidoglycan
- Bacteria - Peptidoglycan is Present
Major Types:
- Archaea - halophiles, thermophiles, methanogens, etc.
- Bacteria - gram positive and gram negative

Viruses

A VIRUS is a nonliving particle made of proteins, nucleic acids, and sometimes lipids that can reproduce only by infecting living cells.

Viruses are not made up of cells! In fact they are smaller than cells and can only be seen using powerful electron microscopes.

Virus Structure:

Viruses can be a variety of different shapes. Though they may not all look the same all viruses will contain genetic information - double or single stranded DNA or RNA - and be surrounded by a protein coat called a CAPSID. Some viruses may have additional lipid mebranes outside of the capsid.

The picture to the left shows three different types of viruses. Notice that though they are shaped differently they all have genetic information (DNA or RNA) and a capsid protein coat.

Viral Infections:

Nearly every type of organism can be infected by viruses! To enter a host cell, most viruses have proteins on their surfaces that bind to receptors on a cell. These proteins "trick" the cell to take in the virus.

Once inside the cell, the virus uses their genetic information to reproduce. Some viruses replicate immediately, while others are initially in an inactive state within the host.

The replication of viruses within cells can destroy the host cell and create new viruses that can infect other cells!

Click the Pictures Below to Learn More!

In a LYTIC INFECTION, a bacterial virus, BACTERIOPHAGE - viruses that infect bacteria - enters a bacterial cell, amkes copies of itself and causes th cell to LYSE. LYSIS is the bursting/destruction of a cell.

During this infection a bacteriophage will bind to the surface of a host bacteria cell and will inject its DNA into the cell. The viral DNA is then turned into mRNA, which is translated into proteins that destroy the host cell's DNA.

Now under the control of the viral DNA, the host cell begins making thousands of copies of virus DNA and virus capsid proteins which are assembled to create new viruses within the host cell. Eventually the cell will lyse (burst) releasing hundreds of viruses that can go infect other cells!

Some bacteria viruses, BACTERIOPHAGES - viruses that infect bacteria- cause a LYSOGENIC INFECTION. During a lysogenic infection a bacterial cell is not immediately taken over. Instead, the virus' nucleic information is inserted/injected into the bacteria cell's genome (DNA). Virus DNA that becomes embedded into bacteria cell's DNA is called a PROPHAGE. This prophage will then be replicated along within the bacteria cell's DNA for many generations.

Influences from the environment such as radiation, heat, and chemicals can activate the propage in the bacteria cell. Once activiated the prophage will remove itself from the host cell DNA and reproduces by forming new virus particles.

The lysogenic cycle now becomes an active lytic cyle infection!

Viral Diseases:

PATHOGENS are microbes that cause disease.

Viruses cause diseases by disrupting an organism's ability to mantain homeostasis.

The table to the right lists some common viral diseases.

Viruses cause disease by directly destroying living cells or by affecting cellular processes in ways that upset homeostasis.

In many viral infections, viruses attack and destroy certain cells in the body, causing the symptoms of the associated disease. Other viruses caus infected cells to change their pattterns of growth and development, sometimes leading to cancer.

Prevention and Treatement:

Many viral disease can be prevented by vaccines from weakened or inactiviated viruses.

VACCINES stimulate the body's immune system to recognize and destroy viruses before they cause disease. Washing your hands, and controlling coughs and sneezes can also help prevent the spread of viruses.

Viral diseases can be very difficult to treat. However, there has been recent progress in the development of antivral drugs/medications. These antiviral drugs/mediciations are created to attack specific enzymes found in viruses that their host cells do not have. This allows the drug to destroy the virus but not harm the host organism's cells.

Viruses vs Cells

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