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Chapter 18: Bacteria and Viruses

Chapter Worksheet

Ch. 18.1 Bacteria

A diverse group of organisms that live in almost every environment.

Diversity of Prokaryotes

Prokaryotes are the most numerous organisms on Earth and are found everywhere. Prokaryote is a Greek word that means before a nucleus.

Once were classified in the kingdom Monera based on their lack of a nucleus and membrane-bound organelles. Now divided into two domains: Bacteria and Archea.
Bacteria (Eubacteria)- prokaryotic organisms that belong to Domain Bacteria.
Bacteria live in nearly every environment on Earth and are important in the human body, industry, and food production.
Archea live in extreme environments and are sometimes called extremophiles; have similarities with eukaryotic cells, such as cytoplasm proteins and histones.


Found almost everywhere except in extreme environments. Have strong cell walls that contain peptidoglycan. Some have a second cell wall, a property that can be used to classify them.
Some bacteria, such as cyanobacteria are photosynthetic.


Found in extreme environments that are hostile to most other forms of life.
Thermoacidophiles live in hot, acidic environments, including sulfur hot springs, thermal vents on the ocean floor, and around volcanoes. Thrive in extreme temperatures and high acidity.
Many are strict anaerobes- they die in the presence of oxygen.

Halophiles live in very salty environments. Usually are aerobic, and some carry out a form of photosynthesis using a protein instead of the pigment chlorophyll.

Methanogens are obligate anaerobes; they cannot live in the presence of oxygen. These use carbon dioxide during respiration and give off methane. Found in sewage treatment plants, bogs, swamps, and the gastrointestinal tract of humans and other animals.

Differences Between Bacteria and Archaea

Bacterial cell walls contain peptidoglycan, archaea do not. There are different lipids in their plasma membranes; and different ribosomal proteins and RNA.
Ribosomal proteins of archaea are similar to those of eukaryotic cells.

Prokaryote Structure

Unicellular, DNA, ribosomes, lack a nuclear membrane and other membrane-bound organelles such as mitochondria and chloroplasts.


Exist as a large, circular chromosome in an area called the nucleoid. Many have at least one smaller piece of DNA called a plasmid, which is also circular.


A layer of polysaccharides around the cell wall. The capsule helps preventing the cell from drying out and help the cell attach to surfaces. Also protects from white blood cells and antibiotics.


Found on the outer surface of some bacteria. Pili- submicroscopic, hairlike structures that are made of protein. Pili are for attaching to surfaces; also can form a bridge to pass copies of plasmids.


Typically 1-10 micrometers long and .7-1.5 micrometers wide. The small size allows nutrients and other substances the cells need to diffuse to all parts of the cell.

Prokaryote Characteristics

Historically, scientists identified prokaryotes using criteria such as shape, cell wall, and movement. Today by comparing DNA, evolutionary relationships can be determined.


Three general shapes: Spherical (round) called cocci (coccus), rod-shaped called bacilli (bacillus), spiral-shaped called spirilli (spirillum).

Cell Walls

All have peptidoglycan; made of disaccharides and peptide fragments.
Gram's staining is a technique that uses dyes to identify the two major types of bacteria; those with/without an outer layer of lipids.

Gram-positive have a large amount of peptidoglycan and appear dark purple.
Gram-negative have a lipid layer and less peptidoglycan and appear light pink.
Some antibiotics work by attacking the cell wall of bacteria, physicians identify th type of cell wall to prescribe the proper antibiotic.


Some are stationary, some move by flagella made of microfilaments unlike the eukaryote microtubules, some glide on a layer of secreted slime.

Reproduction of Prokaryotes

Asexual using binary fission. Binary fission- the division of a cell into two genetically identical cells.
The chromosome replicates, and the original chromosome and the new copy separate. The cell elongates and a new piece of plasma membrane and cell wall forms to separate the cell into two identical cells.

This can occur as quickly as 20min.; in ideal conditions 1 cell can become one billion in just ten hours.

Conjugation- two prokaryotes attach to each other and exchange genetic information. New gene combinations are created by using the Pili to exchange genetic material from one cell to the other.

Metabolism of Prokaryotes

  • Anaerobic prokaryotes do not use oxygen for growth or metabolism.
  • Obligate anaerobes cannot live or grow in the presence of oxygen, they obtain energy by fermentation.
  • Facultative anaerobes can grow either in the presence of oxygen or without it.
  • Obligate aerobes require oxygen to grow.

Prokaryotes can also be classified by how they obtain energy for cellular respiration or fermentation.


Cannot synthesize their own food and must take in nutrients.
Saprotrophs, or saprobes obtain their energy by decomposing organic molecules associated with dead organisms or organic wastes.


Carry out photosynthesis to synthesize organic molecules to use as food. Once thought to be a blue-green algae they have since been renamed cyanobacteria.
These are important as oxygen producers as well as base of food chains.
Thought to be first organisms to release oxygen into Earth's atmosphere.


Autotrophs that do not require light for energy; break down and release inorganic compounds that contain nitrogen or sulfur (ie. ammonia, hydrogen sulfide).

Survival of Bacteria

Bacteria have several ways to survive harsh environmental changes.


Some bacteria form an endospore. Endospore- a dormant cell that can withstand harsh environments. Examples: botulism, anthrax, and tetanus.
Examples of environmental changes: extreme heat, extreme cold, dehydration, ultraviolet radiation.

When conditions become harsh, a spore coat surrounds a copy o the bacterial cell's chromosome and a small part of the cytoplasm. The bacterium itself dies, but the endospore can remain for long periods. When conditions become favorable the endospore germinates into a new bacterial cell.


If a bacterium cannot adapt to new environmental changes then extinction is a possibility. However, bacteria reproduce quickly and in high numbers so mutations can help bacteria survive in changing environments.
Mutations lead to new gene combinations. The new characteristics might allow a bacteria to survive in the changing environment.
Mutations can lead to antibiotic resistant bacteria which may cause diseases that are hard to treat.

Ecology of Bacteria

Most bacteria do not cause disease, and many are beneficial.

Nutrient Cycling and Nitrogen Fixation

Without nutrient recycling all raw materials necessary for life would be used up.

All forms of life require nitrogen. Nitrogen is a key component of amino acids. It is also needed to make DNA and RNA.
Most of Earth's nitrogen is found in the atmosphere in gas form. Some bacteria can use nitrogen gas directly and can use enzymes to convert it into nitrogen compounds by a process called nitrogen fixation.

Some of these bacteria live in soil, others exist in a symbiotic relationship in the root nodules of plants: soybeans, clover, and alfalfa.
The plants are able to use nitrogen gas and create ammonia and other nitrogen compounds. These compounds can then be passed along the food chain.

Normal Flora

Flora are bacteria that live on or in the human body. Flora are harmless and compete with bacteria that is harmful.
Escherichia coli (E. coli) live in our intestines. They produce vitamin K, which humans use in blood clotting.

Some strains of E. coli can cause food poisoning.

Foods and Medicines

Bacteria are used in processing many of the foods we eat: cheese, yogurt, buttermilk, pickles, and chocolate.
Bacteria are also important in the fields of medicine and research. Many fight disease: streptomycin, bacitracin, tetracycline, vancomycine.

Disease-causing Bacteria

Only a small percentage of bacteria cause disease. They do this in two ways:

  1. Multiply quickly and spread.
  2. Secrete a toxin or other substance that causes harm.

Plants can also be infected by bacteria. Some infections can devastate crops.

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Ch. 18.2 Viruses and Prions

Viruses and prions are smaller and less complex than bacteria.


Some viruses are not harmful, others are known to infect and harm all types of living organisms.
Virus- a nonliving strand of genetic material within a protein coat.
Viruses do not exhibit all of the characteristics of life: no organelles, do not use energy, cannot make proteins, cannot move, and they cannot replicate on their own.
Some viral diseases such as genital herpes and AIDS have no cure or vaccine to prevent them.

Virus Size

Most viruses range in size from 5-300 nanometers (nanometer is one billionth of a meter) and requires a electron microscope to study them.

Virus Origin

Origin not known but most accepted theory is that they came from parts of cells and developed the ability to exist outside of the cell. The genetic material of viruses is similar to cellular genes.

Virus Structure

Capsid- the outer layer of all viruses, made of proteins.
Inside the capsid is genetic material; either DNA or RNA.
Viruses are generally classified by the genetic material they contain.

Smallpox is DNA virus. Worldwide vaccinations eradicated the virus.

Viral Infection

To replicate a virus must enter a host cell. The virus attaches to the host cell using specific receptors for different types of viruses. A key reason why many viruses cannot be transmitted between different species.

Once attached, the genetic material of the virus enters the cytoplasm of the host. In some cases, entire virus enters the cell where the capsid is broken down, exposing the genetic material.
Virus then uses the host cell to replicate by either the lytic or lysogenic cycle.

Lytic Cycle (see image)

Once inside the host, the viral genes instruct the host cell to make more viral protein capsids and enzymes needed for replication. The protein coat forms around the nucleic acid of new viruses. The new viruses leave the cell by exocytosis or by causing the cell to lyse (burst).

Viruses that use the lytic cycle often produce active infections with immediate symptoms within 1-4 days after exposure.

Lysogenic Cycle (see image)

Viral DNA inserts into a chromosome of the host cell. Once integrated, the infected cell will have the viral genes permanently.
The viral genes may lay dormant for years until they are activated by some sort of factors.
Activation results in the lytic cycle.

Herpes simplex I is an example of a lysogenic virus, and causes cold sores.


Retroviruses- a virus with RNA instead of DNA and has a complex replication cycle. Human immunodeficiency virus (HIV) is an example.

Retroviruses have a protein capsid that has a lipid envelope surrounding it, which it got from the membrane of a host cell.

RNA and the enzyme reverse transcriptase are inside the virus. Reverse transcriptase transcribes DNA from the viral RNA.

Replication steps:

  1. HIV attaches to a cell.
  2. Virus moves into the cytoplasm and the viral RNA is released.
  3. Reverse transcriptase synthesizes DNA using the viral RNA.
  4. The newly formed DNA moves into the nucleus of the host cell and integrates into a chromosome where it may lay dormant.
  5. Once activated, RNA is transcribed from the viral DNA.
  6. The host cell manufactures and assembles new HIV particles which leave the cell.


Prion- a protein that can cause infection or disease (proteinaceous infectious particle).
Prions normally exist in cells, but their function is not well understood Normal prions are shaped like a coil.
Mutations in the genes that code cause the proteins to be misfolded like apiece of paper folded many times.
Mutated prions are associated with diseases known as transmissible spongiform encephalopathies.

Prion Infection

Mutated prions can cause normal proteins to mutate. These prions infect nerve cells in the brain, causing them to burst.

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Page last updated April 3, 2017.