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Chapter 18 – Bacteria and
Viruses
Section 1 – Bacteria
Section 2 – How Bacteria Affect Humans
Section 3 - Viruses
Section 1 - Bacteria
• Bacteria are the most successful organisms on
Earth.
• Bacteria can be found living almost everywhere
on the globe, even in the most hostile
environment.
• Most species of bacteria are one of three
different shapes:
– Spherical (cocci) – strep throat Streptococcus, form
long chains
– Spiral – Leptospira, infects the liver or brain
– Rod-shaped (bacilli) – Esherichia coli, found in the
intestinal tract and synthesizes vitamin K.
The structure of a bacterial cell is simple.
Bacteria lack cell nuclei, chromosomes, and
membrane-bound organelles.
Bacteria have one of two kinds of cell walls.
– The outer wall is composed of polysaccharides.
– In some bacteria, there is an additional lipid layer
bound to the polysaccharides.
The chemical difference between these two
types of bacterial cell walls is revealed by a
procedure called Gram staining.
• The differences between Gram-positive (stains purple)
and Gram-negative (stains pink) bacteria make it
possible to better diagnose and treat diseases.
• Gram negative bacteria are unaffected by many
antibiotics because the antibiotic cannot penetrate the
additional layer outside the cell wall.
• Bacteria reproduce by splitting in two. Each new cell is
exactly like the parent cell. Genetic material in some
cases is transferred from one bacterium to another
through conjugation.
• Conjugation does not increase the number of bacteria,
but it gives the bacteria more genetic possibilities.
• Bacteria reproduce rapidly – as many as 600,000 in 4
hours.
• Some bacteria are autotrophs. The
autotrophic organisms are either
photosynthetic, capturing solar energy to
make food, or chemosynthetic, using inorganic
molecules to make energy.
• Most bacteria cannot make their own food
and are therefore heterotrophs. They feed on
dead animals and animal wastes, dead plants,
and fallen leaves.
• Some heterotrophic bacteria are parasites.
Section 2 – How Bacteria Affect
Humans
• Many bacteria are beneficial to the Earth.
• Bacteria maintain crucial links with Earth’s
nutrient cycles and are important in the
manufacture of food and life-saving drugs.
• Bacteria decomposers are nutrient recyclers.
Without bacteria, nutrients would be locked
away in the bodies of dead organisms.
• Nitrogen-fixing bacteria is responsible for most of
the nitrogen that is available for plant growth.
These bacteria are located in swellings on the
roots of legumes such as peas, beans, and
peanuts.
Bacteria are used to manufacture food and
drugs through decomposition. We have
learned to use it because it adds flavor.
– Milk converted to yogurt
– Olives changed to be edible
– Cabbage is changed to sauerkraut
– Cucumbers transformed into pickle
Bacteria are used to create drugs like insulin which is
needed for diabetics. Before genetic engineering,
insulin was harvested from the pancreas of animals
killed in slaughterhouses. Being able to produce
insulin has helped to make it more available and
more affordable.
 The human body can become the temporary home for
parasitic bacteria that cause disease and infection. The
disease-causing bacteria is called a pathogen.
 Pathogens are harmful because they damage tissues by
attacking individual cells or by producing toxins that are
released into the hosts body.
 Bacteria are transmitted in a variety of ways:
– By water
– In the air
– In food
– By insects
– By direct human contact
The most common human disease caused by bacteria is
dental caries. Diets high in sugar content promote cavities.
• Pathogenic bacteria can contaminate food and cause food
poisoning. Many cases of the flu are actually food poisoning
from mishandled food.
• The most dangerous kinds of food poisoning:
– Clostridium botulinum- causes botulism due to a toxin
produced by the bacteria. Very toxic causing paralysis and
death. They can grow in canned food.
– Salmonella – found in pork, eggs, poultry and other foods.
Cause diarrhea, vomiting and abdominal cramps. Severe
infections cause dehydration which can lead to death in
the young or the elderly.
– Escherichia coli – this bacteria is commonly found in
human intestines making vitamin K, a new strain causes
severe food poisoning and even death in undercooked
meat.
• The scientific study of disease is called pathology.
• Exotoxins are toxic substances that bacteria secrete
and release into their environment.
• Endotoxins are toxic substances secreted and
released when the bacteria dies.
• Antibiotics affect bacteria by interfering with the
cellular activities and causing death.
• Antibiotic resistance is a big worry for modern
medicine. Populations of bacteria have developed
that are resistant to antibiotics due to mutations and
to misuse of antibiotics.
• Emerging diseases that pass from wild animals to
humans is called zoonosis.
Escherichia coli
Mycobacteria tuberculosis
Borrelia burgdorferi
The spirochete of Lymes Disease
Haemophilus ducreyigenital ulcer
Syphilis
 Many bacterial diseases are controlled due to sanitation and
hygiene. Most industrialized nations filter and chlorinate their
water. Human waste is collected and treated to remove
pathogens before release.
 Developing countries do not have the same sanitation laws
therefore they often have outbreaks of cholera, typhoid and
other diseases caused by contamination.
 Vaccination is a shot that contains pathogens or their toxins
that have been made harmless and that stimulate your
immune system to produce antitoxins that prevent your body
from getting a disease.
 Antibiotics are antibacterial drugs that prevent bacteria from
being able to form new cell walls. Penicillin was the first
antibiotic found to fight bacteria. It was found in green bread
mold.
 Antibiotic resistance is caused by not all the bacteria being
killed when exposed to it.
Section 3: Viruses
Viruses are microscopic particles that invade the cells of plants,
animals, fungi, and bacteria. Viruses often destroy the cells
they invade.
• A virus is not a cell. It is composed of a protein coat (called a
capsid) that protects a core of genetic material.
• Some viruses have DNA as its core and some has RNA.
• Viruses are so small they only have enough genes for the
protein coat and enzymes that allow the virus to take over its
host cell.
• Viruses have some characteristics of living things – like genetic
material, but they lack three things – they are not made of
cells, they cannot make proteins by themselves, and they
cannot use energy. They are only able to reproduce when
inside a living cell.
Adenovirus
Viruses cause many human diseases.
• Chickenpox and shingles are caused by the
same varicella-zoster herpesvirus
• Hepatitis is caused by at least 5 viruses.
Hepatitis A and hepatitis E can be spread by
fecally contaminated food and water.
Hepatitis B, C, and D are spread by sexual
contact, by contact with infected blood, or by
the use of contaminated needles
• Some viruses contain viral oncogenes, genes
that cause cancer by blocking the normal
controls on cell reproduction.
Acquired Immune Deficiency
Syndrome (AIDS)
• The human immunodeficiency virus (HIV)
causes the disease AIDS.
• HIV gradually destroys an infected person’s
immune system.
• HIV is spread by sexual contact, contact with
infected body fluids like blood, and from
mother to fetus.
• The viral RNA enters the cell and takes over
manufacturing viral DNA which in turn release
viral proteins.