CHAPTER OUTLINE
28.1 The Microbial World
Leeuwenhoek improved microscopes so he able to view microscopic life forms in a drop of
water. Louis Pasteur devised the experiments that disproved the theory of spontaneous
generation. Microbiology is the study of microbes, a term that includes the bacteria, archaea,
protists, fungi, viruses, viroids, and prions. Many microbes have beneficial, and sometimes
essential, roles to play in human health and the biosphere. Normal microbiota have beneficial
effects and bacteria and fungi are the decomposers that play essential roles in various nutrient
cycles on Earth.
28.2 Origin of Microbial Life
All life on Earth can be traded back to a single ancestor; the last universal common ancestor is
common to all organisms that live, and have lived, on earth since life began. Organisms acquire
energy through metabolism, respond and interact with their environment, self-replicate, and are
subject to natural selection. The molecules of organisms are called biomolecules and are organic.
Life originated by nonliving matter in four stages: organic monomers, organic polymers,
protobionts, living cells.
Evolution of Monomers
The first stage in the origin of life was the evolution of simple organic molecules from
the inorganic compounds that were present in the Earth’s early atmosphere. The
“primordial soup” hypothesis proposes that early Earth had very little oxygen, but instead
was made up of water vapor, hydrogen gas, methane and ammonia. The Miller-Urey
experiment ran a simulation of early Earth conditions in a closed system and found that a
variety of amino acids and other organic acids had been produced.
Evolution of Polymers
Within a cell’s cytoplasm, organic monomers join to form polymers in the presence of
enzymes. With no enzymes present on early Earth, Fox suggested that the heat of the sun
can cause amino acids to form proteinoids, small polypeptides that have some catalytic
properties. According to the protein-first hypothesis, some of these proteinoids had
enzymatic properties. The RNA-first hypothesis suggests that only the macromolecule
RNA was needed to progress toward formation of the first cell or cells.
Evolution of Protobionts
Before the first true cell, a protobiont or protocell would have emerged and is
characterized as having an outer membrane. According to the membrane-first
hypothesis, liposomes might have provided life’s first membranous boundary, which
existed before any of its other parts.
Evolution of Living Cells
The central dogma of genetics states that DNA directs protein synthesis and that
information flows from DNA to RNA to protein. It is possible that this sequence
developed in stages.
28.3 Archaea
Bacteria and archaea are both prokaryotes, single-celled organisms that lack the nuclein and
membrane-bound cytoplasmic organelles found in eukaryotic cells, but are placed in their own
domains because of molecular and cellular differences.
Archaeal Size and Structure
Archaea usually range from 0.1-15 μm in size. Their genome is a single, closed, circular
DNA molecule, and they reproduce asexually by binary fission. The plasma membrane of
archaea differs markedly from those of bacteria and eukaryotes.
Types of Archaea
Three main types of archaea are distinguished based on their unique habitats and
metabolic activities: halophiles, thermoacidophiles, and methanogens.
Halophiles
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The halophiles are salt lovers and have evolved a number of mechanisms
enabling them to survive in very salty environments. Some perform a type of
photosynthesis.
Thermoacidophiles
The thermoacidophiles are usually isolated from extremely hot, acidic, aquatic
environments such as hot springs, geysers, and underwater volcanoes.
Methanogens
The methanogens, methane-makers, mostly use carbon dioxide and hydrogen as
energy sources, producing methane as a by-product.
28.4 Bacteria
Bacteria are the most common type of prokaryote on Earth.
Bacterial Size and Structure
Most bacteria are between 0.2–10 μm in size, but a few are quite large. Bacteria have
three basic shapes: rod, spherical, and spiral. All bacterial cells have a plasma
membrane and most have a cell wall. Bacteria can be classified by differences in their
cells walls, which are detected using a Gram stain. Most bacteria have a single circular
chromosome located in the nucleoid region.
Bacterial Reproduction and Gene Transfer
Bacteria reproduce asexually by binary fission. They can transfer genes by conjugation,
transformation, and transduction.
Bacterial Metabolism
Bacteria demonstrate a remarkable range of metabolic abilities. Most are
heterotrophs, some are chemoautotrophs, and some photosynthesize.
Bacterial Diseases in Humans
Most types of bacteria do not cause disease, but a significant number do. Pathogenic
microbes often carry genes that code for specific virulence factors that determine the type
and extent of illness they are capable of causing.
Streptococcal Infections
More different types of human disease are caused by bacteria from the genus
Streptococcus than by any other type of bacterium. Streptococcus bacteria cause
pneumonia, meningitis, middle ear infections, tooth decay, the formation of
dental caries, strep throat, mild skin diseases, Scarlet fever, rheumatic fever, and
necrotizing fasciitis.
Staphylococcus aureus and MRSA
A strain of Staphylococcus aureus that is resistant to methicillin, called MRSA, is
killing an increasing number of young, otherwise healthy individuals.
Tuberculosis
Tuberculosis is one of the leading worldwide causes of death due to infectious
disease. It is chronic disease caused by Mycobacterium tuberculosis, a very slow
growing organism. An immune response results in inflammation of the lungs.
Food Poisoning
Two basic types of bacteria cause food poisoning: those that produce toxins
while they are growing in food, and those that cause infections once they are in
the intestine. The symptoms are mainly vomiting and diarrhea, they tend to
appear suddenly within a few hours of ingestion and are usually self-limiting.
Drug Control of Bacterial Diseases
Antibiotics kill or inhibit bacteria by interfering with their unique metabolic
pathways. Growing resistance of bacteria to antibiotics has become a significant
problem.
28.5 Viruses, Viroids, and Prions
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Viruses are not composed of cells. They are obligate parasites, meaning that they can reproduce
only inside a living cell by utilizing that cell’s machinery. Viroids are strands of RNA that can
reproduce inside a cell, and prions are protein molecules that cause other proteins to become
prions.
Viral Size and Structure
Most viruses are much smaller than bacteria. They come in a variety of shapes. A virus
always has at least two parts—a capsid composed of protein subunits, which protects an
inner core of either DNA or RNA. In some viruses, the capsid is surrounded by a
membrane called an envelope.
Viral Reproduction
Viruses infect almost every type of organism on earth and can be host-specific. The life
cycle of a typical enveloped animal RNA virus has six steps: attachment, entry,
replication, biosynthesis, assembly, and budding.
Latency
Some animal viruses can become latent (hidden) inside the host cell.
Retroviruses have a genome of RNA, but are able to convert their
genome into DNA because they contain an enzyme called reverse transcriptase, it
can then be integrated into the host DNA.
Viral Diseases in Humans
Viruses cause many important human diseases. The best protection against most viral
diseases is immunization utilizing a vaccine.
The Common Cold and Influenza
Colds are most commonly caused by rhinoviruses, and the symptoms usually
include a runny nose, mild fever, and fatigue. The flu is caused by the influenza
virus and is characterized by more severe symptoms. The influenza virus can
change rapidly via antigenic drift and antigenic shift.
Measles
Measles is one of the most contagious human diseases. The fatality rate is higher
in less-developed countries than in more-developed ones, but is decreasing due to
vaccinations.
Herpesviruses
Herpesviruses cause chronic infections that remain latent for much of the time.
These include cold sores and fever blisters, chickenpox, and infectious
mononucleosis.
Antiviral Drugs
Because viruses use the machinery of host cells for viral replication, it is difficult
to develop drugs that affect viral replication without harming host cells.
Antibiotics are not effective against viral infections.
Viroids and Prions
Viroids and prions are also acellular pathogens. A viroid consists of a circular piece of
naked RNA. Viroid replication causes diseases in plants, the only known hosts. Prions
are proteinaceous infectious particles that cause degenerative diseases of the nervous
system in humans and other animals. These diseases include scrapie, mad cow disease,
which can infect humans as variant Creutzfeldt-Jakob disease, and chronic wasting
disease.
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