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Bacteria
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| Anatomy of a Simple BacteriumBacteria cells typically are surrounded by a rigid, protective cell wall. The
cell membrane, also called the plasma membrane, regulates passage of materials
into and out of the cytoplasm, the semi-fluid that fills the cell. The DNA,
located in the nucleoid region, contains the genetic information for the cell.
Ribosomes carry out protein synthesis. Many baceteria contain a pilus (plural
pili), a structure that extends out of the cell to transfer DNA to another
bacterium. The flagellum, found in numerous species, is used for locomotion.
Some bacteria contain a plasmid, a small chromososme with extra genes. |
| I | INTRODUCTION |
Bacteria, one-celled organisms visible only through a
microscope. Bacteria live all around us and within us. The air is filled with
bacteria, and they have even entered outer space in spacecraft. Bacteria live in
the deepest parts of the ocean and deep within Earth. They are in the soil, in
our food, and on plants and animals. Even our bodies are home to many different
kinds of bacteria. Our lives are closely intertwined with theirs, and the health
of our planet depends very much on their activities.
Bacterial cells are so small that scientists
measure them in units called micrometers (µm). One micrometer equals a millionth
of a meter (0.0000001 m or about 0.000039 in), and an average bacterium is about
one micrometer long. Hundreds of thousands of bacteria would fit on a rounded
dot made by a pencil.
Bacteria lack a true nucleus, a feature that
distinguishes them from plant and animal cells. In plants and animals the
saclike nucleus carries genetic material in the form of deoxyribonucleic acid
(DNA). Bacteria also have DNA but it floats within the cell, usually in a loop
or coil. A tough but resilient protective shell surrounds the bacterial
cell.
Biologists classify all life forms as either
prokaryotes or eukaryotes. Prokaryotes are simple, single-celled organisms like
bacteria. They lack a defined nucleus of the sort found in plant and animal
cells. More complex organisms, including all plants and animals, whose cells
have a nucleus, belong to the group called eukaryotes. The word
prokaryote comes from Greek words meaning “before nucleus”;
eukaryote comes from Greek words for “true nucleus.” The study of
bacteria is called bacteriology, a branch of microbiology.
Bacteria inhabited Earth long before human
beings or other living things appeared. The earliest bacteria that scientists
have discovered, in fossil remains in rocks, probably lived about 3.5 billion
years ago. These early bacteria inhabited a harsh world: It was extremely hot,
with high levels of ultraviolet radiation from the sun and with no oxygen to
breathe.
Descendents of the bacteria that inhabited a
primitive Earth are still with us today. Most have changed and would no longer
be able to survive the harshness of Earth’s early environment. Yet others have
not changed so much. Some bacteria today are able to grow at temperatures higher
than the boiling point of water, 100oC (212oF). These
bacteria live deep in the ocean or within Earth. Other bacteria cannot stand
contact with oxygen gas and can live only in oxygen-free environments—in our
intestines, for example, or in the ooze at the bottom of swamps, bogs, or other
wetlands. Still others are resistant to radiation. Bacteria have remarkable
abilities to adapt to extreme environments and thrive in parts of Earth that are
inhospitable to other forms of life. Anywhere there is life, it includes
bacterial life.
| II | THE IMPORTANCE OF BACTERIA |
Much of our experience with bacteria involves
disease. Although some bacteria do cause disease, many kinds of bacteria live on
or in the human body and prevent disease. Bacteria associated with the human
body outnumber body cells by ten to one. In addition, bacteria play important
roles in the environment and in industry.
| A | Bacteria and Human Health |
We have all had bacterial diseases. Bacteria
cause many cases of gastroenteritis, sometimes called stomach flu. Perhaps the
most common bacterial disease is tooth decay. Dental plaque, the sticky film on
our teeth, consists primarily of masses of bacteria. These bacteria
ferment (break down) the sugar we eat to produce acids, which over time
can dissolve the enamel of the teeth and create cavities (holes) in the
teeth.
Tooth decay provides a good example of how
multiple factors contribute to bacterial disease. The human body hosts the
bacteria, the diet supplies the sugars, and the bacteria produce the acid that
damages the teeth.
| A1 | Bacteria That Inhabit the Body |
Communities of bacteria form what are
called biofilms on many body surfaces. Dental plaque is a biofilm
covering the teeth. Biofilms also cover the soft tissues of our mouths and the
inner surfaces of our nose, sinuses, throat, stomach, and intestines. Even the
skin has bacterial communities that extend into hair follicles. Bacterial
communities differ in each region of the body, reflecting the environmental
conditions in their specific region. Bacteria that inhabit the surface of the
stomach, for example, must deal with extremely strong acid in the digestive
juices.
Some regions in the interior of the body
are sterile—that is, devoid of living organisms other than the cells of the
body. Sterile regions include the muscles, the blood, and the nervous system.
However, even these regions face constant invasion by bacteria. The body’s
immune system is designed to rid the body of these invaders.
A healthy, balanced community of
bacteria is extremely important for our health. Some of these organisms protect
us from disease-causing organisms that would otherwise infect us. Animals raised
in a completely germ-free environment, without any contact with bacteria, are
highly susceptible to infectious diseases if they are exposed to the outside
world. Bacteria in our bodies also provide us with needed nutrients, such as
vitamin K, which the body itself cannot make. The communities of bacteria and
other organisms that inhabit the body are sometimes called the normal microflora
or microbiota.
| A2 | Disease-Causing Bacteria |
In most cases the bacteria that cause
disease are not part of the bacteria that normally inhabit the body. They are
picked up instead from sick people, sick animals, contaminated food or water, or
other external sources. Bacterial disease also can occur after surgery, an
accident, or some other event that weakens the immune system.
| A2a | Opportunistic Infections |
When the immune system is not
functioning properly, bacteria that usually are harmless can overwhelm the body
and cause disease. These organisms are called opportunistic because they
cause disease only when an opportunity is presented. For example, cuts or
injuries to the skin and protective layers of the body enable normally friendly
bacteria to enter the bloodstream or other sterile parts of the body and cause
infection. Surgery may enable bacteria from one part of the body to reach
another, where they cause infection. A weakened immune system may be unable to
prevent the rapid multiplication of bacteria and other microorganisms.
Opportunistic infections became more
important in the late 20th century because of diseases such as acquired
immunodeficiency syndrome (AIDS), a viral disease that ravages the immune
system. Also contributing to an increase in opportunistic infections is the
wider use of cancer-fighting drugs and other drugs that damage the immune
system.
| A2b | Bacterial Killers |
Some dramatic infectious diseases
result from exposure to bacteria that are not part of our normal bacterial
community. Cholera, one of the world’s deadliest diseases today, is caused by
the bacterium Vibrio cholerae. Cholera is spread in water and food
contaminated with the bacteria, and by people who have the disease. After
entering the body, the cholera bacteria grow in the intestines, often along the
surface of the intestinal wall, where they secrete a toxin (poison). This
toxin causes massive loss of fluid from the gut, and an infected person can die
of dehydration (fluid loss) unless the lost fluids, and the salts they
contain, are replaced. Cholera is common in developing regions of the world that
lack adequate medical care.
Another major bacterial killer is
Mycobacterium tuberculosis, which causes tuberculosis (TB), a disease of
the lungs. Tuberculosis is responsible for more than 2 million deaths per year
worldwide. Although antibiotics such as penicillin fight many bacterial
diseases, the TB bacterium is highly resistant to most antibiotics. In addition,
the TB-causing bacteria readily spread from person to person.
| A2c | New Bacterial Diseases |
While tuberculosis and cholera have
been with us for centuries, in recent decades new bacterial diseases have
emerged. Legionnaires’ disease, a severe form of pneumonia, was first recognized
at an American Legion convention in Philadelphia, Pennsylvania, in 1976. It is
caused by a previously unknown bacterium, Legionella pneumophila, which
is most often transmitted through infected water.
Lyme disease, a form of arthritis
caused by the bacterium Borrelia burgdorferi, was first recognized in
Lyme, Connecticut, in 1975. A bite from a deer tick that carries the bacteria
transmits the disease to human beings.
A food-borne disease that has raised
concern in the United States, Canada, and Western Europe is caused by a
particular variant of the common intestinal bacterium Escherichia coli,
or E. coli for short. Although E. coli is normally present in the
human intestines, the variant E. coli O157:H7 produces toxins that cause
bloody diarrhea and, in some cases, far more severe problems, including kidney
failure and death. A person can become infected by eating contaminated meat.
Thorough cooking kills the bacteria.
Methicillin-resistant
Staphylococcus aureus (MRSA) is a highly opportunistic, drug-resistant
bacteria that originated in hospital settings and then spread widely. First
recognized in the United States in the late 1960s, MRSA attained a broader
public profile in 2007 with the publication of studies attributing to it more
than 19,000 deaths and 94,000 serious infections annually.
| A3 | How the Body Fights Bacterial Disease |
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| Macrophage Engulfing Bacterium A macrophage, in yellow, engulfs and consumes a bacterium. Macrophages are immune cells that wander through the body consuming foreign particles such as dust, asbestos particles, and bacteria. They help protect the body against infection |
After recovering from many bacterial
infections, people have the ability to resist a second attack by the same
bacteria. They can do so because their immune system forms disease-fighting
proteins called antibodies designed to recognize specific bacteria. When next
exposed to those bacteria, the antibodies bind to the surface of the bacteria
and either kill them, prevent them from multiplying, or neutralize their toxin.
Vaccines also can stimulate the immune system to form disease-fighting
antibodies. Some vaccines contain strains of the bacterium that lack the ability
to cause infection; others contain only parts of bacterial cells.
| A4 | Treatment and Prevention of Bacterial Disease |
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| Discovery of Penicillin British bacteriologist Alexander Fleming discovered penicillin in 1928. Penicillin, an important antibiotic derived from mold, is effective against a wide range of disease-causing bacteria. It acts by killing bacteria directly or inhibiting their growth. |
| A4a | Antibiotics |
In many cases the immune system can
wipe out a bacterial infection on its own. But sometimes people become so sick
from a bacterial disease that they require medical treatment. Antibiotics and
other antibacterial drugs are the major weapons against disease-causing
bacteria. Antibiotics act in a number of ways to kill bacteria or suppress their
activity. Over time, however, bacteria can become resistant to antibiotics. As a
result bacterial diseases have become more and more difficult to cure.
In an effort to control antibiotic
resistance, physicians have tried to limit the use of antibiotics. In addition,
they have advocated more vigorous efforts to improve the antibiotics we now have
and to find new agents active against bacteria.
| A4b | Vaccines |
Immunization through vaccines is
important in the prevention of infectious diseases caused by bacteria. Vaccines
expose a human being or other animal to a disease-causing bacterium or its
toxins without causing the disease. As a result of this exposure, the body forms
antibodies to the specific bacterium. These antibodies remain ready to attack if
they meet the bacteria in the future. Some immunizations last a lifetime,
whereas others must be renewed with a booster shot.
Tetanus provides a good example of a
successful vaccine. The bacterium Clostridium tetani, found in soil and
ordinary dirt, produces one of the most lethal toxins known. The toxin affects
nerves, resulting in muscle rigidity and death. Tetanus infection has become
very rare in developed countries such as the United States where nearly everyone
is immunized against the toxin. The vaccine immunizes the body by means of
toxins that have been chemically treated so they are no longer toxic. Health
officials recommend getting a tetanus shot every ten years. In less developed
countries where vaccination is not so common, tetanus is a major cause of death,
especially of babies.
| A4c | Public Health Measures |
Public health measures provide major
controls against infectious disease. Especially important are those measures
leading to ready availability of clean water, safe food, and up-to-date medical
care. Waterborne diseases, such as cholera and typhoid fever, kill an estimated
5 million to 10 million people worldwide each year, according to the United
Nations. Sufficient sources of clean drinking water in developing countries
could help prevent these deaths. Food-safety guidelines can help prevent the
spread of disease through contaminated food. Proper medical care can prevent
transmission of infectious diseases to others. Tuberculosis, for example, kills
more people worldwide every year than any other single disease. But if
identified early, cases of tuberculosis can be treated effectively with
antibiotics and other means, thereby stopping transmission to others.
Maintaining a clean environment for
medical care is also important in preventing the spread of infectious diseases.
For example, medical instruments, such as needles and syringes, must be sterile
and proper infection-control procedures must be followed in hospitals, medical
and dental offices, and industries that use bacteria. However, it is never
possible, or even desirable, to have an environment entirely free of bacteria.
| B | Bacteria and the Environment |
Bacteria play a major role in recycling
many chemical elements and chemical compounds in nature. Without such bacterial
activities as the recycling of carbon dioxide (CO2) life on Earth
would be impossible. Plants use CO2 to grow and in the process
they produce the oxygen humans and other animals breathe. Moreover, we
would drown in garbage and wastes if bacteria did not speed the decomposition of
dead plant and animal matter.
| B1 | Nitrogen Fixation |
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| B2 | The Carbon Cycle |
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Carbon Cycle
Carbon, used by all living organisms,
continuously circulates in the earth’s ecosystem. In the atmosphere, it exists
as colorless, odorless carbon dioxide gas, which is used by plants in the
process of photosynthesis. Animals acquire the carbon stored in plant tissue
when they eat and exhale carbon dioxide as a by-product of metabolism. The
carbon cycle continues after plants and animals die, when bacteria contribute to
the decay process and release carbon dioxide. Although some carbon is removed
from circulation temporarily as coal, petroleum, fossil fuels, gas, and
limestone deposits, respiration and photosynthesis balance to keep the amount of
atmospheric carbon relatively stable. Industrialization, however, has
contributed additional carbon dioxide to the environment.
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Bacteria and fungi (yeasts and
molds) are vital to another process that makes life on Earth possible: the
carbon cycle. They help produce the gas carbon dioxide (CO2), which
plants take from the atmosphere. During a part of the carbon cycle called
photosynthesis, plants turn sunlight and CO2 into food and energy,
releasing oxygen into the atmosphere.
The carbon cycle continues after plants
and animals die, when bacteria help convert the material of which those
organisms are made back into CO2. Bacteria and fungi secrete enzymes
that partially break down dead matter. Final digestion of this matter takes
place within bacterial and fungal cells by the processes of fermentation and
respiration. The CO2 released by this action escapes back into the
atmosphere to renew the cycle.
| B3 | Chemosynthesis |
Bacteria are major players in cycles of
other elements in the environment. Chemosynthetic bacteria use chemical energy,
instead of the light energy used by plants, to change CO2 into
something that other organisms can eat. Chemosynthesis occurs in vents at the
bottom of the ocean, where light is unavailable for photosynthesis but hydrogen
sulfide gas, H2S, bubbles up from below Earth’s crust. Life can
develop around these vents because bacteria use the H2S in changing
CO2 into organic nutrients. The H2S coming up from Earth’s
mantle is extremely hot, but bacteria in these vent communities are adapted to
the high temperatures. Bacteria’s ability to react chemically with sulfur
compounds is useful in certain industrial processes as well.
| B4 | Bioremediation |
Bioremediation refers to the use of
microorganisms, especially bacteria, to return the elements in toxic chemicals
to their natural cycles in nature. It may provide an inexpensive and effective
method of environmental cleanup, which is one of the major challenges facing
human society today.
Bioremediation has helped in cleaning up
oil spills, pesticides, and other toxic materials. For example, accidents
involving huge oil tankers regularly result in large spills that pollute
coastlines and harm wildlife. Bacteria and other microorganisms can convert the
toxic materials in crude oil to harmless products such as CO2. Adding
fertilizers that contain nitrogen, phosphorus, and oxygen to the polluted areas
promotes the multiplication of bacteria already present in the environment and
speeds the cleanup process.
| C | Bacteria in Agriculture and Industry |
Many of bacteria’s beneficial roles in
agriculture have been described in the previous section on Bacteria and the
Environment. By recycling certain chemical elements and compounds, bacteria make
plant and animal life possible. Bacteria’s chemical interactions have also found
uses in industry. In recent decades, scientists have engineered bacterial genes
to produce sought-after substances, such as human insulin, to use in the
treatment of disease.
| C1 | Bacteria in Agriculture |
Through the process of nitrogen
fixation, bacteria turn nitrogen in the air into nutrients that crops and other
plants need to grow. Some of the nitrogen-fixing bacteria attach to the roots of
plants. Through the carbon cycle, bacteria produce the carbon dioxide that
plants require for photosynthesis. Bacteria that live in the stomachs of
cud-chewing animals, such as cows and sheep, help the animals digest
grasses.
Bacteria also can be harmful in
agriculture because of the major diseases of farm animals they cause. Many of
the bacteria that cause infectious diseases in farm animals resemble those that
cause similar human diseases. For example, a variant of the bacterium that
causes human tuberculosis causes tuberculosis in cattle, and it can infect
humans through cow’s milk. To prevent transmission of the disease, milk for
human consumption should be pasteurized (heated at a temperature between
60° and 70°C (140° and 158°F) for a short time. Pasteurization kills most
bacteria in milk.
Other disease-causing bacteria primarily
affect animals other than humans. For example, the bacterium Brachyspira
hyodysenteria causes a type of diarrhea in pigs that can be disastrous for
pig farmers. Many infectious diseases of farm animals also affect wild animals,
such as deer. Wild animals, in turn, can infect domestic animals, including cats
and dogs.
Bacteria in the Food Industry
Bacteria are of major importance in the
food industry. On the one hand, they cause food spoilage and food-borne
diseases, and so must be controlled. On the other hand, they improve food flavor
and nutrition.
The dairy industry provides prime
examples of bacteria’s harmful and helpful roles. Before the introduction of
pasteurization in the late 1800s, dairy products were major carriers for
bacteria that caused such illnesses as tuberculosis and rheumatic heart disease.
Since that time regulation of the dairy industry has greatly reduced the risks
of infection from dairy products.
On the helpful side, bacteria contribute
to the fermentation (chemical breakdown) of many dairy products people
eat every day. Yogurt, considered a healthful food, is produced by bacterial
fermentation of milk. The bacteria produce lactic acid, which turns the milk
sour, hampers the growth of disease-causing bacteria, and gives a desirable
flavor to the resulting yogurt.
Cheese also is produced by fermentation. First, bacteria ferment milk sugar to lactic acid. Then, cheese makers can introduce various microorganisms to produce the flavors they desire. The process is complicated and may take months or even years to complete, but it gives cheeses their characteristic flavors.
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| CHEESE MAKING (SEPARATING CURD FROM MILK) |
Cheese also is produced by fermentation. First, bacteria ferment milk sugar to lactic acid. Then, cheese makers can introduce various microorganisms to produce the flavors they desire. The process is complicated and may take months or even years to complete, but it gives cheeses their characteristic flavors.
The variety of fermented foods we eat
ranges from pickles, olives, and sauerkraut to sausages and other cured meats
and fish, chocolate, soy sauce, and other products. In most of these
fermentations, bacteria that produce lactic acid play major roles.
Alcohol-producing yeasts are the primary fermentors in the manufacture of beer
and wine, but lactic-acid bacteria also are involved, especially in making wine
or cider. Bacteria that produce acetic acid can convert wine, cider, or other
alcoholic beverages to vinegar.
| C3 | Bacteria in Waste Treatment |
Bacteria are very important in sewage
treatment. Standard sewage treatment involves multiple processes. It usually
starts with settling during which large items sink to the bottom. Next, air is
bubbled through the sewage. This so-called aerobic phase encourages oxygen-using
bacteria to break down organic material in the sewage, such as human wastes, to
acids and CO2. Most disease-causing organisms are also killed in this
phase. The sewage sludge left behind is attacked in a subsequent phase by
anaerobic bacteria (bacteria that cannot tolerate oxygen). These bacteria
break down the sludge to produce methane gas, which can then be used as a fuel
to power the treatment facility. In treatment plants today, this anaerobic phase
sometimes precedes the aerobic phase.
Bacteria are also effective in cleaning
up harmful wastes through bioremediation. In this process bacteria and other
microorganisms convert toxic or otherwise objectionable wastes, such as
pesticides and oil spills, to harmless or even useful products.
| C4 | Bacteria in Mineral Extraction |
An interesting industrial process
carried out by bacteria is the recovery of valuable minerals such as copper from
ores. The most important copper ores are copper sulfides, which may contain only
a small percentage of copper. Bacteria of the genera Thiobacillus and
Sulfolobus are able to oxidize sulfides—that is, cause a chemical
reaction of sulfides with oxygen—yielding sulfuric acid. This action produces
the acid conditions necessary to leach (remove) the copper from the ores.
The use of bacteria in extracting minerals, though slow, is environmentally
friendly compared with the standard process of smelting. Smelting requires
energy to heat the ore to extremely high temperatures for extracting minerals,
and it also releases gases that pollute the air.
Some chemical reactions in which
bacteria participate are harmful rather than helpful to industry. Bacteria are
major agents of metal corrosion (wearing away) through the formation of
rust, especially on metals containing iron. During the early stages of rust
formation, hydrogen is produced, and it acts to slow the rusting process.
However, certain bacteria use the hydrogen as a nutrient with the result that
they greatly speed up rust formation.
| C5 | Bacteria in Biotechnology |
Bacteria have been at the center of
recent advances in biotechnology—the creation of products for human benefit
through the manipulation of biological organisms. Biotechnology itself dates
back at least as far as ancient Egyptian civilization. Paintings on the walls of
Egyptian tombs depict the brewing of beer, which uses microorganisms in the
fermentation process. However, the existence of bacteria did not become known
until the development of sufficiently powerful microscopes in the late 1600s.
During the centuries that followed, scientists became aware that living
organisms were responsible for many biotechnological processes.
Biotechnology grew steadily during the
20th century. In the 1970s scientists used information about replication of
viruses and bacteria and about DNA synthesis (manufacture) to begin the
genetic engineering of bacterial cells. When scientists combined human DNA with
the DNA in bacterial cells, recombinant DNA technology was born. Human DNA is
the “recombinant.” DNA contains the instructions for creating proteins. With
their recombinant DNA, bacteria became factories for turning out human proteins,
such as the hormone insulin or antibodies that fight disease. Because they
multiply so rapidly, bacteria produce multiple copies of proteins in a short
time. The process of taking genetic information from one organism and placing it
in a different organism was patented by American biochemists Stanley Cohen and
Herbert Boyer in 1980. The genetic revolution was underway.
| C6 | Other Industrial Roles |
Bacteria play a role in the production
of other products, including certain plastics and enzymes used in laundry
detergents. They also produce many antibiotics, such as streptomycin and
tetracycline. Since the 1980s, bacteria have gained importance in the production
of many bulk chemicals, including ethanol, a form of alcohol made from fermented
corn. Ethanol is an ingredient of gasohol, a fuel that burns more cleanly than
gasoline and uses less petroleum. Chemical production using bacteria and other
microorganisms results in less pollution to the environment than standard
chemical production. The growth of genetic engineering has opened the way to
even greater use of bacteria in large-scale industrial manufacturing and
environmentally friendly processes.
| III | CHARACTERISTICS OF BACTERIA |
Bacteria are so small that they can be seen
only under a microscope that magnifies them at least 500 times their actual
size. Some become visible only at magnifications of 1,000 times. They are
measured in micrometers (µm) and average about 1 to 2 µm in length. One
micrometer equals one-millionth of a meter (0.0000001 m or about 0.000039
in).
Bacteria not only have many uses, they also
occur in diverse shapes and types. As a group they carry out a broad range of
activities and have different nutritional needs. They thrive in a variety of
environments.
| A | Types of Bacteria |
Scientists use various systems for
classifying bacteria into different types. One of the simplest systems is by
shape. Other systems depend on oxygen use, source of carbon, and response to a
particular dye.
| A1 | Classification by shape |
Most bacteria come in one of three
shapes: rod, sphere, or spiral. Rod-shaped bacteria are called bacilli.
Spherical bacteria are called cocci, and spiral or corkscrew-shaped
bacteria are called spirilla. Some bacteria come in more complex shapes.
A hairlike form of spiral bacteria is called spirochete (see
Spirochetes). Streptococci and staphylococci are well-known disease-causing
bacteria among the cocci.
| A2 | Aerobic and Anaerobic Bacteria |
Scientists also classify bacteria
according to whether they need oxygen to survive or not. Aerobic bacteria
require oxygen. Anaerobic bacteria cannot tolerate oxygen. Bacteria that live in
deep ocean vents or within Earth are anaerobic. So are many of the bacteria that
cause food poisoning.
| A3 | Autotrophic and Heterotrophic Bacteria |
All bacteria require carbon for growth
and reproduction. Bacteria called autotrophs (“self-feeders”) get their carbon
from CO2. Most bacteria, however, are heterotrophs (“other feeders”)
and derive carbon from organic nutrients such as sugar. Some heterotrophic
bacteria survive as parasites, growing within another living cell and using the
nutrients and cell machinery of their host cells. Some autotrophic bacteria,
such as cyanobacteria, use sunlight to produce sugars from CO2.
Others depend instead on energy from the breakdown of inorganic chemical
compounds, such as nitrates and forms of sulfur.
| A4 | Gram-Positive and Gram-Negative Bacteria |
Another system of classifying bacteria
makes use of differences in the composition of cell walls. The difference
becomes clear by means of a technique called Gram’s stain, which identifies
bacteria as either gram-positive or gram-negative. After staining, gram-positive
bacteria hold the dye and appear purple, while gram-negative bacteria release
the dye and appear red. Gram-positive bacteria have thicker cell walls than
gram-negative bacteria. Knowing whether a disease-causing bacterium is
gram-positive or gram-negative helps a physician to prescribe the appropriate
antibiotic. The stain is named for H. C. J. Gram, a Danish physician who
invented it in 1884.
| A5 | The Cell and Its Structure |
The cell wall generally determines the
shape of the bacterial cell. The wall is a tough but resilient shell that keeps
bacterial cells from drying out and helps them resist environmental stress. In
some cases the cell wall protects the bacterium from attack by the body’s
disease-fighting immune system cells. Some bacteria do not have much of a cell
wall, while others have quite thick structures. Many species of bacteria move
about by means of flagella, hairlike structures that project through the cell
wall. The flagellum’s rotating motion propels the bacterial cell toward
nutrients and away from harmful substances.
Like all cells bacteria contain the
genetic material DNA. But bacterial DNA is not contained within a nucleus, as is
DNA in plant and animal cells. Most bacteria have a single coil of DNA, although
some bacteria have multiple pieces. Bacterial cells often have extra pieces of
DNA called plasmids, which the cell may gain or lose without dying. Surrounding
the DNA in a bacterial cell is cytoplasm, a watery fluid that is rich in
proteins and other chemicals. A cell membrane inside the wall holds together the
DNA and the constituents of the cytoplasm. Most activities of the bacterial cell
are carried out within the cytoplasm, including nutrition, reproduction, and the
manufacture of proteins.
| B | How Bacteria Function |
Bacterial cells, like all cells, require
nutrients to carry out their work. These nutrients must be water soluble to
enter through pores in the cell wall and pass through the cell membrane into the
cytoplasm. Many bacteria, however, can digest solid food by secreting chemicals
called exoenzymes into the surrounding environment. The exoenzymes help break
down the solid food outside the bacteria into water-soluble pieces that the cell
wall can absorb. Bacterial cells use nutrients for a variety of life-sustaining
biochemical activities known collectively as metabolism.
| B1 | Anabolism and Catabolism |
The metabolic activities that enable
the cell to function occur in two ways: anabolism and catabolism. Simply put,
anabolism is the manufacture of complex molecules from simple ones, and
catabolism is the breakdown of complex molecules into simple ones. Cells use the
energy from catabolism for all their other tasks, including growth, repair, and
reproduction.
A single bacterial cell takes up small
molecules from the environment by means of specific transport proteins in the
cell membrane. In the case of more complex molecules, such as proteins or
complex carbohydrates, bacteria first secrete digestive enzymes into the
environment to break the nutrients down into smaller molecules, which are
transported across the membrane. Enzymes (proteins that speed chemical
reactions) within the cytoplasm then digest the molecules further. This
breakdown, called catabolism, results in energy transfer through the processes
of respiration and fermentation. During metabolism, some of the small molecules
are converted into the molecules the cell needs to synthesize
(manufacture) its own proteins, nucleic acids (building blocks of DNA), lipids
(fatty substances), and polysaccharides (sugars and starches). The metabolic
processes for synthesis of these complex cells are anabolism.
| B2 | Adaptation to Environmental Stress |
All organisms have some capacity to
adapt to environmental stress, but the extent of this adaptive capacity varies
widely. Heat, cold, high pressure, and acid or alkaline conditions can all
produce stress. Bacteria easily adapt to environmental stress, usually through
changes in the enzymes and other proteins they produce. These adaptations enable
bacteria to grow in a variety of conditions. Gradual exposure to the stress, for
example, may enable bacteria to synthesize new enzymes that allow them to
continue functioning under the stressing conditions or that enhance their
capacity to deal with the stressing agent. Or they may resist environmental
stress in other ways. Some bacteria that live in extremely acidic conditions can
pump out acid from their cell.
Extremophiles are organisms that can
grow in conditions considered harsh by humans. Some kinds of bacteria thrive in
hydrothermal vents on the ocean floor or in oil reservoirs within Earth, at high
pressures and temperatures as high as 120oC (250oF). Other
kinds can live at temperatures as low as –12oC (10oF) in
Antarctic brine pools. Other bacteria have adapted to grow in extremely acid
conditions, where mines drain or minerals are leached from ores and sulfuric
acid is produced. Others grow at extremely alkaline or extremely salty
conditions. Still others can grow in the total absence of oxygen. Bacteria able
to function in these extreme conditions generally cannot function under
conditions we consider normal.
| B3 | Reproduction and Survival |
Bacteria reproduce very rapidly.
Replication in some kinds of bacteria takes only about 15 minutes under optimal
conditions. One bacterial cell can become two in 15 minutes, four in 30 minutes,
eight in 45 minutes, and so on. Bacteria would quickly cover the entire face of
the globe if their supply of nutrients was unlimited. Fortunately for us,
competition for nutrients limits their spread. In the absence of sufficient
nutrients, however, many bacteria form dormant spores that survive until
nutrients become available again. Spore formation also enables these bacteria to
survive other harsh conditions.
| B3a | Binary Fission |
The simplest sort of bacterial
reproduction is by binary fission (splitting in two). The bacterial cell
first grows to about twice its initial size. Toward the end of that growth, the
cell membrane forms a new membrane that extends inward toward the center of the
cell. The cell wall follows closely behind, bisecting the cell. The membrane
then seals to divide the enlarged cell into two small cells of equal or nearly
equal size, and a new wall forms between the membranes.
The growth and division of a
bacterial cell has two main phases. In one phase, the cell replicates its DNA
and makes all the other molecules needed for the new cell. The second phase—cell
division—occurs when DNA replication stops. In the bacterium Escherichia
coli replication takes about 40 minutes and cell division lasts about 20
minutes. The entire cycle takes about an hour. Yet the time for one cell to
become two cells still takes only about 20 minutes. How is this possible? The
cell does not wait for one cycle of replication to end before it starts another.
Thus, a rapidly growing bacterial cell is carrying out multiple rounds of
replication at the same time.
| B3b | Spore Formation |
In response to limited nutrients or
other harsh conditions, many bacteria survive by forming spores that resist the
environmental stress. Spores preserve the bacterial DNA and remain alive but
inactive. When conditions improve, the spore germinates (starts growing)
and the bacterium becomes active again.
The best-studied spores form within
the bodies of Bacillus and Clostridium bacteria, and are known as endospores.
Clostridium botulinum spores cause deadly botulism poisoning. Endospores
have thick coverings and can resist environmental stress, especially heat. Even
boiling in water does not readily kill them. But they can be killed by heating
in a steel vessel filled with steam at high temperature and high pressure.
Endospores can live for centuries in their dormant state.
Some bacteria form other types of
spores. These spores are usually dormant but not as heat resistant or long-lived
as endospores. Some aquatic bacteria, for example, attach to surfaces and
produce swarmer cells during division. The swarmer cell swims away to attach to
another surface and give rise to still more swarmer cells. Still other bacteria
survive by forming colonies made up of millions of cells that act in a
coordinated way to keep the organism alive.
| B3c | Genetic Exchange |
Bacterial cells often can survive by
exchanging DNA with other organisms and acquiring new capacities, such as
resistance to an antibiotic intended to kill them. The simplest method of DNA
exchange is genetic transformation, a process by which bacterial cells take up
foreign DNA from the environment and incorporate it into their own DNA. The DNA
in the environment may come from dead cells. The more the DNA resembles the
cell’s own DNA, the more readily it is incorporated.
Another means of genetic exchange is
through incorporation of the DNA into a virus. When the virus infects a
bacterial cell, it picks up part of the bacterial DNA. If the virus infects
another cell, it carries with it DNA from the first organism. This method of DNA
exchange is called transduction.
Transformation and transduction
generally transfer only small amounts of DNA, although bacterial geneticists
have worked to increase these amounts. Many bacteria are also capable of
transferring large amounts of DNA, even the entire genome (set of genes),
through physical contact. The donor cell generally makes a copy of the DNA
during the transfer process so it is not killed. This method of exchange is
called conjugation. DNA exchange enables bacteria that have developed
antibiotic-resistant genes to rapidly spread their resistance to other
bacteria.
| IV | CLASSIFICATION AND STUDY OF BACTERIA |
Scientists long had difficulty classifying
bacteria in relation to each other and in relation to other living things.
Because bacteria are so small, scientists found it nearly impossible to identify
characteristic structures on or in the organisms that would help in
classification. For many years bacteria were considered to be plants and named
according to the botanical system of classification, by genus and species. For
example, Escherichia coli belongs to the genus Escherichia and to
the species coli within that genus. The genus name starts with a capital
letter; the species name, with a small letter. Both are written in italic
letters. For convenience, people often use only the letter of the genus name, as
in E. coli, for example.
| A | A New Classification System |
The development of the field of molecular
phylogeny in the 1970s changed our view of bacteria. Phylogeny relates organisms
through their evolutionary origins. In molecular phylogeny, scientists look for
similarities in the molecules of organisms to figure out relationships.
Initially, scientists looked at proteins, which are made up of long strings of
amino acids. They figured that if a particular protein in two organisms
contained exactly the same amino acids in the same order, then the two were very
closely related or even identical. If there were only a few differences, the
organisms were closely related. The more differences there were, the more
distant the relationship would be.
Carl Woese, a microbiologist at the
University of Illinois, discovered that it was easier to work with nucleic
acids, such as DNA and RNA. He found that the best molecules were ribonucleic
acid molecules from ribosomes (rRNA). Ribosomes are the biochemical machines
inside cells that coordinate the synthesis of proteins. It was relatively easy
to obtain rRNA, to identify its chemical building blocks known as nucleotides,
and to determine the order of the nucleotides in the molecule. Because rRNA
shows relatively little variation from one generation to the next, it proved to
be an excellent tool for determining evolutionary relationships.
Molecular phylogeny indicated that there
are three major groups, or kingdoms, of organisms. One kingdom, called
Eukaryotae, consists of all organisms with a true nucleus and includes all
plants and animals. The two other kingdoms, called Archaea and Eubacteria,
consist of prokaryotic bacteria without a true nucleus. Archaea, or
archaeabacteria, were once classified with other bacteria and the two kingdoms
share many characteristics. Many of the archaea are extremophiles and can live
in extremely hot, salty, or acid environments, but so can many eubacteria.
The classification of bacteria into two
kingdoms, a system proposed by Woese, is based almost entirely on the structure
of ribosomal RNA. But it appears to agree with other findings regarding the
basic structures of the organisms, their metabolism, and their evolution.
| B | Sequencing Bacterial DNA |
Amazing advances in technology have
enabled scientists to sequence the entire genome of many bacteria—that is,
identify the nucleotides that make up the DNA and the order in which the
nucleotides are arranged. This knowledge, and the sciences that have developed
around it, will enable scientists to harness the useful capabilities of bacteria
in agriculture, industry, and other fields and to develop new drugs. In one
example, scientists have turned bacteria into factories for producing the
hormone insulin by inserting human insulin-producing genes into bacteria. The
insulin produced can be used to treat human diabetes.
Insulin is a protein, and genes govern the
production of proteins by a cell. The study of protein production will help
scientists understand the process of disease at a cellular level and help them
develop new means of combating diseases. As scientists study how bacteria attach
to and enter healthy cells, cause illness, and spread, they are learning useful
details about the molecular structure of cells.
| V | EVOLUTION OF BACTERIA |
The oldest fossils of bacteria-like organisms
date back as many as 3.5 billion years, making them the oldest-known fossils.
These early bacteria could survive in the inhospitable conditions when Earth was
young, extremely hot, and without oxygen. With the help of molecular phylogeny,
scientists have pieced together a view of the evolution of bacteria. They
believe that the kingdoms Archaea and Eubacteria had a common ancestor but
separated very early on, a few billion years ago. Archaea may be the most common
organisms on Earth today. Many of them can live without oxygen and without
sunlight and inhabit such places as deep-sea vents. However, scientists
currently know much more about the kingdom Eubacteria than the kingdom Archaea,
because humans have more contact with disease-causing Eubacteria, such as
streptococci and Escherichia coli, and with Eubacteria such as
lactobacilli used in food processing and other industries.
Over time, bacteria evolved to capture energy
from the Sun’s light and thereby carry out the process of photosynthesis,
converting sunlight into nutrients. Next they developed the sort of
photosynthesis that plants today carry out by splitting water molecules to
produce oxygen. With oxygen available, organisms that require it, such as
animals, could inhabit Earth.
Recent discoveries suggest that Eukaryotae
(plants and animals) probably evolved from Eubacteria. Many of the
organelles (structures within the cytoplasm) of plant and animal cells
are actually bacterial. Among organelles derived from bacteria that invaded
plant or animal cells are mitochondria and chloroplasts.
Mitochondria in plants and animals convert nutrients into energy-storage
molecules. Chloroplasts house the photosynthetic machinery of plant cells. Not
only do bacteria live on us and in us, but we ourselves are in a way partly
bacterial.
| VI | SCIENTIFIC STUDY OF BACTERIA |
Before the development of the microscope,
some people speculated that small, invisible particles caused diseases and
fermentations. But not until the late 1600s did anyone actually see bacteria. In
the 1670s Dutch lens maker Antoni van Leeuwenhoek first saw what he called “wee
animalcules” under his single-lens microscopes. Leeuwenhoek noticed cells of
different shapes within a variety of specimens, including scrapings from his
teeth and rainwater from gutters. His findings laid the foundation for the
growth of microbiology.
The microscope was improved over the
following centuries, but bacteria still appeared as tiny objects, even with
magnifications of 1,000 times. In the 1930s, the first electron microscopes were
developed. Using beams of electrons instead of light, these microscopes could
magnify objects at least 200 times more than light microscopes could. With
magnifications of 200,000 times actual size, it became possible to see
structures within bacterial cells in detail.
Early studies of bacteria were difficult.
In any environment many types of bacteria compete and cooperate, and all this
activity makes it nearly impossible to figure out what each organism is doing.
The first step was to separate different types of bacteria. One way of isolating
bacteria was to grow them on a solid surface. Scientists first used kitchen
foods, such as a potato slice cut with a sterile knife, on which to grow
bacteria that attack plants. This method was not very convenient, however.
The perfect medium (environment) for
growing bacteria also came from the kitchen, although its usefulness was
demonstrated in the laboratory of German scientist Robert Koch. The medium was
agar, a gel-forming substance that comes from seaweed. A coworker of Koch’s
noted that his wife’s puddings remained solid in summer heat, whereas the
gelatin on which he grew bacteria dissolved or got eaten by the bacteria. The
firm puddings contained agar.
Agar dissolves in water only at
temperatures close to boiling. When it cools, it forms a stable gel. Most
bacteria cannot digest it. Bacteriologists could transfer a bacterial specimen
onto a plate of agar using sterile wires or loops, and obtain a colony of
organisms. If more than one type of bacteria formed a colony, the scientists
could repeat the process, growing each type on a separate agar plate to obtain a
pure culture (laboratory-grown specimen) for study. They could also add
nutrients to agar to provide the bacteria with the food they need for growth. In
addition, they could add substances to suppress the growth of unwanted bacteria
but not the growth of those the bacteriologist wished to isolate. Growing
bacteria on agar has become routine in laboratories.
Bacteriologists have become accustomed to
studying individual types of bacteria in pure cultures. In nature, however,
bacteria usually live in diverse communities, often with hundreds of types of
organisms. These communities form sticky masses called biofilms on soil
particles, ocean debris, plants and animals, and just about any solid or liquid
surface. In our bodies, biofilms develop on teeth, on the soft tissues of the
mouth and throat, on the membrane lining the nose and sinuses, in the gut, and
on all other exposed body surfaces. In nature, organisms form microbial mats on
surfaces between water and air. In sewage treatment, bacteria clump together in
masses. All these communities are highly diverse, harboring many kinds of
organisms. They can be compared to cities in which the different members have
different functions, all important to maintaining the community.
Bacteriologists are realizing more and more
the need to move from studying pure cultures containing only a single species to
the study of communities in biofilms and microbial mats. The growth of molecular
biology and the capacity to study bacteria in molecular detail have demonstrated
that the bacterial world is far more diverse than previously thought. It seems
possible that we currently have discovered only a small fraction of existing
types of bacteria in the world. Perhaps as many as 95 percent of total types
remain unknown.
Recent information on the true diversity of
bacteria comes from a study published in 2006 that used a new DNA-identification
technique to study microbes taken from the ocean. Scientists found more than
20,000 types of bacteria in a liter of sea water—over ten times the biodiversity
predicted. Much of the diversity came from rare bacteria that had not been
detected in previous studies of marine microbes. Samples were taken at eight
sites in the Atlantic Ocean and Pacific Ocean from a wide range of depths and
environments, including the North Sea and hydrothermal vents. The work will be
expanded in the future to sample marine microbes from more than 1,000 ocean
sites with even more types of environments. The international research is being
conducted as part of the global Census of Marine Life, a ten-year project that
began in 2000. The newly recognized complexity of ocean bacteria could lead to a
much greater gene pool for a range of scientific work.
Scientists have already sequenced the
entire genome for many bacteria. Researchers can cut pieces from bacterial DNA
and replicate it in many copies. Through DNA transfer, the pieces can be
inserted in bacterial cells. The cells with the new DNA may then start to make
new proteins they were unable to make previously. Thus, bacteria can be
genetically engineered to make a whole range of products and to develop new
functions. Genetic engineering has opened up a new world of biology and a
tremendous opportunity to explore bacteria and other microorganisms and to
benefit humanity from the resulting knowledge.
