Bacteria.
Dr. Sayeed Ahmad D.
I. Hom. (London)Bacteria are simple organisms that consist of one
cell. They are among the smallest living things. Most bacteria measure
from 0.3 to 2.0 microns in diameter and can be seen only through a
microscope. (One micron equals 0.001 millimeter or 1/25,400 inch.)
Scientists classify bacteria as prokaryotes.Bacteria exist almost everywhere. There are thousands
of kinds of bacteria, most of which are harmless to human beings. Large
numbers of bacteria live in the human body but cause no harm. Some
species cause diseases, but many others are helpful.
The importance of bacteria
Helpful bacteria. Certain kinds of bacteria live in
the intestines of human beings and other animals. These bacteria help in
digestion and in destroying harmful organisms. Intestinal bacteria also
produce some vitamins needed by the body.Bacteria in soil and water play a vital role in
recycling carbon, nitrogen, sulfur, and other chemical elements used by
living things. Many bacteria help decompose (break down) dead organisms
and animal wastes into chemical elements. Other bacteria help change
chemical elements into forms that can be used by plants and animals. For
example, certain kinds of bacteria convert nitrogen in the air and soil
into nitrogen compounds that can be used by plants.A chemical process called fermentation, used in
making alcoholic beverages and cheese and many other foods, is caused by
various bacteria. Sewage treatment plants use bacteria to purify water.
Bacteria are also used in making some drugs.Bacterial cells resemble the cells of other living
things in many ways, and so scientists study bacteria to learn about
more complex organisms. For example, the study of bacteria has helped
researchers understand how certain characteristics are inherited. Most
types of bacteria reproduce quickly. This rapid reproduction enables
scientists to grow large quantities for research.
Harmful bacteria
.
Some species of bacteria cause diseases in human
beings. These diseases include cholera, gonorrhea, leprosy (Hansen’s
disease), pneumonia, syphilis, tuberculosis, typhoid fever, and whooping
cough. The bacteria enter a human being’s body through its natural
openings, such as the nose or mouth, or through breaks in the skin. In
addition, air, food, and water carry bacteria from one person to
another. Harmful bacteria prevent the body from functioning properly by
destroying healthy cells.Certain bacteria produce toxins (poisons), which
cause such diseases as diphtheria, scarlet fever, and tetanus. Some
toxins are produced by living bacteria, but others are released only
after a bacterium dies. A form of food poisoning called botulism is
caused by toxins from bacteria in improperly canned foods.Bacteria that usually live harmlessly in the body may
cause infections when a person’s resistance to disease is low. For
example, if bacteria in the throat reproduce faster than the body can
dispose of them, a person may get a sore throat.Bacteria also cause diseases in other animals and in
plants. Anthrax is a bacterial disease that infects many animals,
especially cattle and sheep. Plant diseases caused by bacteria include
fire blight, which occurs in apple and pear trees, and soft rot, which
decays some fruits and vegetables. Bacteria also cause growths called
crown galls, which attack various plants.
Protection against harmful bacteria
.
Many bacteria live on the skin and in the mouth,
intestines, and breathing passages. But the rest of the body tissues are
normally free of bacteria. The skin, and the membranes that line the
digestive and respiratory systems, prevent most harmful bacteria from
entering the rest of the body. When harmful bacteria do enter the body,
white blood cells surround and attack them. Also, the blood produces
antibodies, substances that kill or weaken the invaders. Toxins are
neutralized by certain antibodies called antitoxins. Sometimes the body
cannot make its own antitoxins fast enough. In such cases, a physician
may inject an antitoxin from an animal, such as a horse or rabbit, or
from another person.Dead or weakened bacteria are used in making drugs
called vaccines, which can prevent the diseases caused by those species
of bacteria. Vaccines are injected into the body, causing the blood to
produce antibodies that attack the bacteria. Some vaccines protect the
body from infection for several years or longer. Drugs called
antibiotics are made from microorganisms that inhabit the air, soil, and
water. Antibiotics can kill or weaken disease-causing bacteria. However,
extensive use of antibiotics may encourage the spread of bacteria
resistant to the drugs. The drugs then become ineffective.People use chemicals called antiseptics to prevent
bacteria from growing on living tissues. Other chemicals, known as
disinfectants, are used to destroy bacteria in water and on such items
as clothing and utensils. Bacteria can also be killed by heat, and so
heat is often used to sterilize food and utensils.
The structure of bacteria
Nearly all kinds of bacteria are enclosed by a tough
protective layer called a cell wall. The cell wall gives the bacterium
its shape and enables it to live in a wide range of environments. Some
species are further enclosed by a capsule, a slimy layer outside the
cell wall. The capsule makes the cell resistant to destructive
chemicals. All bacteria have a cell membrane, an elastic, baglike
structure just inside the cell wall. Small molecules of food enter the
cell through pores in this membrane, but large molecules cannot pass
through. Inside the membrane is the cytoplasm, a soft, jellylike
substance. The cytoplasm contains chemicals called enzymes, which help
break down food and build cell parts.Like the cells of all living things, bacterial cells
contain DNA (deoxyribonucleic acid). DNA controls a cell’s growth,
reproduction, and all other activities. The DNA of a bacterial cell
forms an area of the cytoplasm called the nucleoid. In all other
organisms except cyanobacteria (blue-green algæ), the DNA is in the
nucleus, a part of the cell separated from the cytoplasm by a membrane.Scientists generally divide bacteria into groups
according to shape. Round bacteria are called cocci, and rod-shaped ones
are bacilli. Bacteria that look like bent rods are vibrios. There are
two types of spiral-shaped bacteria, spirilla and spirochetes. Two or
more bacteria linked together may be described by the prefixes diplo-
(pair), staphylo- (cluster), or strepto- (chain). For example,
streptococci are a type of round bacteria linked together in chains.
The life of bacteria
Where bacteria live. Bacteria live almost everywhere,
even in places where other forms of life cannot survive. The air, water,
and upper layers of soil contain many bacteria. Bacteria are always
present in the digestive and respiratory systems and on the skin of
human beings and other animals.Certain bacteria, called aerobes, require oxygen to
live, but others, known as anaerobes, can survive without it. Some
anaerobes can exist either with or without oxygen. Other anaerobes
cannot live with even a trace of oxygen in their environment.Some bacteria protect themselves against a lack of
food, oxygen, or water by forming a new, thicker cell membrane inside
the old one. The cell material surrounding the new membrane dies. The
remaining organism becomes inactive and is called a bacterial spore.
Bacterial spores may live for decades or even longer because they can
resist extremely high or low temperatures and other harsh conditions. If
food, oxygen, and water again become available, the spores change back
into active bacteria.
How bacteria move
.
Bacteria are carried long distances by air and water
currents. Clothing, utensils, and other objects also carry bacteria.
Various kinds of bacteria have flagella (thin hairs) that enable them to
swim. Some species that lack flagella move by wriggling.
How bacteria obtain food
.
Most kinds of bacteria, called heterotrophic
bacteria, feed on other organisms. Some species, known as autotrophic
bacteria, manufacture their own food. For example, photosynthetic
bacteria make food from carbon dioxide, sunlight, and water. Certain
bacteria may be autotrophic or heterotrophic, depending on the food
available. The majority of heterotrophic bacteria feed on dead
organisms. Others are parasites. Some parasitic bacteria cause little or
no harm to the host organism, but others cause diseases.
How bacteria reproduce
.
Most bacteria reproduce asexually–that is, each cell
simply divides into two identical cells by a process called binary
fission. Most bacteria also reproduce quickly, and some species double
their number every 20 minutes. If one of these cells were given enough
food, over a billion bacteria would be produced in 10 hours. Industrial
and laboratory processes often produce such enormous numbers of
bacteria. But in nature, bacteria lack an adequate food supply to
maintain such a high rate of reproduction.When bacteria reproduce by binary fission, the DNA in
each of the two resulting cells is identical to the DNA in the original
bacterium. Some bacteria can exchange DNA by a kind of simple sexual
process called conjugation. Conjugation involves the direct transfer of
DNA from one type of bacterial cell, called a male, to another type,
called a female. DNA also may be transferred by viruses. Bacteria also
may pick up fragments of DNA from dead bacterial cells. By transferring
DNA, bacterial cells transfer individual traits. For example, bacterial
cells that are resistant to certain antibiotics may transfer this
characteristic to nonresistant bacterial cells.Scientists have developed techniques that allow them
to isolate fragments of DNA responsible for particular traits. Inserting
these fragments into different bacteria, called recombinant DNA
technology, produces useful new kinds of bacteria. For example, some of
these bacteria chemically break down oil and also help clean up oil
spills. Others are used to make substances with medical applications,
such as insulin.
History
The first living things on the earth probably
included simple forms of bacteria. The oldest known fossils are those of
bacteria that lived about 31/2 billion years ago. Some scientists
believe certain bacteria gradually developed into multicelled organisms
that were the ancestors of the more complex plants and animals of today.Bacteria were first described in the mid-1670’s by
Anton van Leeuwenhoek, a Dutch amateur scientist. For many years,
scientists believed that bacteria came from nonliving matter. But in the
late 1800’s, the French chemist Louis Pasteur showed that only living
things can produce living things. Pasteur and Robert Koch, a German
physician, helped develop the science of bacteriology (the study of
bacteria).
Types of Bacteria
Microbiologists broadly classify bacteria according
to their shape: spherical, rod-shaped, and spiral-shaped. Pleomorphic
bacteria can assume a variety of shapes. Bacteria may be further
classified according to whether they require oxygen (aerobic or
anaerobic) and how they react to a test with Gram’s stain. Bacteria in
which alcohol washes away Gram’s stain are called gram-negative, while
bacteria in which alcohol causes the bacteria’s walls to absorb the
stain are called gram-positive.
TYPE —– CHARACTERISTICS
Acetic acid
Rod-shaped, gram-negative, aerobic; highly
tolerant of acidic conditions; generate organic acids
Actinomycete
Rod-shaped or filamentous, gram-positive,
aerobic; common in soils; essential to growth of many plants;
source of much of original antibiotic production in pharmaceutical
industry
Coccoid
Spherical, sometimes in clusters or strings,
gram-positive, aerobic and anaerobic; resistant to drying and
high-salt conditions; Staphylococcus species common on
human skin, certain strains associated with toxic shock syndrome
Coryneform
Rod-shaped, form club or V shapes,
gram-positive, aerobic; found in wide variety of habitats,
particularly soils; highly resistant to drying; include Arthrobacter,
among most common forms of life on earth
Endospore-
forming
Usually rod-shaped, can be gram-positive or
gram-negative; have highly adaptable, heat-resistant spores that
can go dormant for long periods, possibly thousands of years;
include Clostridium (anaerobic) and Bacillus
(aerobic)
Enteric
Rod-shaped, gram-negative, aerobic but can live
in certain anaerobic conditions; produce nitrite from nitrate,
acids from glucose; include Escherichia coli, Salmonella
(over 1000 types), and Shigella
Gliding
Rod-shaped, gram-negative, mostly aerobic;
glide on secreted slimy substances; form colonies, frequently with
complex fruiting structures
Lactic acid
Gram-positive, anaerobic; produce lactic acid
through fermentation; include Lactobacillus, essential in
dairy product formation, and Streptococcus, common in
humans
Mycobacterium
Pleomorphic, spherical or rod-shaped,
frequently branching, no gram stain, aerobic; commonly form yellow
pigments; include Mycobacterium tuberculosis, cause of
tuberculosis
Mycoplasma
Spherical, commonly forming branching chains,
no gram stain, aerobic but can live in certain anaerobic
conditions; without cell walls yet structurally resistant to
lysis; among smallest of bacteria; named for superficial
resemblance to fungal hyphae (myco- means
“fungus”)
Nitrogen-fixing
Rod-shaped, gram-negative, aerobic; convert
atmospheric nitrogen gas to ammonium in soil; include Azotobacter,
a common genus
Propionic acid
Rod-shaped, pleomorphic, gram-positive,
anaerobic; ferment lactic acid; fermentation produces holes in
Swiss cheese from the production of carbon dioxide
Pseudomonad
Rod-shaped (straight or curved) with polar
flagella, gram-negative, aerobic; can use up to 100 different
compounds for carbon and energy
Rickettsia
Spherical or rod-shaped, gram-negative,
aerobic; cause Rocky Mountain spotted fever and typhus; closely
related to Agrobacterium, a common gall-causing plant
bacterium
Sheathed
Filamentous, gram-negative, aerobic;
“swarmer” (colonizing) cells form and break out of a
sheath; sometimes coated with metals from environment
Spirillum
Spiral-shaped, gram-negative, aerobic; include Bdellovibrio,
predatory on other bacteria
Spirochete
Spiral-shaped, gram-negative, mostly anaerobic;
common in moist environments, from mammalian gums to coastal
mudflats; complex internal structures convey rapid movement;
include Treponemapallidum, cause of syphilis
Sulfate- and
Sulfur-reducing
Commonly rod-shaped, mostly gram-negative,
anaerobic; include Desulfovibrio, ecologically important in
marshes
Sulfur- and
iron-oxidizing
Commonly rod-shaped, frequently with polar
flagella, gram-negative, mostly anaerobic; most live in neutral
(nonacidic) environment
Vibrio
Rod- or comma-shaped, gram-negative, aerobic;
commonly with a single flagellum; include Vibrio choleræ,
cause of cholera, and luminescent forms symbiotic with deep-water
fishes and squids
References:
MS Encarta Encyclopædia.
World Book 2003
Copyright © Dr. Sayeed Ahmad
2004