THE MICROBIAL WORD
For many people the words germ and microbe
bring to mind a group of tiny creatures that do not quite fit into any of the
categories in that old question is it animal vegetables or mineral microbes
also called microorganisms are minute living things that individually are
usually too small to be seen with the unaided eye. The group includes bacteria
fungi protozoa and microscopic algae it also includes viruses those noncellular
entities sometimes regarded as being at the border between life and nonlife you
will be introduced to each of these groups of microbes shortly. We tend to
associated these small organism only with major diseases such as AIDS
uncomfortable infections or such common inconvenience as spoiled food. However
the majority of microorganisms make crucial contributions to the welfare of the
world's inhabitants by helping to maintain the balance of living organisms and
chemicals in our environment. Marine and freshwater microorganisms form the
basis of the food chain in oceans lakes and rivers. Soil microbes help break
down wastes and incorporate nitrogen gas from the air into organic compounds
thereby recycling chemical elements in the soil water and air. Certain microbes
play important roles in photosynthesis a food and oxygen generating process that
is critical to life on Earth. Human and many other animals depend on the
microbes in their intestines for digestion and the synthesis of Some vitamins
that their bodies require including some B vitamins for metabolism and vitamins
k for blood clotting. Microorganisms also have many commercial applications.
They are used in the synthesis of such chemical products as acetone organic
acids enzymes alcohols and many drugs. The process by which microbes produce acetone and butanol was discovered in 1914
by Chaim Weizmann a Russian born chemist working in England. The food industry also uses microbes in
producing vinegar sauerkraut pickles alcoholic beverages green olives soy sauce
buttermilk cheese yogurt and bread. In addition enzymes from microbes can
now be manipulated such that the microbes produce substances they normally do
not synthesized these substances include cellulose digestive aids and drain
cleaner plus important therapeutic substances such such as insulin. Though only
a minority of microorganisms are pathogenic practical knowledge of microbes is
necessary for medicine and the related health sciences. For example hospital
workers must be able to protect patients from common microbes that are normally
harmless but pose a threat to the sick and injured. Today we understand that
microorganisms are found almost everywhere. Yet not long ago before the
invention of the microscope microbes were unknown to scientists. Thousands of
people died in devastating epidemics the causes of which were not understood.
Entire families died because vaccination and antibiotics were not available to
fight infections. We can get an idea of how our current concepts of
microbiology developed by looking at a few of the historic milestones in
microbiology that have changed our lives before doing this however we will
first take a look at the major groups of microbes and how they are named and
classified.
TYPES OF MICROORGANISMS
BACTERIA
Bacteria are relatively simple single
celled organisms. Because their genetic material is not enclosed in a special
nuclear membrane bacterial cells are called Prokaryotes from greek words
meaning prenucleus. Prokaryotes include both the bacteria and the archaea.
Bacterial cells generally appears in one of several shapes. Coccus and spiral
are among the most common shapes but some bacteria are star shaped or square.
Individual bacteria may form pairs chains cluster or other groupings such
formations are usually characteristics of the particular genus or species of
bacteria. Bacteria are enclosed in cell walls that are largely composed of a
carbohydrates and protein complex called peptidoglycan. Bacteria generally
reproduce by dividing into 2 equal cells this process is called binary fission.
For nutrition most bacteria use organic chemicals which in nature can be
derived from either dead or living organisms. Some bacteria can manufacture
their own food by photosynthesis and some can derive nutrition from inorganic
substances. Many bacteria can Swim by using moving appendages called flagella
ARCHAEA
Like bacteria archaea consist
of the prokaryotic cells but if they have cell walls the walls lack
peptidoglycan. Archaea often found in extreme environment are divided into
three main group. The mathanogens produces methane as a waste products from
respiration. The extreme halophiles live in extremely salty environment such as
great salt lake and the dead sea. The extreme thermophiles live in hot
sulfurous water such as hot springs at Yellowstone national park.
Archaea are
not known to cause diseases in humans.
FUNGI
Fungi organism whose cells have
a distinct nucleus containing the cells genetic material surrounded by a
special envelope called the nuclear membrane. Organism in the kingdom Fungi may
be unicellular or multicellular. Large multicellular fungi such as mushrooms
may look somewhat like plants but they cannot carry out photosynthesis as most
plants can. True fungi have cell walls composed primarily of a substance called
chitin.The unicellular forms of fungi yeasts are oval microorganisms that are
largest than bacteria. The most typical fungi are molds. Molds form visible
masses called mycelia which are composed of long filaments that branch and
intertwine. The cottony growths sometimes found on bread and fruit are mold
mycelia. Fungi can reproduce sexually or asexually. They obtain nourishment by
absorbing solutions of organic material from their environment whether soil sea
water fresh water or an animal or plant host. Organism called slime molds have
characteristics of both fungi and amoebas.
PROTOZOA
Protozoa are unicellular
eukaryotic microbes. Protozoa move by pseudopods flagella or cilia. Amoebas
move by using extensions of their cytoplasm called pseudopods. Other protozoa
have long flagella or numerous shorter appendages for locomotion called cilia.
Protozoa have a variety of shapes and live either as free entities or as
parasites that absorb or ingest organic compounds from their environment.
Protozoa can reproduce sexually or asexually.
ALGAE
Algae are photosynthetic
eukaryotes with a wide variety of shapes and both sexually and asexual
reproductive forms. The algae of interest to microbiologist are usually
unicellular. The cell walls of many algae like those of plants are composed of
a carbohydrates called cellulose. Algae are abundant in fresh and salt water in
soil and in association with plants. As photosynthesis algae need light water
and carbon dioxide for food production and growth but they do not generally
require organic compounds from the environment. As a result of photosynthesis
algae produce oxygen and carbohydrates that are then utilized by other
organisms including animals. Thus they play an important role in the balance of
nature.
VIRUSES
Viruses are very different from
the other microbial groups mentioned here. They are so small that most can be
seen only with an electron microscope and they are acellular. Structurally very
simple a virus particle contains a core made of only 1 type of nucleic acid
either DNA or RNA. This core is surrounded by a protein coat. Sometimes the
coat is enclosed by an additional layer a lipids membrane called an envelope.
All living cells have RNA and DNA can carry out chemical reactions and can
reproduce as self sufficient units. Viruses can reproduce only by using the
cellular machinery of other organisms. Thus on the one hand viruses are
considered to be living when they multiply within host cells they infect. In
this sense viruses are parasites of other forms of life. On the other hand
viruses are not considered to be living because outside of living hosts they
are inert.
(a).
 |
| ROD SHAPED BACTERIUM HAEMOPHILUS INFLUENZAE |
The road shaped bacterium
Haemophilus influenzae one of the bacterial causes of pneumonia.
(b).
Mucor a common bread mold is a
type of fungus when released from sporangia spores that land on a favorable
surface germinate into a network of hyphae that absorb nutrients.
(c).
 |
| AMOEBA |
An amoeba a protozoan
approaching a food particle.
 |
| ALGA VOLVOX |
The pond alga volvox
 |
| VIRUSES |
(e).
Several human immunodeficiency
viruses the causative agent of AIDS budding from a CD4 lymphocyte.
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