Life began on a small scale. The first organisms were single cells. Sometimes
small groups of cells formed. Eventually, these microorganisms evolved into
complex multicellular organisms. The fact that microorganisms still exist today
in many forms is a testament to the quality of this original life form.
Microorganisms have adapted to inhabit almost every corner of the world. They
live in the oceans and lakes, where they provide a valuable food source for
larger organisms. They live on land where they may the decay of dead organic
material, recycling valuable nutrients. Many even live within other, larger
organisms that they may help or hinder.
Humans have several reasons to be interested in the study of microorganisms.
Many microorganisms cause disease in humans.
Bacteria and
fungi can be parasites of humans, causing
anything from food poisoning to athletes foot to malaria. All
viruses are pathogenic, or disease-causing.
Viruses are responsible for deadly diseases such as AIDS and polio, as well as
milder forms like the common cold. Some viruses have even been implicated in the
development of cancer.
We also have several positive relationships with microorganisms. Some soil
bacteria fix atmospheric nitrogen, making it available to the plants we eat.
Certain fungi grow symbiotically with plant roots, increasing their ability to
obtain food and moisture from the soil. Others fungi are themselves quite
tasty. Bacteria and protists that live in
our intestines help us gain nutrition from food.
Microorganisms have also gained importance as tools in the scientific world.
Since most have simple life cycles and can reproduce rapidly, they make ideal
model organisms. Topics like genetics that are difficult to study in larger
organisms because of the time and expense involved in raising them can easily be
studied in microorganisms living in petri dishes by the billions. As gene
therapy gains importance for the study and treatment of disease, characterizing
viruses that may be used to transport genes is becoming a priority for some
scientists.
Overall, microorganisms are some of the most important living creatures. Their
roles as producers and recyclers makes them vital in most ecosystems. A greater
understanding of these tiny creatures is vital for the study and preservation of
our natural environment.
The following topics discuss the various types of microorganisms in depth. Since
many microorganisms are unicellular, it is recommended that you be familiar with
basic cell biology and
reproduction.
A Note on Phylogeny and Taxonomy
Figure %: a comparison of taxonomic (a) and phylogenetic (b) classifications
In the following discussion, microorganisms have been divided into the Kingdoms
Monera, Protista, and Fungi, as well as the classification Viruses. These
designations come from the taxonomic 5 kingdom system commonly used in texts.
However, readers should be aware that such classification is often innaccurate
in terms of relatedness, or phylogeny. Recent research using molecular
biological techniques has given us a wealth of information about the relatedness
of different groups. For example, we now know that in the Kingdom Monera,
eubacteria and archaebacteria are only as closely related to each other as each
is to the other kingdoms. Other differences between the taxonomic and
phylogenetic classifications of kingdoms will be discussed in each section.
