d. Others. There have been many important contributions to microbiology and
many esteemed workers in this field; however, Leeuwenhoek, Koch, and Pasteur are
the individuals who were most important in establishing microbiology as a science.
CLASSIFICATION OF ORGANISMS
a. Animal. Most animal cells have flexible outer walls, and therefore, they are
usually able to move independently.
b. Plant. Plant cells usually have rigid outer walls of cellulose, which surround
the living protoplasm within the cell and maintain the cell shape. Unlike animals, most
plants are either attached to one spot by their roots or are transported by their
environment, as, for example, the liquids in which they are suspended. Plants get their
energy from the process known as photosynthesis, with chlorophyll acting as a catalyst.
c. Protista. The only basis for an organism being classified as a member of the
kingdom Protista is its simple biological organization, characterized by a lack of
extensive tissue formation. However, some organisms have a tendency to form tissues,
and it is difficult to classify these organisms with great certainty. The kingdom includes
both microscopic unicellular organisms and very large multicellular forms. Included in
the kingdom Protista are all bacteria, fungi, rickettsiae, viruses, and protozoa.
(1) Fungi. Fungi are organisms that are not photosynthetic. They are
multicellular, consisting of long filaments.
(2) Bacteria. Bacteria are one-celled microorganisms. They differ in shape
from ball-or round-shaped, to rod-shaped, to curved or coil-shaped. Shape is an
important characteristic of bacteria because it plays a key role in laboratory
identification. The ball- or round-shaped bacteria are called cocci; the rod-shaped are
known as bacilli; and we call the curved or coil-shaped ones spirochetes.
(3) Rickettsiae. These are bacteria that can survive only in living cells.
Unlike most infectious agents, rickettsiae require an intermediate host or vector in order
to be transmitted from one host to another. Some of the common vectors include fleas,
lice, ticks, and mites. Rickettsiae are smaller than most other bacteria. They may
appear as cocci, diplococci (cocci in pairs), or short bacilli.
(4) Viruses. A virus is a noncellular, submicroscopic particle that lives and
reproduces in living cells. Like bacteria, viruses have many sizes and shapes that are
significant factors in their laboratory identification. In size, the virus is the smallest of all
infectious agents and must be viewed with an electronic microscope. Filtration is the
method usually employed to separate bacteria from viruses, which may be present in
animal or plant fluid. Filters catch the bacteria but let the viruses pass through.
(5) Protozoa. A protozoan is a microscopic, unicellular organism without
chlorophyll. They (protozoa) get their energy from organic matter.