Biologic Classification Of Bacteria
All living things are classified in a scheme wherein categories represent successively dependent and related groups. The highest possible levels are designated kingdoms. For years, the traditional two were the plant and animal kingdoms. Today there is a change in this approach.
The lower forms of life variably incorporate features of both plants and animals and do not show the dramatic differences of the higher forms. The lower forms of plants and animals are best studied with the aid of the microscope. It is difficult to define many microbes as either plant or animal, and since bacteria and other microbes long classified as plants have been more closely studied under the microscope, the inconsistencies appear even greater. Because of this fact, a third biologic compartment with equivalent rank to the plant and animal kingdoms has been advocated to sift out the simpler units, designating them as protists. These organisms can be examined more carefully under the microscope. Basically most protists are one-cell units and remain so throughout their life history. Even if they pile cells up in large plantlike masses, their component cells remain the same and do not differentiate.
There is an alternate scheme of classification of living things, again based on complexity of structure but this time focused directly on the nucleus. It is apparent that “higher” organisms possess a true nucleus while “lower” ones do not. To differentiate between the higher and the lower forms of organisms, it is best to examine them under the microscope. Thus two distinct taxonomic categories emerge. The obvious nucleus in the higher forms is complete, with the expected number of chromosomes and mitotic apparatus. These organisms are termed “eucaryotic” (using the Greek word which means true nucleus). Eucaryotae, as a category (or kingdom), would be one to contain some protists and the plants and animals.
As observed closely under the microscope, in the lower forms of life, nuclear function is carried out by only a single chromosome, devoid of any membrane. Lower forms are small and less complex in other ways. For instance they do not contain such membrane-bounded organelles as mitochondria. They are designated “procaryotic.” The second category or kingdom, Procaryotae, encompasses all bacteria and a small group of blue-green algae (algae demonstrate plantlike photosynthesis). These organisms can be viewed well under the microscope.
Procaryotes are distinctive. They possess certain unique components in their cell walls, and they display remarkable capabilities with regard to carbon storage, nitrogen fixation, obligate anaerobiosis, and derivation of energy from oxidation of inorganic compounds. .The distinct appearance of these organisms can be observed closely with the aid of the microscope.
Microorganisms usually surveyed in a treatise of microbiology include not only unicellular procaryotes, but also fungi (best known as plants), and lower forms of animal life, such as the unicellular protozoa and a restricted number of the more complex multicellular or metazoan animals. These organisms can be carefully scrutinized under the microscope.
Fungi are classified into Thallophyta, one of the four divisions of the plant kingdom. Thallophytes, or thallus plants (Greek thallos, young shoot or branch), are defined as simple forms of plant life that do not differentiate into true roots, stems, or leaves. Thallus plants include the algae, which contain chlorophyll, and the fungi, which do not. Algae, many of which are microscopic, are shaped like bacteria and form pond scums and seaweeds. The term fungi as ordinarily used refers to molds, yeasts, and certain related microorganisms. These organisms are best studied with the use of the microscope.
CLASSIFICATION OF BACTERIA
The classification of bacteria is difficult. This applies both to the separation of bacteria into groups and to the placing of certain organisms into the proper group. Biologic classification is based largely on morphology, but the morphology of bacteria as a whole is so uniform that it is useful only in dividing bacteria into comparatively large groups. Shape has been an important factor in general classification, but for more exact identification such criteria as staining reactions, cultural characteristics, biochemical and physiologic behavior, genetic analyses, animal inoculations, and immunologic differences must be used. The shapes and forms of the different kinds of bacteria can be viewed well using the microscope.
We adhere to the scientific classification embodied in Bergey’s Manual of Determinative Bacteriology with certain important exceptions. This is the one most generally accepted in the United States.
