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Microorganisms. source: Tecnico Lisboa

Microorganisms: Naming & Classification


The system of nomenclature (naming) for organisms in use today was established in 1735 by Carolus Linnaeus. Scientific names are latinized because Latin was the language traditionally used by scholars. Scientific nomenclature assigns each organism two names—the genus (plural: genera) is the first name and is always capitalized; the specific epithet (species name) follows and is not capitalized. The organism is referred to by both the genus and the specific epithet, and both names are underlined or italicized. By custom, after a scientific name has been mentioned once, it can be abbreviated with the initial of the genus followed by the specific epithet.

Scientific names can, among other things, describe an organism, honor a researcher, or identify the habitat of a species. For example, consider

Source: Microbiology: An Introduction, 13th edition, Tortora et. al.

Staphylococcus aureus, a bacterium commonly found on human skin. Staphylo- describes the clustered arrangement of the cells; -coccus indicates that they are shaped like spheres. The specific epithet, aureus, is Latin for golden, the color of many colonies of this bacterium. The genus of the bacterium Escherichia coli (esh′er-IK-e¯-ah KO¯ -lI¯, or KO¯ -le¯) is named for a physician, Theodor Escherich, whereas its specific epithet, coli, reminds us that E. coli live in the colon, or large intestine.

Types of Microorganisms

In health care, it is very important to know the different types of microorganisms in order to treat infections. For example, antibiotics can be used to treat bacterial infections but have no effect on viruses or other microbes. Here is an overview of the main types of microorganisms.


Bacteria (singular: bacterium) are relatively simple, singlecelled (unicellular) organisms. Because their genetic material is not enclosed in a special nuclear membrane, bacterial cells are called prokaryotes (pro¯-KAR-e-o¯ts), from Greek words meaning prenucleus. Prokaryotes include both bacteria and archaea.

Bacterial cells generally appear in one of several shapes. Bacillus (bah-SIL-lus) (rodlike), illustrated in coccus (KOK-kus) (spherical or ovoid), and spiral (corkscrew or curved) are among the most common shapes, but some bacteria are starshaped or square. Individual bacteria may form pairs, chains, clusters, or other groupings; such formations are usually characteristic of a particular genus or species of bacteria. Bacteria are enclosed in cell walls that are largely composed of a carbohydrate and protein complex called peptidoglycan.

(By contrast, cellulose is the main substance of plant and algal cell walls.) Bacteria generally reproduce by dividing into two 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.


Like bacteria, archaea (ar-KE¯-ah) consist of prokaryotic cells, but if they have cell walls, the walls lack peptidoglycan. Archaea, often found in extreme environments, are divided into three main groups. The methanogens produce methane as a waste product from respiration. The extreme halophiles (halo = salt; philic = loving) live in extremely salty environments such as the Great Salt Lake and the Dead Sea. The extreme thermophiles (therm = heat) live in hot sulfurous water, such as hot springs at Yellowstone National Park. Archaea are not known to cause disease in humans.


Fungi (singular: fungus) are eukaryotes (u¯-KAR-e¯-o¯ts), organisms whose cells have a distinct nucleus containing the cell’s genetic material (DNA), surrounded by a special envelope called the nuclear membrane. Organisms in the Kingdom Fungi may be unicellular or multicellular. Large multicellular fungi, such as mushrooms, may look somewhat like plants, but unlike most plants, fungi cannot carry out photosynthesis. True fungi have cell walls composed primarily of a substance called chitin. The unicellular forms of

Types of microorganisms
Types of microorganisms. Source: Microbiology: An Introduction, 13th edition, Tortora et. al.

fungi, yeasts, are oval microorganisms that are larger than bacteria. The most typical fungi are molds. Molds form visible masses called mycelia, which are composed of long filaments (hyphae) 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 organic material from their environment—whether soil, seawater, freshwater, or an animal or plant host. Organisms called slime molds are actually ameba-like protozoa.


Protozoa (singular: protozoan) are unicellular eukaryotic microbes. Protozoa move by pseudopods, flagella, or cilia. Amebae  move by using extensions of their cytoplasm called pseudopods (false feet). 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 (organisms that derive nutrients from living hosts) that absorb or ingest organic compounds from their environment. Some protozoa, such as Euglena (u¯-GLE¯-nah), are photosynthetic. They use light as a source of energy and carbon dioxide as their chief source of carbon to produce sugars. Protozoa can reproduce sexually or asexually.


Algae (singular: alga) are photosynthetic eukaryotes with a wide variety of shapes and both sexual and asexual reproductive forms. The algae of interest to microbiologists are usually unicellular. The cell walls of many algae are composed of a carbohydrate called cellulose. Algae are abundant in freshwater and saltwater, in soil, and in association with plants. As photosynthesizers, 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 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 (that is, they are not cells). Structurally very simple, a virus particle contains a core made of only one type of nucleic acid, either DNA or RNA. This core is surrounded by a protein coat, which is sometimes encased by a lipid 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 only 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 they are inert outside living hosts.

Multicellular Animal Parasites

Although multicellular animal parasites are not strictly microorganisms, they are of medical importance and therefore will be discussed in this text. Animal parasites are eukaryotes. The two major groups of parasitic worms are the flatworms and the roundworms, collectively called helminths. During some stages of their life cycle, helminths are microscopic in size. Laboratory identification of these organisms includes many of the same techniques used for identifying microbes.

Classification of Microorganisms

Before the existence of microbes was known, all organisms were grouped into either the animal kingdom or the plant kingdom. When microscopic organisms with characteristics of animals and plants were discovered late in the seventeenth century, a new system of classification was needed. Still, biologists couldn’t agree on the criteria for classifying these new organisms until the late 1970s.

In 1978, Carl Woese devised a system of classification based on the cellular organization of organisms. It groups all organisms in three domains as follows:
1. Bacteria (cell walls contain a protein–carbohydrate complex called peptidoglycan)

2. Archaea (cell walls, if present, lack peptidoglycan)
3. Eukarya, which includes the following:
● Protists (slime molds, protozoa, and algae)
● Fungi (unicellular yeasts, multicellular molds, and mushrooms)
● Plants (mosses, ferns, conifers, and flowering plants)
● Animals (sponges, worms, insects, and vertebrates)


About Fahmida Akter Bristi

I am currently doing my Bachelor degree. I love to write by exploring knowledge that is new to me. Hope this effort of mine benefits you all. Right now, I am the head of Project R. Franklin & Project Waksman in Society & Science Foundation. Knock me anytime. Email:

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