Microorganisms differ from each other not only in size, but also in structure, habitat, metabolism, and many other characteristics. While we typically think of microorganisms as being unicellular, there are also many multicellular organisms that are too small to be seen without a microscope. Some microbes, such as viruses, are even acellular (not composed of cells).
Microorganisms are found in each of the three domains of life: Archaea, Bacteria, and Eukarya. Microbes within the domains Bacteria and Archaea are all prokaryotes (their cells lack a nucleus), whereas microbes in the domain Eukarya are eukaryotes (their cells have a nucleus). Some microorganisms, such as viruses, do not fall within any of the three domains of life. In this section, we will briefly introduce each of the broad groups of microbes.
Bacteria (singular: bacterium) are relatively simple, single-celled (unicellular) organisms. Because their genetic material is not enclosed in a special nuclear membrane, bacterial cells are called prokaryotes (pro-kar’e-ots), from Greek words meaning prenucleus. Prokaryotes include both bacteria and archaea.
Bacterial cells generally appear in one of several shapes. Bacillus (ba-sil’lus) (rodli ke), 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-a) 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 (eu-kar’e-ots), 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 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 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 solutions of organic material from their environment- whether soil, seawater, fresh water, or an animal or plant host. Organisms called slime molds have characteristics of both fungi and amoebas.
Protozoa (singular: protozoan) are unicellular eukaryotic microbes. Protozoa move by pseudopods, flagella, or cilia. Amoebas 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. Protozoa can reproduce by means of 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 fresh and salt water, 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 (not cellular). 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. Sometimes the coat is encased by an additional layer, 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, 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 living hosts, they are inert.
Multicellular Animal Parasites
Although multicellular animal parasites are not strictly microorganisms, they are of medical importance and therefore will be discussed in this text. Animals are eukaryotes. The two major groups of parasitic worms are the flatworms and the roundworms, collectively called hehninths. 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.