Types of Microorganisms
An infection is caused by the infiltration of a disease-causing microorganism known as a pathogen. Some pathogenic microorganisms infect humans but not other animals and plants. Some pathogenic microorganisms that infect animals or plants also infect humans.
Pathogenic microorganisms make headlines and play an important role in history. Legendary gunfighter John “Doc” Holliday is famous for his escapades in the Wild West. He dodged countless bullets, showing that he was the best of the best when it came to gun fighting, yet Mycobacterium tuberculosis took down Doc Holliday quietly, without firing a shot. M. tuberculosis is the bacterium that causes tuberculosis. This bacterium affects the lung tissue when droplets of respiratory secretions or particles of dry sputum from a person who is infected with the disease are inhaled by an uninfected person.
Yersinia pestis nearly conquered Europe in the fourteenth century with the help of the flea. Y. pestis is the microorganism that caused the black plague and killed more than 25 million Europeans. You might say that Y. pestis launched a sneak attack. First, it infected fleas that were carried into populated areas on the backs of rats. Rodents traveled on ships and then over land in search of food. Fleas jumped from rodents and bit people, transmitting the Y. pestis microorganism into the person’s bloodstream.
In an effort to prevent the spread of Y. pestis, sailors entering Sicily’s seaports had to wait 40 days before leaving the ship. This gave time for sailors to exhibit the symptoms of the black plague if the Y. pestis microorganism had infected them. Sicilians called this quarantenaria. Today we know it as quarantine. Sailors who did not exhibit symptoms were not infected and were free to disembark.
Campers and travelers sometimes become acquainted with Giardia lamblia, Escherichia coli, or Entameba histolytica. Travelers who become infected typically do not die but come down with a bad case of diarrhea.
Not all microorganisms are pathogens. In fact, many microorganisms help to maintain homeostasis in our bodies and are used in the production of food and other commercial products. For example, flora are microorganisms found in our intestines that assist in the digestion of food and play a critical role in the formation of vitamins such as vitamins B and K. They help by breaking down large molecules into smaller ones.
What Is a Microorganism?
Microorganisms are the subject of microbiology, which is the branch of science that studies microorganisms. A microorganism can be one cell or a cluster of cells that can be seen only by using a microscope.
Microorganisms are organized into six fields of study: bacteriology, virology, mycology, phycology, protozoology, and parasitology.
Bacteriology is the study of bacteria. Bacteria are prokaryotic organisms. A prokaryote is a onecelled organism that does not have a true nucleus. Many bacteria absorb nutrients from their environment, and some make their own nutrients by photosynthesis or other synthetic processes. Some bacteria can move freely in their environment with the help of flagella whereas others are stationary. Bacteria occupy space on land and can live in aquatic environments and in decaying matter. They can even cause disease. Bacillus anthracis is a good example. It is the bacterium that causes anthrax.
Virology is the study of viruses. A virus is an infectious agent composed of a nucleic acid core surrounded by a protein coat. Viruses are not considered living organisms. They lack independent metabolism and can only reproduce within the cells of a living host. An example of a virus is the varicella-zoster virus, which is the virus that causes chickenpox in humans.
Mycology is the study of fungi. A fungus is a eukaryotic organism, often microscopic, that absorbs nutrients from its external environment. Fungi are not photosynthetic but they do possess a cell wall for structure and support. A eukaryote is a microorganism whose cells have a nucleus, cytoplasm, and organelles. These include yeasts and some molds. Tinea pedis, better known as athlete’s foot, is caused by the Trichophyton rubrum fungus.
Phycology is the study of algae. Algae are eukaryotic photosynthetic organisms that transform sunlight into nutrients using photosynthesis. A photosynthetic eukaryote is a microorganism whose cells have a nucleus, nuclear envelope, cytoplasm, and organelles, called chloroplast, that are able to carry out photosynthesis.
Protozoology is the study of protozoa, animal-like single-cell microorganisms. Many protozoa obtain their food by engulfing or ingesting smaller organisms. An example is Amoeba proteus.
Parasitology is the study of parasites. A parasite is an organism that lives at the expense of another organism or host. Parasites that cause disease are called pathogens. Examples of parasites are bacteria, viruses, protozoa, and many animals such as round worms, flatworms, and arthropods (insects).
Keeping track of the study of organisms can be difficult. Remember: Bacteriology is the study of bacteria, and bacteria are prokaryotic organisms. Virology is the study of viruses, and viruses are infectious entities composed of a nucleic acid surrounded by a protein coat. Mycology is the study of fungi, and fungi are eukaryotic organisms that absorb nutrients from their external environment. Phycology is the study of algae, and algae are eukaryotic photosynthetic organisms. Protozoology is the study of protozoa, and protozoa are motile (usually) single-celled eukaryotic organisms. Parasitology is the study of parasites, and a parasite is an organism that lives and benefits at the expense of another host organism.
What’s in a Name: Naming and Classifying
Carl Linnaeus developed the system for naming organisms in 1735. This system is referred to as binominal nomenclature. Each organism is assigned two latinized names because Latin or Greek was the traditional language used by scholars. The first name is called the genus. The second name is called the specific epithet, which is the name of the species and is not capitalized. The genus and the epithet appear underlined or italicized. The name itself describes the organism. For example, Staphylococcus aureus is a very common bacterium. Staphylococcus is the genus, and aureus is the epithet (or species). In this case, the genus describes the appearance of the cells. Staphylo means a clustered arrangement of the cells, and coccus signifies that the cells are spheres. In other words, this means a cluster of sphere-like cells. Aureus is the Latin word for golden, which means that the cluster of sphere-like cells has a golden hue. Sometimes an organism is named for a researcher, as is the case with Escherichia coli, better known as E. coli. The genus is Escherichia, which is named for Theodor Escherich, a leading microbiologist. The epithet is coli, which implies that the bacterium lives in the colon (large intestine).
Organisms were classified into either the animal kingdom or the plant kingdom before the scientific community discovered microorganisms in the seventeenth century. It was at that time scientists realized that this classification system was no longer valid. Carl Woese developed a new classification system that arranged organisms according to genetic relationships. However, it wasn’t until 1978 that scientists agreed on the new system for classifying organisms, and it took 12 more years before the new system was published. The new system utilizes domains which are longer than kingdoms.
Woese divided three classification groups into three domains:
Eubacteria- Bacteria that have peptidoglycan in their cell walls. (Peptidoglycan is a large meshlike molecule composed of repeating subunits of the sugar derivatives of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), as well as several different amino acids.
Archaea-Prokaryotes that have cell walls are not composed of peptidoglycan.
Eukarya-Organisms from the following kingdoms:
Protista-Algae, protozoa, slime molds (Note: This is in the process of changing.)
Fungi-One-celled yeasts, multicellular molds, and mushrooms
Plantae-Moss, conifers, ferns, flowering plants
Animalia-Insects, worms, sponges, and vertebrates