CLASSIFICATION OF MICROORGANISMS
Bacteria are very simple, single-celled organisms that are found virtually everywhere. The natural habitats of bacteria include freshwater, saltwater, soil, and other living organisms. Most bacteria are not harmful to us, and within their normal environments, they have the vital role of decomposing dead organic mate-rial and recycling their nutrients. However, a number of bacteria cause human diseases, including strep throat, pneumonia, and meningitis.
Viruses are not cells; they are even smaller and simpler in structure than the bacteria. All viruses are parasites because they can reproduce only within the living cells of a host. Therefore, all viruses cause dis-ease. Common human viral diseases are influenza, the common cold, and chickenpox.
Protozoa are single-celled animals such as amoe-bas. Most protozoa are free living in freshwater or saltwater, where they consume bacteria, fungi, and one another. Human protozoan parasites include those that cause malaria, amebic dysentery, and giardiasis, another intestinal infection.
Fungi may be unicellular or multicellular. Molds and mushrooms are familiar fungi. They decompose organic matter in the soil and freshwater and help recycle nutrients. Fungal diseases of people include yeast infections, ringworm, and more serious diseases such as a type of meningitis.
Worms are multicellular animals. Most are free liv-ing and non-pathogenic; within the soil they consume dead organic matter or smaller living things. Worm infestations of people include trichinosis, hookworm disease, and tapeworms.
Arthropods (the name means “jointed legs”) are multicellular animals such as lobsters, shrimp, the insects, ticks, and mites. Some insects (such as mos-quitoes and fleas) are vectors of disease; that is, they spread pathogens from host to host when they bite toobtain blood. Ticks are also vectors of certain diseases, and some mites may cause infestations of the skin.
We refer to bacteria and all other living things using two names (binomial nomenclature), the genus and the species. The genus name is placed first, is always capitalized, and is the larger category. The species name is second, is not capitalized, and is the smaller category. Let us use as examples Staphylococcus aureus and Staphylococcus epidermidis. These two bacteria are in the same genus,Staphylococcus, which tells us that they are related or similar to one another. Yet they are different enough to be given their own species names: aureus or epidermidis. It may be helpful here to think of our own names. Each of us has a family name, which indicates that we are related to other members of our families, and each of us has a first name indi-cating that we are individuals in this related group. If we wrote our own names using the method of bino-mial nomenclature, we would write Smith Mary and Smith John.
In scientific articles and books, for the sake of con-venience, the genus name is often abbreviated with its first letter. We might read ofS. aureus as a cause of a food poisoning outbreak or see E. coli (E. For Escherichia) on a lab report as the cause of a patient’s urinary tract infection. Therefore, it is important to learn both genus and species names of important pathogens.