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 material and recycling their nutrients. However, a number of bacteria cause human diseases, including strep throat, pneumonia, and meningitis.
Each of us has a natural population of microorganisms living on and within us. This is our normal flora. These microbes may be further categorized as residents or transients. Resident flora are those species that live on or in nearly everyone almost all the time. These residents live in specific sites, and we provide a very favorable environment for them. Some, such as Staphylococcus epidermises, live on the skin. Others, such as E. coli, live in the colon and small intestine. When in their natural sites, resident flora do not cause harm to healthy tissue, and some are even beneficial to us. However, residents may become pathogenic if they are introduced into abnormal sites. If E. coli, for example, gains access to the urinary bladder, it causes an infection called cystitis. In this situation, E. coli is considered an opportunist, which is a normally harmless species that has become a pathogen in special circumstances.
An infectious disease is one that is caused by microorganisms or by the products (toxins) of microorganisms. To cause an infection, a microorganism must enter and establish itself in a host and begin reproducing. Several factors determine whether a person will develop an infection when exposed to a pathogen. These include the virulence of the pathogen and the resistance of the host. Virulence is the ability of the pathogen to cause disease. Host resistance is the total of the body’s defenses against pathogens. Our defenses include aspects of innate immunity such as intact skin and mucous membranes, and the sweeping of cilia to clear the respiratory tract, as well as adequate nutrition, and the adaptive immune responses of our lymphocytes and macrophages.
COURSE OF AN INFECTIOUS DISEASE
When a pathogen establishes itself in a host, there is a period of time before symptoms appear. This is called the incubation period. Most infectious diseases have rather specific incubation periods. Some diseases, however, have more variable incubation periods, which may make it difficult to predict the onset of illness after exposure or to trace outbreaks of a disease.
One of the most effective drugs used to treat serious, systemic mycoses, amphotericin B, has great potential to cause serious side effects. Patients receiving this medication should have periodic tests of liver and kidney function. Newer medications include ketoconazole, fluconazole, and caspofungin, which are less toxic to the recipient and may prove to be as effective as amphotericin B. Superficial mycoses such as ringworm may be treated with certain oral medications. Taken orally, the drug is incorporated into living epidermal cells.
Protozoa are unicellular animals, single cells that are adapted to life in freshwater (including soil) and saltwater. Some are human pathogens and are able to form cysts which are resistant, dormant cells that are able to survive passage from host to host. Intestinal protozoan parasites of people include Entamoeba histolytica, which causes amebic dysentery, and Giardia lamblia, which causes diarrhea called giardiasis.
Most worms are simple multicellular animals. The parasitic worms are even simpler than the familiar earthworm, because they live within hosts and use the host’s blood or nutrients as food. Many of the parasitic worms have complex life cycles that involve two or more different host species.
The flukes are flatworms that are rare in most of North America but very common in parts of Africa and Asia. People acquire these species by eating aquatic plants or raw fish in which the larval worms have encysted. Within the person, each species lives in a specific site: the intestine, bile ducts, or even certain veins. Although rarely fatal, these chronic worm infestations are often debilitating, and the host person is a source of the eggs of the fluke, which may then infect others.
The preceding discussion is an introduction to microorganisms and human disease, and is only part of the story. The rest of this story is the remarkable ability of the human body to resist infection. Although we are surrounded and invaded by potential pathogens, most of us remain healthy most of the time. The immune responses that destroy pathogens and enable us to remain healthy are describe. Also in that chapter are discussions of vaccines. The development of vaccines represents the practical application of our knowledge of pathogens and of immunity, and it enables us to prevent many diseases. The availability of specific vaccines is noted in the tables of bacterial and viral diseases that follow.