Home   >   Medicine   >   Infectious Diseases
Overview - Microbes


Drawing of streptococci bacteria.

Microbes belonging to the bacteria group are made up of only one cell. Bacteria inhabit a variety of environments. Psychrophiles, or cold-loving bacteria, can live in the subfreezing temperature of the Arctic. Thermophiles are heat-loving bacteria that can live in extreme heat, such as in the hot springs in Yellowstone National Park. Extreme thermophiles, or hyperthermophiles, thrive at 235 degrees Fahrenheit near volcanic vents on the ocean floor. Many bacteria prefer the milder temperature of the healthy human body.

Like humans, some bacteria (aerobic bacteria) need oxygen to survive, but others (anaerobic bacteria) do not. Amazingly, some can adapt to new environments by learning to survive with or without oxygen.

Like all living cells, each bacterium requires food for energy and building materials. There are countless numbers of bacteria on Earth - most are harmless and many are even beneficial to humans. In fact, less than 1 percent of them cause diseases in humans. For example, harmless anaerobic bacteria, such as Lactobacilli acidophilus, live in human intestines, where they help to digest food, destroy disease-causing microbes, fight cancer cells, and give the body needed vitamins. Healthy food products, such as yogurt, sauerkraut, and cheese, are made using bacteria.

Certain bacteria give off toxins that can seriously affect our health. Botulism, a severe form of food poisoning, affects the nerves and is caused by toxins from Clostridium botulinum bacteria. Under certain circumstances, however, bacterial toxins can be helpful. Several vaccines that protect us from getting sick are made from bacterial toxins. One type of pertussis vaccine, which protects infants and children from whooping cough, contains toxins from Bordetella pertussis bacteria. This vaccine is safe and effective and causes fewer reactions than other types of pertussis vaccine.


Computer graphic of influenza virus cutaway.

Viruses are among the smallest microbes, much smaller even than bacteria. Viruses are not cells. They consist of one or more molecules of DNA or RNA, which contain the virus's genes surrounded by a protein coat. Viruses can be rod-shaped, sphere-shaped, or multisided. Some look like tadpoles.

Unlike most bacteria, most viruses do cause disease because they invade living, normal cells, such as those in the human body. They then multiply and produce other viruses like themselves. Each virus is very particular about which cell it attacks. Various human viruses specifically attack particular cells in the body's organs, systems, or tissues, such as the liver, respiratory system, or blood cells.

Although types of viruses behave differently, most survive by taking over the machinery that makes a cell work. Briefly, when a single virus particle, a "virion", comes in contact with a cell it likes, it may attach to special landing sites on the surface of that cell. From there, the virus may inject molecules into the cell, or the cell may swallow up the virion. Once inside the cell, viral molecules such as DNA or RNA direct the cell to make new virus offspring. That's how a virus "infects" a cell.

Viruses can even "infect" bacteria. These viruses, called bacteriophages, may help researchers develop alternatives to antibiotic medicines for wiping out bacterial infections.

Many viral infections do not result in disease. For example, by the time most people in the United States become adults, they have been infected by cytomegalovirus (CMV). Most of these people, however, do not develop CMV disease symptoms. Other viral infections can result in deadly diseases, such as HIV infection, which causes acquired immunodeficiency syndrome (AIDS).


Drawing of penicillin mold fungus

A fungus is actually a primitive vegetable. Fungi can be found in air, in soil, on plants, and in water. Thousands, perhaps millions, of different types of fungi exist on Earth. The most familiar ones to us are mushrooms, yeast, mold, and mildew. Some live in the human body, usually without causing illness. In fact, only about half of all types of fungi cause disease in humans. Those conditions are called mycoses.

Mycoses can affect skin, nails, body hair, internal organs such as the lungs, and body systems such as the nervous system. Aspergillus fumigatus, for example, can cause aspergillosis, a fungal infection in the respiratory system.

Some fungi have made our lives easier. Penicillin and other antibiotics, which kill harmful bacteria in our bodies, are made from fungi. Other fungi, like certain yeasts, also can be beneficial. For example, when a warm liquid like water and a food source are added to certain yeasts, the fungus ferments. The process of fermentation is essential for making healthy foods like some breads and cheeses.


Drawing of a malaria parasite

Protozoa are a group of microscopic one-celled animals. Protozoa can be parasites or predators. In humans, protozoa usually cause disease. Some protozoa, like plankton, live in water environments and serve as food for marine animals, such as some species of whales. Protozoa also can be found on land in decaying matter and in soil, but they must have a moist environment to survive. Termites wouldn't be able to do such a good job of digesting wood without these microorganisms in their guts.

Malaria is caused by a protozoan parasite. Another protozoan parasite, Toxoplasma gondii, causes toxoplasmosis in humans. This is an especially troublesome infection in pregnant women because of its effects on the fetus and in people with HIV infection or other immune deficiency.



National Institute of Allergy and Infectious Diseases, USA.