Microbes: In Sickness and in Health

by National Institute of Health

What Are Microbes?

Microbes are tiny organisms - too tiny to see without a microscope, yet they are abundant on Earth. They live everywhere - in air, soil, rock, and water. Some live happily in searing heat, while others thrive in freezing cold. Some microbes need oxygen to live, but others do not. These microscopic organisms are found in plants and animals as well as in the human body.

Some microbes cause disease in humans, plants, and animals. Others are essential for a healthy life, and we could not exist without them. Indeed, the relationship between microbes and humans is delicate and complex. In this booklet, we will learn that some microbes keep us healthy while others can make us sick.

Most microbes belong to one of four major groups: bacteria, viruses, fungi, or protozoa. A common word for microbes that cause disease is "germs." Some people refer to disease-causing microbes as "bugs." "I've got the flu bug," for example, is a phrase you may hear during the wintertime to describe an influenza virus infection.

Since the 19th century, we have known microbes cause infectious diseases. Near the end of the 20th century, researchers began to learn that microbes also contribute to many chronic diseases and conditions. Mounting scientific evidence strongly links microbes to some forms of cancer, coronary artery disease, diabetes, multiple sclerosis, and chronic lung diseases.

Bacteria

Microbes belonging to the bacteria group are made up of only one cell. Under a microscope, bacteria look like balls, rods, or spirals. Bacteria are so small that a line of 1,000 could fit across the eraser of a pencil. Life in any form on Earth could not exist without these tiny cells. Scientists have discovered fossilized remains of bacteria that date back more than 3.5 billion years, placing them among the oldest living things on Earth. Bacteria can inhabit a variety of environments, including extremely hot and cold areas. 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. Others (anaerobic bacteria), however, 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 bacteria cause diseases in humans. For example, harmless anaerobic bacteria, such as Lactobacilli acidophilus, live in our 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.

Some bacteria produce poisons called toxins, which also can make us sick.

Are Toxins Always Harmful?

Certain bacteria give off toxins that can seriously affect your 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.

Viruses

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 viruses look like tadpoles.

Unlike most bacteria, most viruses do cause disease because they invade living, normal cells, such as those in your 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 your body's organs, systems, or tissues, such as the liver, respiratory system, or blood.

Although types of viruses behave differently, most survive by taking over the machinery that makes a cell work. Briefly, when a piece of a virus, called 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 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 preventing and treating 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 AIDS (acquired immunodeficiency syndrome) or Ebola hemorrhagic fever.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.