Bacteria

Bacteria are one-celled microscopic organisms. Some kinds of bacteria cause disease in humans and animals, but many others are beneficial. Bacteria dispose of organic waste, enrich the soil, and are used to make wine, beer, vinegar, cheese, and yogurt. In human beings, certain beneficial bacteria live in the intestines, where they assist in digestion.

Bacteria are different from viruses in that they are able to multiply outside a living cell, whereas viruses can grow and multiply only in living cells. Bacteria are different from other causes of infection -- protozoa (one-celled animals) and fungi (plantlike organisms) -- in two ways: They have a primitive nucleus (center where genetic material is located), rather than a well-defined one with an enclosing membrane and chromosomes; and they reproduce by a simple process called transverse binary fission (splitting in two), rather than by the more complex processes seen in higher organisms.

Types of Bacteria
One way of classifying bacteria is by shape:

• Rod-shaped bacteria are known as bacilli. They often have waving projections known as flagella, which they use to propel themselves. Some bacilli form thick-walled cells known as spores, which can survive for long periods even after the parent bacteria have been killed by freezing, disinfectants, or other forces. When conditions are favorable, the spores are able to generate new bacteria. Typhoid fever is caused by bacilli.
• Round or egg-shaped bacteria are known as cocci. They occur singly (micrococci), in chains (streptococci -- the cause of strep throat), in pairs (diplococci -- the cause of a common type of pneumonia), or in irregular bunches (staphylococci -- a cause of many skin infections). Cocci do not form spores, nor do they usually move about.
• Bacteria also exist as comma-shaped organisms called vibrios -- the organisms responsible for cholera. Spiral-shaped organisms are called spirochetes; one such organism causes syphilis.

Another way to classify bacteria is by whether they can live in the presence of air. Those that can are called aerobic; those that cannot are called anaerobic. Some can live either with or without air and are called facultative anaerobes. Tetanus (lockjaw) is an example of a disease caused by anaerobic bacteria that are commonly found in the soil. They pose no danger to humans unless they enter the body through a wound, particularly a puncture wound (such as one made by a nail). The air then cannot get to the organisms to destroy them, and so they multiply within the body, unless the infected person has been vaccinated against tetanus.

How Bacteria Are Transmitted
Disease-causing bacteria enter the body in many different ways. Those that cause pneumonia and sore throat are carried in droplets that an infected person sneezes or coughs into the air. The bacteria are then inhaled and deposited on the mucous membranes of the throat or lungs of a healthy person, where they multiply and eventually cause disease, unless checked by the body's immune system.

The bacteria that cause intestinal diseases such as cholera and typhoid can be transmitted in foods that have been handled by an infected person or in water tainted by body wastes from an infected individual. Another important route for infection is any break in the skin, which is why it is important to clean cuts as soon as possible.

Bacteria and the Body
Once inside the body, bacteria do their damage in two main ways: by direct destruction of tissue and by producing toxins. Certain white blood cells, known as lymphocytes, produce antitoxins (which neutralize the toxins of the bacteria) and antibodies (which destroy the invading bacteria themselves). Once created, the antibodies against a specific disease can persist in the body or be reproduced when needed, providing continuous immunity -- sometimes for life. Still other white blood cells, known as phagocytes, entrap and destroy bacteria.