Germs are generally considered to be bacteria, viruses, protozoa, helminths/worms and fungi.

Bacteria are so small that they can only be seen with the aid of a microscope.

These are single-cell organisms that do not contain a nucleus. They may invade the body through the respiratory tract by inhalation of small droplets as in the case of tuberculosis or they may be ingested by mouth and attack the gastrointestinal tract and cause nausea and vomiting as seen in food poisoning.

Other organisms may enter the body by a puncture site under the skin, causing an abscess which would act as a local infection. If bacteria make their way into the blood stream, multiple organs may become infected.

Bacteria have certain characteristics that aid in causing disease.

Some are able to attach to our cells or tissue and cause disease. Some bacteria may destroy normal cells; an example of this would be tuberculosis.

Others may produce toxins or a poison substance, and the classic examples would be food poisoning and tetanus.

Not all bacteria cause disease or illness, and it is believed that only about 1 percent are harmful.

Many bacteria are harmless or are made harmless by our immune system; some are helpful particularly those that normally live in our gastrointestinal tracts. It has been estimated that there can be up to 40 million bacterial cells in a gram of soil.

The biomass of all bacteria on earth is greater than that of all plants and animals combined. There are 10 times the number of bacterial cells in the human flora as there are total human cells.

Bacteria are able to reproduce rapidly, and each individual organism may reproduce once in 20 minutes or less. Therefore, if potato salad is left out overnight and starts with only one bacterial cell at 9 p.m., there could be as many as 134 million organisms by 6 a.m.

Other than our immune systems, antibiotics are an alternate source of resisting the illness caused by bacteria.

Antibiotics must harm the bacterial cell wall but not human cells. They may alter bacterial cell wall synthesis or their folic acid synthesis or even DNA and protein synthesis.

Penicillin was the first antibiotic widely used that interfered with synthesis of bacterial cell walls.

Sulfa drugs kill bacteria by inhibiting certain metabolic pathways. These bacterial cells require the B vitamin known as folic acid, and they, unlike humans, are capable of making folic acid.

Sulfa drugs interfere with bacterial cell folic acid synthesis; if this process is blocked, the bacterial cell will die or no longer grow.

Tetracycline inhibits protein synthesis by crossing into the cell wall and once arriving in the cells cytoplasm binds to sites that blocks RNA protein synthesis.

Ciprofloxacin, on the other hand, blocks DNA synthesis and is helpful in treating anthrax.

Because of the widespread use of antibiotics today, particularly in the beef and cattle industry, there is concern that bacteria will become resistant to many of our presently effective antibiotics.

Some bacteria are predators and kill other microorganisms then consume them and absorb their nutrients. One example is an organism called Vampirococcus.

David Keisler is a gastroenterologist and internist in Aiken.