Bacteria are single-celled organisms that are probably the oldest form of life on our planet. They are neither plant nor animal but belong to a special category all their own called the prokaryotes. They are the simplest cells that exist on earth: their basic structure is a membrane wrapped around a collection of DNA strands. They do not have a nucleus and are microscopically tiny, but usually live in large colonies and can reproduce quickly if conditions are right. Bacteria come in different shapes, the most common being rods, circles, and spirals. They are amazingly resilient as a life form. There are some kinds of bacteria than can withstand boiling temperatures, freezing cold, or vacuum-conditions that would kill any higher organism.
Although we often think of bacteria in terms of sickness or infection, only a few kinds of bacteria actually cause disease. In fact we are continuously surrounded by bacteria and even host billions of them in our bodies. There are at least 500 kinds of bacteria that live in our digestive system and help us digest foods and synthesize vitamins. Similar numbers colonize our skin. Healthy bacteria in the digestive and vascular systems and on the skin can help fight off "bad" bacteria that might make us sick.
The bacteria than can cause disease are called pathogens. A bacterial infection occurs when harmful pathogens enter the body or sometimes when bacteria that normally live harmlessly in the body get out of balance. Bacteria can cause disease by invading tissues where they don't belong, or by producing toxins, or both. The body's immune system will fight against invading pathogens, and this usually causes pain, fever, and swelling at the site of the infection. If the pathogens have attacked the gastrointestinal system, vomiting and diarrhea may occur. Some bacterial infections are spread by insect bites, some by direct contact with another infected person, some through water or food.
Bacterial infections have been a major cause of disease and death in humans throughout history. Bacteria are responsible for such infections as tuberculosis, plague, syphilis, cholera, typhoid fever, diphtheria, tetanus, anthrax and leprosy. Secondary bacterial pneumonia, which can occur when a person is sick and the immune system is weakened by the original illness, is a major killer in times of flu epidemics. Salmonella food poisoning is a bacterial infection. Staph infections, a growing concern in hospitals and other health care facilities where the immune systems of patients are likely to be weak, are caused by bacteria and can range from mild to fatal. Bacterial infections can enter the bloodstream and poison the entire body that way, a condition called bacteremia or blood poisoning. Long-term bacterial infections, such as peptic ulcer disease or some sexually transmitted conditions, can even cause the later development of cancer, by causing chronic inflammation and cellular turnover that predisposes to eventual malignancy.
Throughout most of human history, the only defense against bacterial infections was the immune system. A person would either overcome the infection and recover, live with chronic illness or dysfunction, or die. The immune system fights bacterial infection by producing antibodies to the bacteria. These are proteins in the blood that help the white blood cells, which fight infection, to recognize the offending type of bacteria. It typically takes two weeks from the time of infection before the immune system is putting out sufficient antibodies to battle the infection. The antibody-producing cells have a kind of memory that remembers the ability to produce that kind of antibody, so a person is said to have acquired immunity to the particular bacteria. If it ever enters the body again, the immune system will attack it immediately, before it can cause disease.
Acquired immunity can also be induced by exposure to a weakened or killed form of the bacteria-a vaccination. This was the first of the two great weapons developed in the past 200 years to fight bacterial infections. A vaccination may have to be repeated periodically as a "booster." Because the actual inoculant is not the full-strength bacteria-which would cause illness-the immune response is not necessarily as strong as it would be if you had contracted the infection and then recovered. But it is generally enough to boost the immune system response and trigger attack on the bacteria.
The second and greatest weapon in the medical arsenal against bacterial infections is the development of antibiotic medicines. Antibiotics are products of bacteria, fungi, and other microorganisms that inhibit or outright kill other microorganisms. With the development of penicillin in 1928, human medicine took a huge step forward. Antibiotics seemed to offer the "miracle cure" for bacterial infections that had plagued humankind for thousands of years. Many different forms of antibiotics have since been developed that target many strains of bacteria.
Unfortunately, in recent decades antibiotic use became so widespread that it was expected by patients and overprescribed by doctors, even for viral or other diseases on which it had no effect. Bacteria, like other living organisms, are opportunists. Living in an environment of low-grade exposure to antibiotics, they have mutated and become resistant to particular antibiotics. Bacterial infections once vulnerable to the older antibiotics are no longer harmed by them. Scientists warn of "superbugs" that are resistant to multiple types of antibiotics. Of particular concern already are antibiotic-resistant strains of tuberculosis and the sometimes-deadly MRSA, or Methicillin-resistant Staphylococcus aureus. Medical science cannot rest in keeping one jump ahead of bacterial infections, lest the miracle cure becomes useless through our own mismanagement.