Antibiotics
Antibiotics are drugs derived wholly or partially from certain microorganisms and are used to treat bacterial or fungal infections. They are ineffective against viruses. Antibiotics either kill microorganisms or stop them from reproducing, allowing the body's natural defenses to eliminate them. Antifungal and antiviral drugs are discussed elsewhere (see Section 17, Chapter 197 and Section 17, Chapter 198).
Selecting an Antibiotic
Each antibiotic is effective only against certain bacteria. In selecting an antibiotic to treat a person with an infection, a doctor makes a best guess as to which bacterium is responsible. For some infections, doctors know that only certain types of bacteria may be responsible. If there is one antibiotic that is predictably effective against all of these bacteria, further testing is not needed. For infections that may be caused by many different types of bacteria or by bacteria whose susceptibility to antibiotics is not predictable, a laboratory will be asked to identify the infecting bacterium from samples of blood, urine, or tissue taken from the person (see Section 17, Chapter 188). The infecting bacterium is then tested for susceptibility to a variety of antibiotics. These tests generally take a day or two to yield results and thus cannot guide the initial choice of which antibiotic is given.
Antibiotics that are effective in the laboratory do not necessarily work in an infected person, however. The effectiveness of the treatment depends on how well the drug is absorbed into the bloodstream, how much of the drug reaches the sites of infection in the body, and how quickly the body eliminates the drug. In selecting which antibiotic to use, a doctor also considers the nature and seriousness of the infection, the drug's possible side effects, the possibility of allergies or other serious reactions to the drug, and the cost of the drug.
Combinations of antibiotics are sometimes needed to treat severe infections, particularly in the first days when the bacterium's sensitivity to antibiotics is not known. Combinations are also important for certain infections in which the bacterium rapidly develops resistance to a single antibiotic. Infections caused by more than one bacterium, in which each bacterium is susceptible to a different antibiotic, are also treated with a combination of antibiotics.
See the drug table Antibiotics.
Antibiotic Resistance
Bacteria, like all living organisms, change over time in response to environmental challenges. Because of the widespread use and misuse of antibiotics in modern society, bacteria are constantly exposed to these agents. Although many bacteria die when exposed to antibiotics, some develop resistance to the drugs' effects. For example, 50 years ago the bacterium Staphylococcus aureus (a common cause of skin infections) was very sensitive to penicillin. Over time, strains of Staphylococcus aureus developed an enzyme able to break down penicillin, making the drug ineffective. Researchers responded by developing a form of penicillin that the enzyme could not split, but after a few years the bacteria adapted and became resistant to even this modified penicillin. Other bacteria have developed resistance to antibiotics using different mechanisms.
Medical researchers continually work to ensure that there are effective drugs to combat bacteria. Taking antibiotics only when necessary (not for viral infections such as a cold or the flu) and for the full prescribed course helps limit the development of antibiotic-resistant bacteria.
Taking Antibiotics
For severe bacterial infections, antibiotics are usually first given by injection. When the infection is under control, antibiotics can then be taken by mouth. Less severe infections can be treated from the start with oral antibiotics. Antibiotics need to be taken until the infecting organism is eliminated from the body, which may be days after the symptoms disappear. Antibiotics are rarely given for fewer than 5 days (an exception is certain uncomplicated urinary tract infections). Discontinuing treatment too soon can result in a relapse of infection or the development of antibiotic-resistant bacteria.
A doctor, nurse, or pharmacist can explain how the prescribed antibiotic should be taken. Some antibiotics must be taken on an empty stomach, whereas others may be taken with food. Metronidazole, a common antibiotic, causes an unpleasant reaction with alcohol. Also, some antibiotics can interact with other drugs a person may be taking, possibly reducing the effectiveness or increasing the side effects of the antibiotic or the other drugs. Some antibiotics make the skin sensitive to sunlight.
In addition to treating existing infections, antibiotics are sometimes used to prevent infections (prophylaxis). Antibiotics may be used to prevent meningitis in people who have been exposed to someone with meningitis. Some people with abnormal or artificial heart valves take antibiotics before dental and surgical procedures to prevent bacteria from infecting the damaged valves. Prophylactic antibiotics may also be given to people who have a weakened immune system, such as people with leukemia, people receiving chemotherapy for cancer, or people with AIDS. People undergoing surgery that has a high risk of introducing infection (such as major orthopedic or intestinal surgery) may also be given antibiotics. To be effective, and to avoid the development of resistance in bacteria, prophylactic antibiotic therapy is used for only a short time.
Home Antibiotic Therapy
Usually, antibiotics are given by mouth and the duration of treatment does not cause hardship. However, the treatment of some infections--such as those involving bone (osteomyelitis) or the heart (endocarditis)--requires antibiotics to be given intravenously for a long time, often 4 to 6 weeks. If the person has no other conditions that need treatment in the hospital and is feeling relatively well, intravenous antibiotics may be administered at home. Short intravenous (IV) catheters inserted into small veins in the arm or hand (such as are used in most routine hospital procedures) do not last more than 3 days, so a special type of IV catheter inserted into a large central vein may be needed. Some devices for infusing antibiotics are simple enough that people and their families can learn to operate them on their own. In other cases, a visiting nurse must come to the home to administer each dose. In either situation, careful supervision is required to assist the person and watch for possible complications and side effects.
People who receive antibiotics at home through an IV catheter are at increased risk of developing an infection at the site where the catheter is inserted and in the bloodstream. Pain, redness, and pus at the catheter insertion site, or chills and fever (even in the absence of problems at the insertion site) are signs that a catheter-related infection may have developed.
Side Effects and Allergies
Common side effects of antibiotics include upset stomach, diarrhea, and, in women, vaginal yeast infections. Some side effects are more severe and, depending on the antibiotic, may disrupt the function of the kidneys, liver, bone marrow, or other organs. Blood tests are used to monitor such adverse reactions.
Some people who receive antibiotics develop colitis, an inflammation of the large intestine. The colitis results from a toxin produced by the bacterium Clostridium difficile, which grows unchecked when other antibacteria are killed by the antibiotics.
Antibiotics can also cause allergic reactions. Mild allergic reactions consist of an itchy rash or slight wheezing. Severe allergic reactions (anaphylaxis) can be life threatening and usually include swelling of the throat, inability to breathe, and low blood pressure.
Many people tell their doctor that they are allergic to an antibiotic when in fact they have only experienced side effects from it that are not allergy-related. The distinction is important because people who are allergic to an antibiotic should not be given that drug or one closely related to it. However, people who have experienced minor side effects can usually take related drugs or even continue taking the same one. The doctor can determine the significance of any unpleasant reaction a person has to an antibiotic.
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