Antibiotic

 Antibiotics are drugs used to treat bacterial infections. They are ineffective against viral infections and most other infections. Antibiotics either kill microorganisms or stop them from reproducing, allowing the body's natural defenses to eliminate them.

• Although doctors try to use antibiotics for specific bacterial infections, they sometimes start antibiotics without waiting for tests that identify the specific bacteria.

• Bacteria can develop resistance to the effects of antibiotics.

• Antibiotics can have side effects, such as upset stomach, diarrhea, and, in women, vaginal yeast infections.

• Some people are allergic to certain antibiotics.

Antibiotics are grouped into classes based on their chemical structure. However, antibiotics within each class often affect the body differently and may be effective against different bacteria.

Classes of antibiotics include the following:

• Aminoglycosides

• Carbapenems

• Cephalosporins

• Fluoroquinolones

• Glycopeptides and lipoglycopeptides (such as vancomycin)

• Macrolides (such as erythromycin and azithromycin)

• Monobactams (aztreonam)

• Oxazolidinones (such as linezolid and tedizolid)

• Penicillins

• Polypeptides

• Rifamycins

• Sulfonamides

• Streptogramins (such as quinupristin and dalfopristin)

• Tetracyclines

Carbapenems, cephalosporins, monobactams, and penicillins are subclasses of beta-lactam antibiotics, a class of antibiotic characterized by a chemical structure called a beta-lactam ring.

Other antibiotics that do not fit into the classes listed above include chloramphenicol, clindamycin, daptomycin, fosfomycin, lefamulin, metronidazole, mupirocin, nitrofurantoin, and tigecycline.

Selecting an Antibiotic

Each antibiotic is effective only against certain bacteria. In selecting an antibiotic to treat a person with an infection, doctors estimate which bacteria are likely to be the cause. For example, some infections are caused only by certain types of bacteria. Sometimes one antibiotic is predictably effective against all of the bacteria that are most likely to be causing an infection and so further testing may not be needed.

If infections may be caused by many different types of bacteria or by bacteria that are not predictably susceptible to antibiotics, a laboratory is asked to identify the infecting bacteria from samples of blood, urine, or tissue taken from the person (see Diagnosis of Infectious Disease). The infecting bacteria are then tested for susceptibility to a variety of antibiotics. Results of these tests usually take a day or two and thus cannot guide the initial choice of antibiotic. In such cases, doctors typically start treatment with an antibiotic that is effective against the bacteria most likely to be causing the infection. When test results are back, doctors change the antibiotic if needed.

Antibiotics that are effective in the laboratory do not necessarily work in an infected person. The effectiveness of the treatment depends on

• How well the drug is absorbed into the bloodstream (for drugs taken by mouth)

• How much of the drug reaches the sites of infection in the body (see Drug Distribution)

• How quickly the body eliminates the drug (see Drug Elimination)

These factors may vary from person to person, depending on other drugs being taken, other disorders present, and the person’s age.

In selecting an antibiotic, doctors also consider the following:

• The nature and seriousness of the infection

• The status of the person's immune system (how well it can help the drug fight the infection)

• The drug’s possible side effects

• The possibility of allergies or other serious reactions to the drug

• The cost of the drug

Doctors also consider how hard it may be for people to take antibiotics for the entire time prescribed and complete the full course of treatment. People may find it more difficult to complete treatment if the drug must be taken very often or only at specific times (such as before meals, during meals, or after meals).

Combinations of antibiotics may be needed to treat the following:

• Severe infections, particularly during the first days when the bacteria's susceptibility to antibiotics is not known

• Certain infections caused by bacteria that rapidly develop resistance to a single antibiotic

• Infections caused by more than one type of bacteria if each type is susceptible to a different antibiotic

Antibiotic Resistance

Bacteria, like all living organisms, change over time in response to environmental challenges. Because of the widespread use and misuse of antibiotics, bacteria are constantly exposed to these drugs. Although many bacteria die when exposed to antibiotics, some develop resistance to the drugs’ effects. For example, 50 years ago, Staphylococcus aureus (a common cause of skin infections) was very sensitive to penicillin. But over time, strains of this bacteria 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 break down, but after a few years, the bacteria adapted and became resistant to this modified penicillin. Other bacteria have also developed resistance to antibiotics.

Medical research continues to develop drugs to combat bacteria. But people can help prevent the development of resistance in bacteria by

• Taking antibiotics only when necessary (that is, people should take antibiotics only for infections caused by bacteria, not for those caused by viruses such as a cold or the flu)

• Not asking doctors to prescribe antibiotics for viral infections, such as the common cold or the flu

https://www.msdmanuals.com/en-in/home/infections/antibiotics/overview-of-antibiotics