Antibiotics are medications used to treat infectious diseases caused by bacteria. Penicillin was the first antibiotic discovered (in 1927). Since that time, more than 150 types of antibiotics have been developed. These drugs have dramatically lowered rates of death and illness associated with infectious disease.

Bacteria are tiny, single-cell living organisms. They can be found everywhere in the environment, including both on and within the human body. Most bacteria are harmless, and some are beneficial. However, certain bacteria cause infection when they are introduced into the human body. Once inside, bacteria reproduce rapidly, disrupting normal function and causing illness.

Antibiotics work to eliminate these microorganisms by halting their ability to grow and reproduce. Each type of antibiotic is effective against certain types of bacteria. About 90 percent of the antibiotics used today are created by chemically isolating them from bacteria, although a few are completely manufactured. “Broad spectrum” agents are antibiotics that harm or kill a wide variety of infectious bacteria.

Antibiotics can harm bacteria in several ways. Most antibiotics fall into one of the following categories:

• Cell membrane inhibitors. These antibiotics directly kill bacteria cells by causing the outer membrane of the cell to break down. Most medications of this kind are used as topical antibiotics, such as skin ointments or creams. These types of antibiotics are not effective when administered systemically (provided to the entire body through the bloodstream, such as when taken in pill form).
  
  
• Cell wall synthesis inhibitors. These antibiotics will not harm existing bacteria cells, but they prevent new bacteria cells from forming. Infections grow and spread because bacteria cells multiply rapidly, doubling their presence in the body once every 20 minutes. When bacteria divide in order to multiply, they create a wall or container for the new cell. Antibiotics considered cell wall synthesis inhibitors prevent bacteria from forming this cell wall, thus preventing bacteria from multiplying.
  
  
• Protein synthesis inhibitors. These antibiotics prevent bacteria from converting proteins to energy, effectively starving bacteria to death.
  
  
• Nucleic acid synthesis inhibitors. These antibiotics prevent bacteria cells from growing by interrupting communication between bacteria and its deoxyribonucleic acid (DNA) – the genetic material of the bacteria that tells it how to make proteins to survive and reproduce.
  
  
• Competitive inhibitors. These antibiotics are mostly synthetic (manufactured) and are similar to the material in bacteria that regulate growth. Once this medication reaches bacteria, it replaces the bacteria's own growth-regulating material, preventing an infection from spreading.

Antibiotics are typically administered in oral form, although they also may be applied as topicals (applied directly to the skin) or injections, including intravenous injections (which are injected directly into the bloodstream). Antibiotics are taken until the infecting organism is completely eliminated from the body, which often does not occur until several days after symptoms cease. For this reason, antibiotics are rarely prescribed for periods of fewer than five days.

Different antibiotic drugs or combinations of drugs may be used to treat various conditions. Generally, treatments may be classified according to their effectiveness in the following manner:

• First-line treatments: The first medication prescribed for treatment of a given diagnosis. First-line drugs are the most frequently used medications due to their low risk of toxicity (overdose), relatively low cost and broad availability.
  
  
• Second-line treatments: The second medication provided if the patients have either relapsed following first–line treatment or were unresponsive to the initial drug used. Second-line treatments have a comparatively higher risk of side effects than first-line treatments.
  
  
• Third-line treatments: These medications are provided when resistance develops to a second-line drug. Third-line treatments carry a greater risk of toxicity and other side effects than first- or second-line treatments. Consequently, they are only given when the potential benefits of a third-line medication are believed to outweigh its risks.

After a physician identifies the type of bacterium responsible for a person’s illness, a specific type of antibiotic known to be effective against that particular bacterium will be prescribed. In some cases, infection is the result of several different types of bacteria or a bacterium that does not have an established track record of being treated effectively by a certain type of antibiotic. In such instances, a physician may order laboratory tests to examine the effect of various antibiotics on the bacterium. When a promising drug is identified, it will be prescribed to the patient.