In common usage, an antibiotic is a drug that kills certain kinds of bacteria, but which is generally harmless to the host and is used to treat infection. The term was originally used to describe only antibacterial formulations derived from living organisms but is now used in reference to synthetic antimicrobials such as the Sulfonamides.

In general, the term can also apply to substances that affect prions, viruses, fungi, worms or any other intracellular or extracellular parasite, but the antibacterial kind are the most common. Generally, the antibiotics are not effective in viral infections.

Table of contents
1 History
2 Classes
3 Side Effects
4 Antibiotic misuse
5 Antibiotic resistance


The first antibiotic to be discovered was penicillin. Alexander Fleming had been culturing bacteria on an agar plate with an accidental fungal contamination, and noticed that the culture medium around the mould was free of bacteria. He had previously worked on the antibacterial properties of lysozyme, and so was predisposed to make the correct interpretation of what he saw: that the mold was secreting something that stopped bacterial growth. Though he was unable to produce the pure material (the beta-lactam ring in the penicillin molecule was not stable under the purification methods he tried), he reported it in the scientific literature. Since the mold was of the genus Penicillium, he named this compound penicillin. With the increased need for treating wound infections in World War II, resources were poured into investigating and purifying this compound, and a team led by Howard Florey succeeded in producing large quantities of the purified active ingredient. Antibiotics soon came into widespread use.

The discovery of antibiotics, along with anesthesia and the adoption of hygienic practices by physicians (for example, washing hands and using sterilized instruments) revolutionized medicine - it has been said that this is the greatest advance in health since modern sanitation. They are often called "magic bullets": drugs which target bugs without greatly harming the host.


There are many way to classify antibiotics.

One such classification is by chemical structure:

Another such classification is by their mechanism of action (that is, the mechanism by which they selectively poison bacterial cells:

  • antibiotics which interfere with cell-wall synthesis''
    • beta-lactams, including penicillins and cephalosporins; mono-lactams, such as Imipenem; vancomycin, bacitracin
  • antibiotics which interfere with bacerial protein synthesis
    • antibiotics which bind to the 50S ribosomal unit''
      • Lincosamides/lincosides including clindamycin and lincomycin; chloramphenicol
    • antibiotics which interfere the 30S ribosomal unit
      • tetracyclines; aminoglycosides including gentamicin
  • drugs with inhibit folate sytheisis
    • sulfonamides and tirmethoprim
  • drugs which interfere with DNA synthesis
    • rifampin, metronidazole, quinolines, novobiocin
  • drugs which interfere with cell membrane function
    • polymyxin B, Gramicidin

Antibiotics can also be classified by the organisms against which they are effective, and by the type of infection in which they are useful, which depends on the sensitivities of the organisms that most commonly cause the infection and the concentration of antibiotic obtainable in the affected tissue.

Side Effects

Side effects range from slight headache to a major allergic reaction. One of the more common side effects is constipation, as the antibiotic will also kill the good bacteria which dwell inside the human digestive system. Also, the physician, prior to prescribing an antibiotic will ensure that the patient is not taking any drugs which might have a negative interaction.

Antibiotic misuse

Common forms of antibiotic misuse include taking an antibiotic for an inappropriate condition, in particular the use of antibiotics for viral infections; and not taking the entire course of the antibiotic, usually because the patient feels better before the infection is cured.

There is debate over the appropriateness of including antibiotics in the diet of healthy farm animals. Opponents of this practice point out that it leads to antibiotic resistance, including in bacteria that infect humans. The practice continues in many places, however, because feeding livestock antibiotics promotes weight gain, and thus makes economic sense for the individual farm or ranch.

Antibiotic resistance

One side effect of misusing antibiotics is the development of antibiotic resistance by bacteria. By 1984 half the people with active tuberculosis in the United States had a strain that resisted at least one antibiotic. Between 1985 and 1991 tuberculosis increased 12 per cent in the US and 300 per cent in Africa where HIV and TB are often found together.