Anti means against and bio means life. An antibiotic is a semi synthetic or synthetic substance which inhibits the growth of micro-organisms, like bacteria, fungi, or protozoa.
Earlier antibiotic was defined as an agent with biological activity against living organisms. Nowadays, antibiotics are defined as substances possessing anti-bacterial, anti-fungal, or anti-parasitical activity. Antibacterial, antimicrobial and antibiotic are now used synonymously and have capability to kill the organisms causing infections or slow down their growth. Depending upon the causative organisms of various infectious diseases, antibiotic(s) are classified antibacterial, antifungal antiprotozoal and antiviral. Antibiotics are also classified according to the spectrum of their activity. Broad spectrum antibiotics have effect on vide variety of microorganisms, while narrow spectrum antibiotics acts on a few microorganisms. Another classification of antibiotics is based on their mode of action. Antibiotics which cause the death of causative organisms are called bactericidal while those inhibiting the growth of microorganisms are known as bacteriostatic antibiotics.
An ideal antibiotic has selective toxicity (i.e. the drug is harmful to disease causing microorganisms with little/no adverse effects on patients). The lesser is selective the drug, the more are the possibilities and severity of the occurrence of the adverse effects. Adverse effects occur in the form of allergic reactions (allergic reactions can range from rashes to anaphylaxis), hypersensitivity to the drug, cross-sensitivity, toxic reactions at high doses, their interactions with food and drugs, and changes in normal intestinal flora.
Use of Antibiotics can reduce or kills the microorganisms constituting normal intestinal flora with the occurrence of resistant bacteria which results in super infections.
Some microorganisms have inherent resistance to wards particular antibiotic while some can acquire resistance. If the resistant microorganisms are suspected than more than one antibiotic are used to treat the infection.
When antibiotics do not act on microorganisms then it is said that microorganism is resistant to that antibiotic, which can be inherent (i.e. the natural resistance where the causative microorganisms were never sensitive to the antibiotic) and acquired resistance develops with prolonged use of low doses of an antibiotic.
There is also a concept of cross-resistance. Cross-resistance is the resistance developed by microorganisms to wards more than one antibiotic, i.e. if microorganism is resistant to a particular class of antibiotic, then the same microorganism would be resistant to another type of antibiotic belonging to the same class.
Penicillin was the first antibiotic to be discovered is historically significant because that was the first antibiotic effective against serious infections. Penicillins are used in the modern era also in the form of penicillinase resistant Penicillin, natural penicillins, Penicillin G, (benzylpenicillin), Penicillin V etc., although microorganisms are now resistant to it. Penicillin is usually used by injection. Tablets and capsules of penicillins are not available. Thereafter many researches have given very useful oral antimicrobial drugs like sulphonamides, tetracyclines, ampicillin, bcampicillin, amoxicillin, macrolide compounds (erythromycin, clindamycin, azithromycin), streptomycin, ethambutol, rifampicin, aminoglycosides, cephalosporins, metronidazole, tinidazole ornidazole, griseofulvin, fluconazole, ketoconazole, and many more which are successfully used to treat various types of infections due to bacteria, protozoa and fungus. New generation of antimicrobials have broader spectrum of activity and comparatively lesser adverse effects.
Antibiotics are considered as lifesaving drugs. Alexander Fleming was the discoverer of penicillin. It was the most efficacious lifesaving drug of the world. He was awarded Nobel Prize for his work on antibiotics and bacteriology. He also developed the methods for testing the antibiotics. His work was so concrete that even modern manufacturers of antibiotic use the similar techniques.