What are antibiotics?

The word antibiotic is derived from two Greek words anti meaning against and bios meaning life. Antibiotics may be defined as the compounds produced synthetically, semi-synthetically or by various species of microorganism that have the capability to kill or inhibit the growth of other microorganism.

The term antibiotic and antibacterial are used synonymously. Generally antibiotics or antibacterials are the drugs used to treat infections caused by bacteria.

Bacteria (singular = bacterium) are the simplest and smallest tiny organisms possessing cellular organization that can sometimes cause illness to animals and human beings, such as meningitis, syphilis, anthrax, tuberculosis, pneumonia etc.

Not all bacteria are harmful but even some are good for us. When harmful bacteria attack our body, our immune system become active and destroy them before they multiply and cause symptoms of the infection.

Mild infections are also eradicated by immune system, but in case it fails to fight off infection we need some help from outside the body. Antibiotics provide this aid to destroy invading microorganisms and eradicate infections.

How do antibiotics work?

All antibiotic drugs are effective in the treatment of infectious diseases because of their selective toxicity. It means that the drug is harmful to an invading microorganism without harming the cells of the host.

Selective toxicity is due to biochemical differences that exist between microorganisms and the host.

Concentration of the drug should be carefully controlled to attack the microorganism while still being tolerated by the host. All antibiotics work either as bacteriostatic or bactericidal.

What are bacteriostatic drugs?

Bacteriostatic drugs stop bacteria from multiplying. They inhibit the growth of microorganisms temporarily.

The therapeutic success of these agents depends upon the participation of defence mechanism of the host . The effect may be reversible i.e. when the drug used is stopped , the organisms will resume growth and infection or disease will reoccur in immunocompromised patient.

What are bactericidal drugs?

These drugs kill the microorganisms . The treatment with bactericidal drugs becomes mendatory in case of those infections that cannot be controlled or eradicated by defence mechanism of host. Examples are, beta-lactam antibiotics, cephalosporins, penicillines and aminoglycosides.

However the term bactericidal and bacteriostatic are relative and not absolute. As some times prolong treatment, higher dose of bacteriostatic agents can cause death of microorganism (chlormphenicol and maningococci) while bacctericidal drugs may fail to kill the microbes e.g penicillin and enterococci.
Bactericidal drugs can be divided into two subgroups,

1- Drugs that exhibit concentration dependent killing. For these drugs, the rate and extent of killing increases with increase in concentration above MBC (minimal bactericidal concentration). Maximum peak concentration of these drugs results in increased efficacy and decreased resistance.

2- Drugs that exhibit time dependent killing. Increase in concentration of these drugs does not result in increased killing of susceptible microorganisms. Bactericidal activity is directly related to time above MBC (minimal bactericidal concentration) and not the concentration of the drug.