Antibiotic

The animal body has an exceptional capacity to respond effectively to bacterial invasion. Most infections develop and resolve without any detectable signs or symptoms. Therefore, many infections do not require chemotherapy. However, some infections do require treatment. The aim of chemotherapy is to use the selective toxicity of a drug to assist the patient by rapidly and effectively controlling the bacterial infection without causing harmful effects to the host.

Antibiotics are the primary drugs used in antibacterial chemotherapy. The term antibiosis was first introduced by Louis Pasteur in 1877, opposing the prevailing belief in universal coexistence of microorganisms. At the time, it was known that one microorganism could inhibit the growth of another, both inside and outside the body. A notable observation was the failure of typhoid bacteria to grow in purified cultures where other organisms had grown.

Pseudomonas aeruginosa played a major role in this context, and purified cultures of this organism formed the basis of an effective commercial preparation (pyocyanase), which was mistakenly thought to be an enzyme. Another idea that led to the emergence of antibiotics was the understanding that all organic matter is eventually broken down by soil microorganisms. Testing soil samples for microbes that might contain enzymes capable of attacking pathogens proved valuable. This reasoning led to the discovery of aminoglycosides, tetracyclines, erythromycin, and chloramphenicol, all of which later gained extensive clinical use.

The first true antibiotic was tyrothricin, isolated in 1939 from a protein-free extract of Bacillus brevis. Tyrothricin consists of two peptides—tyrocidine and gramicidin. Although it protected mice from pneumococcal infection, it was found to be too toxic for general use and was restricted to topical application.

The accidental discovery of the antibacterial effect of penicillin by Fleming in 1929, and its introduction into medicine in the early 1940s, marked the real beginning of the antibiotic era. A plate of agar seeded with staphylococci had been left on a laboratory bench for five weeks; contamination with Penicillium notatum resulted in the production of significant amounts of penicillin, creating zones of inhibition where nearby bacteria had been destroyed.

The isolation of 6-aminopenicillanic acid in 1959 was another key milestone. Although this compound itself lacked antibacterial activity, it served as the core structure for the development of all synthetic penicillins. The same approach was later applied to cephalosporins, resulting in many clinically useful derivatives. As a result, the requirement for a natural source of antibiotics has become largely obsolete, since numerous synthetic antibacterial agents exist—many of which evolved into drug classes such as sulfonamides. For this reason, it is more accurate to refer to them collectively as antibacterial drugs.

Antibacterial drugs have no therapeutic purpose other than helping eliminate invading microorganisms. Although they are considered pharmacologically inert, they are not harmless and can produce significant toxic effects. It is important to recognize that while antibacterial drugs specifically treat bacterial infections, supportive measures such as antitoxins, vitamins, minerals, and anti-inflammatory agents may also be essential in managing the consequences of infection.