Antibiotic

The animal body has an exceptional ability to respond effectively to bacterial invasion. Most infections develop and resolve without any visible symptoms or signs. Therefore, many infections do not require chemotherapy. However, some infections do require treatment. The purpose of chemotherapy is to use the selective toxicity of a drug to help the patient rapidly and effectively control a 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, contradicting the then-common belief in universal microbial coexistence. It was already known that one microorganism could inhibit the growth of another, both inside and outside the body. What drew attention was the observation that typhoid bacteria failed to grow in purified cultures where other organisms thrived.

Pseudomonas aeruginosa was particularly effective in this regard, and purified cultures of this organism led to a commercially successful preparation known as pyocyanase, which was mistakenly believed to be an enzyme. Another line of reasoning that contributed to the development of antibiotics was the understanding that all organic matter is eventually degraded by soil microorganisms. This encouraged researchers to examine soil samples for microbes that might produce enzymes capable of attacking pathogenic organisms. This approach led to the discovery of aminoglycosides, tetracyclines, erythromycin, and chloramphenicol, all of which later achieved widespread clinical use.

The first true antibiotic was tyrothricin, isolated in 1939 from a protein-free extract of Bacillus brevis. Tyrothricin contains two peptides—tyrocidine and gramicidin. Although it protected mice from pneumococcal infections, it proved far 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 true beginning of the antibiotic era. A plate of agar contaminated with Penicillium notatum had been left on a bench for five weeks; the mold produced a significant amount of penicillin, inhibiting nearby staphylococcal colonies and creating a clear zone where bacteria had been destroyed.

The isolation of 6-aminopenicillanic acid in 1959 was another major milestone. Although it lacked antibacterial activity itself, it served as the core structure for producing all synthetic penicillins. A similar approach was taken with cephalosporins, leading to numerous clinically useful derivatives. Today, a natural source is no longer essential, as many synthetic antibacterial agents exist—often considered part of the broader class of antibacterial drugs, including medications such as sulfonamides.

Antibacterial drugs have no therapeutic purpose other than helping eliminate invading microorganisms. Although pharmacologically inert, they are not harmless and can cause significant toxic effects. It is also important to note that although antibacterial drugs are used specifically to treat bacterial infections, additional measures—such as antitoxins, supportive medications (e.g., vitamins, minerals), and anti-inflammatory drugs—may also be crucial in managing the consequences of infection.