Antibiotics Classification and Mechanisms of Action

Antibiotics Classification and Mechanisms of Action

Antibiotics are chemical drugs of natural, semi-synthetic, or synthetic origin, which in minimal concentrations inhibit the replication or cause the death of susceptible microbial agents in inner compartments of the body.

Discovery of antibiotics occurred in 1928-1929, when A. Fleming has demonstrated the obvious antibacterial action of Penicillium notatum fungal culture. In 1940 H. Florey and E. Chain have obtained stable penicillin product (its sodium salt). Later in 1943 A. Schatz and S. Waksman discovered new antibiotic streptomycin. Since that time a tremendous number of antibiotics has been described, investigated and applied for different medical goals.

Antibiotics must be in accordance with several requirements:

  1. High antimicrobial activity and selectivity in doses, non-toxic for the patient.
  2. Efficient therapeutic action in tissues and organs, low level of inactivation by tissue proteins and enzymes.
  3. Absence or slow development of side effects.
  4. Long period of metabolism (prolonged effect).
  5. Slow growth of microbial resistance to the antibiotic.
  6. High effectiveness of the drug with the low cost of therapy.
  7. The drug must be compliant for different practical applications and stable in storage.

Unfortunately, none of known antibiotics satisfies all these requirements completely.

Classification of Antibiotics

Antibiotics are classified according to their origin.

Antibiotics of natural origin are obtained from various sources.

Vast number of antimicrobial agents is produced by actinomycetes. Streptomycin is obtained from S. griseus, chloramphenicol is derived from the cultural fluid of a strain of S. venezuelae, tetracycline is produced by S. aureofaciens, erythromycin is derived from S. erythreus, nystatin has been extracted from the culture of S. noursei. Kanamycin is produced by S. kanamycetius. Amphotericins (A and B) are antimycotic antibiotics obtained from S. nodosum.

Antibiotics synthesized by other bacteria comprise gramicidin (derived from the culture of Bacillus brevis), polymyxins B, E or M from Paenibacillus polymyxa and some others.

Fungal antibiotics are of wide use. Penicillin is produced by fungi Penicillium notatum or Penicillium chrysogenum.

Some antimicrobial substances are obtained from plants. This broad group of antimicrobials is termed phytoncides.

Finally, some substances of animal origin can possess antimicrobial activity. For instance, enzyme lysozyme is capable of hydrolyzing bacterial cell walls.

Semisynthetic and synthetic antibiotics comprise a great number of modern drugs.

Different semisynthetic penicillins are obtained on the basis of penicillin nucleus, 6-aminopenicillanic acid, by substitution of the lateral radicals (methicillin, oxacillin, dioxacillin, ampicillin, etc.). Similarly, modern cephalosporins are synthesized on the basis of 7-amino-cephalosporanic acid, the nucleus of cephalosporin (ceftazidime, cefotaxime, cefepime, etc.)

Finally, many antibiotics are the products solely of chemical synthesis. Among them are isoniazid, nalidixic acid, cycloserine, pirazinamide, etc. The synthetic drugs are sulphonamides.

According to the character of action, antibiotics are divided into bactericidal and bacteriostatic.

Antimicrobials drugs are considered to be “bactericidal” if they kill affected microorganisms. If antimicrobials only inhibit the growth of susceptible bacteria they are called “bacteriostatic”. Growth inhibition results in microbial killing by host immune system.

According to spectrum of action antibiotics are divided into drugs with broad spectrum of action (affecting both gram-positive and gram negative bacteria, rickettsiae, chlamydiae etc.) and with narrow spectrum of action (e.g., inhibiting only particular group of gram-positive or other microbes).

Antibiotics Classification and Mechanisms of Action

The Mechanisms of Antibiotics Action

Antibiotics produce various deleterious effects against microbial cells. These effects are based on specific molecular mechanisms of antibiotic action. Among them are:

  1. inhibition of cell wall synthesis;
  2. impairment of the normal function of cell membrane;
  3. inhibition of protein synthesis;
  4. impairment of nucleic acid synthesis.