metabolites are meant products of microorganisms (also plants) which are not essential for basic metabolic processes such as reproduction and growth [3]. On the other hand, in the case of many secondary compounds, pieces of evidence of their role in the metabolism of the producer have been brought. These compounds often function as the so-called signal molecules, used to control the producer s metabolism. One of the functions attributed to antibiotics is a suppression of the competing microorganisms in the environment. Thus the antibiotic-producing microorganisms have an advantage in competing for nutrients with the other microorganisms but antibiotic activity is only one from many other biological activities of secondary microbial products. However, the function of antibiotics in the environment can be observed only with difficulty.
1.1.1 Use of antibiotics in human, veterinary and plant medicine
Antibiotics are very often used in medicine for suppression of pathogenic bacteria, fungi and viral diseases. Their use marked a revolution in medicine, saved millions of lives and helped reduce some, rather frequent diseases such as tuberculosis. An efficient, antiprotozoan antibiotic, however, has not yet been discovered. Antibacterial antibiotics are sometimes used in the case of viral diseases to protect the weakened macroorganism against a subsequent bacterial infection. As mentioned in the introductory part, some antibiotics are also used as cancerostatics or for curing some other illnesses.a similar way as in human medicine, antibiotics are also employed in veterinary medicine. Besides, antibiotics are added to various feeding mixtures used in poultry and animal farming to keep the animals in good health. If the antibiotics are used, higher farming yields are often reached. However, the administration of antibiotics should be stopped a certain time before the animal is slaughtered and the meat consumed since the residues of antibiotics should not enter the human diet. To avoid the production of strains resistant to the antibiotics used in human medicine, special antibiotics allowed to be employed in veterinary medicine and animal production have been singled out and are no more used in human medicine (chlortetracycline, bacitracin, tylosin, etc.).
Side effects of antibiotics. In addition to their positive effects, antibiotics can also have negative effects. Besides various allergies linked with the use of antibiotics, the human organism can sometimes suffer a damage when treated with them. Sometimes toxic compounds can be formed when antibiotics are transformed in the organism. Tetracyclines, that form complexes with calcium, can, for example, inflict damage on the formation of tooth enamel in children, on the condition they are frequently used during the period of teeth growth. A number of newly discovered antibiotics can not be used for therapy because of their excessive toxicity. Fortunately, the first antibiotic to be used on a massive scale, penicillin, has relatively moderate side effects on the human organism. . In spite of variety of their structures, antibiotics are synthesized from simple building units amino acids, acetate, propionate, sugars, nucleotides which are used in living organisms for the biosynthesis of cellular structures. According to their structure and type of biosynthesis, antibiotics are classified to form several groups.
1.1.2 Peptide and peptide-derived antibiotics
Peptides. Microorganisms produce a number of peptides that have the biological activity. In contrast to biologically active peptides of higher organisms, where they function as hormones, the function of microbial peptides in microorganisms is not known. They are included in the group of secondary metabolites. They differ from the biologically active peptides of higher organisms in having often D-amino acids in their molecules. Besides their antibiotic activity, another interesting feature of the peptide antibiotics is the fact that they are not synthesized on ribosomes, as other peptides, but on enzyme complexes called peptide synthetases [3-4].
Chemical structure. Amino acids linked by the peptide bond form the basal structure of any peptide antibiotic. The peptide chain is often cyclic or branched. In addition to L-amino acids, other compounds can also be present in the molecule, such as D-amino acids, organic acids, pyrimidines and sugar molecules. Numbers of derivatives are known to exist in the case of some peptide antibiotics, that differ in both amino acid substitutions and substituents bound to the amino acids.
The linear molecule of gramicidin A and the cyclic molecule of gramicidin S belong to the structurally simplest peptide antibiotics. ...
