tabolism of the producing microorganism.
Polyketide synthase genes of microorganisms producing various polyketides have also been hybridized. As a result, a great similarity of polyketide synthases from various streptomycetes was evidenced and new polyketide antibiotics were synthetized.
. 1.4 Search for new antibiotics
Isolation from nature. At present several thousands of compounds having some biological activity have been obtained from microorganisms isolated from nature. As the probability of finding a new compound that would be usable as a new antibiotic is as little as one in ten thousand, a great number of microorganisms have to be checked. A rough estimation says that about 100000 microorganisms is screened for the presence of biologically active compounds per year. Well equipped laboratories study about 30 different biological activities. The requirements for new antibiotics result from the occurrence of resistant strains of pathogenic microorganisms, that are no more sensitive to known antibiotics used in the clinical practice. It is mostly big pharmaceutical companies that look for new antibiotics. Their search for new compounds is highly automated. The selection methods used and the methods of detection of the biological activity are normally not published.
Preparation of a new antibiotic and its introduction into the clinical practice requires cooperation of scientists from various scientific disciplines. They can be divided into three groups: microbiology (colection of source samples, isolation of diverse microbes, fermentation to enhance the production, taxonomy), pharmacology (target selection, screen design, high-troughput screening, identification of active compounds, efficacy studies, mechanism of action), chemistry (active compounds identification, characterization/replication, isolation/purification, structure elucidation). Producers of antibiotics and other biologically active compounds.The majority of the known antibiotics are produced by actinomycetes, fungi and by moulds. With an increasing spectrum of efficiency of microbial metabolites, new, non-traditional sources of such compounds have been looked for. Tropical soils have an enormous biodiversity and they are a rich source of new antibiotics [14].
The tests of other biological than antibiotic activities require sophisticated methods. This is true especially when enzyme inhibitors are looked for. Thus, Ogawara chose a tyrosine protein kinase associated with the malignant transformation of the cell caused by retroviruses as the target in a biochemical screen and found genistein, an isoflavone from Pseudomonas, exhibiting a specific inhibitory activity. Production of target enzymes using recombinant DNA methodology has dramatically expanded the number of potential targets that can be feasibly screened. A screen for the inhibitors of HIV reverse transcriptase is an example. The enzyme was produced in Escherichia coli, purified by affinity chromatography, and used to test natural products for the activity.
1.1.5 Semisynthetic and synthetic antibiotics
After the structures of the antibiotics discovered had been determined and microbial strains resistant to them detected, possible variations of the molecules of known antibiotics were studied. Several methods have been used to accomplish such variations.
Biosynthetic antibiotics. unspecificity of the enzyme systems able to synthesize antibiotics was used, together with the addition of precursors to the growth medium. Thus, the reaction equilibrium was shifted to promote the production of the derivative required. In this way, penicillins with different side chains were prepared. Addition of amino acids to the growth medium can affect the amino acid composition of polypeptide antibiotics. The individual derivatives of penicillin and cephalosporin have slightly different antimicrobial spectra and are capable of suppression of microorganisms resistant to other derivatives.
Semisynthetic antibiotics.of a part of the antibiotic molecule can be accomplished chemically or enzymatically. In this way, semisynthetic penicillins, cephalosporins, tetracyclines, etc. were prepared. The production of semisynthetic penicillins and cephalosporins was facilitated by the fact that 6-amino penicillanic and 7-amino cephalosporanic acids (Fig. 18) could be easily prepared. The side chain is removed by the action of an enzyme or by a chemical hydrolysis and to the amino group in position 6 (penicillins) or 7 (cephalosporins), that was made free in the previous step, another acyl is bound chemically or enzymatically. In such a way, various penicillins and cephalosporins have been prepared to be effective against microorganism...
