ranes of resistant strains are often less sensitive to the effect of antibiotics. This kind of resistance is thought to be connected with the content of phospholipids in the cell. In Bacillus colistinus, a colistin producer, the content of phospholipids in the cell-free extract increased with the sensitivity to the antibiotic.Another way how the antibiotic producers can avoid the effect of their products is by situating the distal enzymes of the antibiotic biosynthetic pathway (synthases) outside the cell, most often in the periplasm. In Streptomyces aureofaciens, a higher proportion of the outside terminal tetracycline synthase was found in production strains under high production conditions in periplasm, as compared to low production conditions [18].
Resistance in pathogenic microorganisms. Shortly after antibiotics were introduced into clinical practice on a massive scale, strains of hitherto-sensitive microorganisms started to appear, that required the use of much higher antibiotic concentrations or, even, were completely resistant to these antibiotics. The resistant strains originated from clones that survived the antibiotic treatment, especially if the treatment was terminated before all pathogenic microorganisms were killed or the antibiotic was applied at sublethal doses. There are several ways how microorganisms can gain resistance .In most resistant microorganisms, the main mechanisms of resistance are detoxification or inactivation of the antibiotic, change of the target site, blocking of the transport of the antibiotic out of the cell.
Penicillins and cephalosporins are degraded using three ways :) by the enzyme penicillin amidase that cleaves the amidic bond by which the side chain is bound to the? -lactam ring,
b) by the enzyme acetyl esterase that hydrolyzes the acetyl group at C - 3 on the dihydrazine ring of cephalosporins,) by the enzyme? -lactamase that catalyzes hydrolysis of the? -lactam ring of penicillins and cephalosporins.Penicillin amidases are rarely used by microorganisms to build up resistance to? -lactam antibiotics. They are often employed for the synthesis of semisynthetic antibiotics. Acetyl esterase is also not important from the point of view of antibiotic resistance. In most cases,? -lactam Antibiotics are inactivated by? -lactamase That destroys one of the important sites for their antibiotic activity; the damage is irreversible.
1.2.1 Regulation of antibiotic production
Overproduction of secondary metabolites. Microorganisms produce in natural environment only small amount of antibiotics. They have to control the antibiotic synthesis since secondary metabolites at high concentrations are mostly toxic even for their producers. Using high-yielding strains and optimization of fermentation condition we can reach many times higher production. In that case we speak about overproduction . Production of antibiotics in factories are at present several thausands higher as production of original strains isolated from nature but this high production is reached only when high-yilding strain is used and special conditons of cultivation are kept. The main factors influence production of antibiotics are discused in next chapters. phases of microbial culture. A culture of a microorganism capable of antibiotic production, where the overproduction of the antibiotic is taking place, includes several growth phases representing a number of physiological states.
Preparatory phase (lag phase) - the culture is adapting to the new environment, the growth is slow and, evidently, regulatory proteins are being synthesized that, on the basis of the information from the environment, activate the expression of the respective genes during cultivation.phase - the culture grows intensively, usualy a low amount of antibiotic is synthesized.phase - growth rate and proteosynthesis slowed down; the antibiotic production is started. The enzymes of secondary metabolism are intensively synthesized .phase - growth is practically ceased, dry weight of microorganism is constant, the antibiotic is intensively synthesized. producers mostly belong to filamentous microorganisms, which means that, in their culture, cells of various age and at different stages of development are present. The microorganisms grow in pellets, inside which the cultivation conditions differ from those on the pellet surface (nutrient concentrations, oxygen concentration, etc.). An increase in dry weight does not have to always mean the biomass growth since, in streptomycetes, often a thickening of the cell wall or glycocalyx formation occur that increase the dry weight value without rising the cell number. The individual cells can thus be at different stages of development, ie in different physiological state...
