Metabolites, the intermediate products formed as a result of metabolic reactions catalyzed by different enzymes. Metabolites are usually classified into two Primary metabolites and Secondary metabolites. Secondary metabolites released from microorganisms are generally low molecular mass products and are not essential for the growth of the microorganism. However, these can have diverse important functions in nature. A secondary metabolite is called “secondary” merely because it has no apparent involvement in the vegetative

growth of the producing culture and, not because it is produced after growth. Probably the most important use of microbial secondary metabolites has been as anti-infective drugs or antibiotics. The secondary metabolites are usually formed during the late growth phase of the producing microorganisms. Synthesis of secondary metabolites can be greatly influenced by manipulating the type of nutrients and their concentration in the formulation of culture media. Actinomycetes particularly Streptomyces are major sources of novel secondary

metabolites with a range of biological activities that may ultimately find application as anti-infective, anticancer agents, or other pharmaceutically useful compounds. Though the nature of secondary metabolism is dependent on genetic information but its expression can be influenced greatly by environmental manipulations. Thus, secondary metabolism occurs by the exhaustion of a nutrient, or addition of an inducer and/or by a decrease in growth rate. Growth rate control appears to be important in secondary metabolism and may be the overriding factor in the cases where the nutrient limitation is needed for the production of secondary metabolites. The delay often seen before the onset of secondary metabolism was probably established by evolutionary pressures or selection pressure (environmental conditions). Many secondary metabolites have antibiotic activity and could kill the producing culture if made too early. Most of the secondary metabolites are formed via enzymatic pathways rather than by a ribosomal mechanism. Secondary metabolism is highly regulated by various factors, particularly carbon and nitrogen sources. Regulation by the carbon source – Glucose, which is an excellent carbon source for the growth of many bacteria, interferes with the formation of many secondary metabolites. In media containing a mixture of a rapidly used carbon source and slowly used carbon sources,

the rapidly used carbon source is used first to produce cells but very little or no secondary metabolite is formed. Regulation by the nitrogen source – It has been seen that many secondary metabolic pathways are negatively affected by nitrogen sources though they were seen to be favorable for the growth of the microorganisms. e.g. ammonium salts. Thus, these value-added secondary metabolites can be obtained from the microorganism by regulation of the metabolic pathways.