A new era of exploitation of microbial metabolites

In the past history of the pharmaceutical industry, secondary metabolites have been screened almost exclusively for antimicrobial activities. This biased and narrow view has severely limited the potential application of microbial metabolites. Fortunately, this situation is changing and we are now entering into a new era in which microbial metabolites are being applied to diseases heretofore only subjected to synthetic compounds. This new approach is the application of microbial secondary metabolites to diseases that are not caused by other bacteria or fungi. For years, major drugs such as hypotensive and anti-inflammatory agents that are used for non-infectious diseases have been strictly synthetic products. Similarly, major therapeutics for parasitic diseases in animals (for example, coccidiostats and anthelminthics) resulted strictly from screens of chemically synthesized compounds followed by molecular modification. However, today fermentation products such as monensin and lasalocid dominate the coccidiostat market. The avermectins, another group of streptomycete products, have high activity against helminths and arthropods. Indeed, their activity appears to be an order of magnitude greater than previously discovered anthelminthic agents, the vast majority of which are synthetic compounds. Umezawa's group in Japan has isolated many microbial products with important pharmacological activities by screening with simple enzymic assays. There is much interest in a natural inhibitor of intestinal glucosidase, which is produced by an actinomycete of the genus Actinoplanes. The aim is to decrease hyperglycaemia and triacylglycerol synthesis in adipose tissue, liver and the intestinal wall of patients with diabetes, obesity and type IV hyperlipidaemia. Another natural compound of interest is mevinolin, a fungal product which acts as a cholesterol-lowering agent in animals. Mevinolin is produced by Aspergillus terreus. In its hydroxyacid form (mevinolinic acid), mevinolin is a potent competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase from liver. It is clear that, although the microbe has contributed greatly to the benefit of mankind, we have merely scratched the surface of the potential of microbial activity.