**1.2 Antibiotics and antimicrobial agents**

From the beginning until now, the humankind has always been faced with a problem of spreading of infectious diseases. Today, more than 150 compounds make arsenal of antimicrobial substances used in the treatment of infectious diseases. Antibiotics are defined as low molecular weight organic natural products (secondary metabolites or idiolites) made by microorganisms, which are active at low concentrations against other microorganisms. There are estimations that among 12,000 antibiotics known, approximately 55% are produced by Streptomyces, 11% by other Actinomycetes, 12% from other bacteria and 22% from filamentous fungi (Inouye et al., 2004). In its broadest definition an antibacterial is an agent that interferes with the growth and reproduction of bacteria. Unlike antibiotics, antibacterials are not used as medicine for humans or animals, but are now most commonly described as agents used to disinfect surfaces and eliminate potentially harmful bacteria found in products such as soaps, detergents, health and skincare products and household cleaners.

Since Alexander Fleming`s discovery, in 1928, of the first antibiotic, called penicillin, produced by the mold *Penicillium chrysogenum*, a real revolution in medicine with a new era of antibiotics have started. Later, the entire group of β-lactam antibiotics (penicillins and cephalosporins) was discovered, followed by the Waxman`s discovery of streptomycin derived from Streptomyces bacteria, used in a treatment of tuberculosis), and then tetracyclines, quinolones, antifungal metabolites, antiparasitic substances and more recently antiviral drugs such as acyclovir. In 1971, the second significant antibiotic cyclosporin A and C were isolated from fungal organism *Hypocladium inflatum gams* (*Tolypocladium inflatum*) which is the asexual state of the pathogen of beetles *Elaphocordyceps subsessilis* (Petch) G.H. Sung, J.M. Sung & Spatafora). Its immunosupresive activity was revealed in 1976 by J.F. Borel and was approved for use 1983 in order to reduce the risk of organ rejection in transplant surgery (Upton, 2001 as cited in Giovannini, 2006).
