Antibiotics are a group of natural or synthetic compounds that destroy bacteria (bactericidal) or inhibit their growth (bacteriostatic). Antibiotics that are sufficiently nontoxic to the host are used as chemotherapeutic agents in the treatment of infectious diseases of humans, and animals. Nature produces an amazing variety and number of products. In this section we will concentrate on antibiotics and its natural sources. About 100,000 secondary metabolites of molecular weight less than 2500 have been characterized, which are mainly produced by microbes and plants (Roessner and Scott, 1996); Out of which around 50,000 are from microorganisms (Fenical and Jensen, 1993; Berdy, 1995).
The selective action exerted on pathogenic bacteria and fungi by microbial secondary metabolites ushered in the antibiotic era, and for 50 years we have been benefited from this remarkable property of “wonder drugs” such as penicillins, cephalosporins, tetracyclines, aminoglyco sides, chloramphenicol, and macrolides, among others. The successes were so impressive that these antibiotics were virtually the only drugs utilized for chemotherapy against pathogenic microorganisms. By 1996,the world market for antimicrobials amounted to $23 billion and involved some 150 to 300 products, natural, semisynthetic, or synthetic. The $8 billion US antimicrobial market in 1995 included cephalosporins (45%), penicillins (15%), quinolones (11%), tetracyclines (6%) and macrolides (5%) (Strohl, 1997).
About 20 years ago, the difficulty and high cost of isolating novel structures and antimicrobial agents with new mode of action for such uses became apparent, and the field looked like it might enter a phase of decline. Indeed, the number of anti-infective investigational new drugs (INDs) declined by 50% from the 1960s to the late 1980s (DiMasiet al., 1994). However, it was realized that compounds which possess antibiotic activity also possess other activities, that some of these had been quietly exploited in the past, and that such broadening of scope should be exploited and expanded in the future. Thus, a broad screening of antibiotically active molecules for antagonistic activity against pathogenic organisms other than microorganisms, as well as for other pharmacological applications, was proposed in order to yield new and useful lives for “failed antibiotics”. A large number of in vitro laboratory tests were developed to help detect, isolate, and purify useful compounds. Much of this emphasis was brought about by Umezawa (1982), who pointed out the potential importance of enzyme inhibitors as drugs. Fortunately, we entered into a new era in which microbial metabolites were applied to diseases i.e., diseases not caused by bacteria and fungi. Let us see some of the bacterial antibiotics in detail in this chapter.
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