Antibiotics
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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