Abstract

Antibiotics are chemical substances produced by microorganisms and other living systems that in low concentrations are capable of inhibiting the growth of bacteria or other microorganisms. Many thousands of semisynthetic variations of the naturally occurring antibiotics have been prepared. Only relatively few have become commercial products for human and veterinary uses. The more common mechanisms of action of antibiotics include inhibition of bacteria cell-wall biosynthesis, inhibition of protein, ribonucleic acid (RNA), or deoxyribonucleic acid (DNA) synthesis, and damaging of membranes. Cell-wall biosynthesis is a target present in bacteria, but not in mammalian cells. Thus the -lactams, which inhibit bacterial cell-wall biosynthesis, are very effective against bacterial infections in humans and are relatively nontoxic. In contrast, antibiotics that damage DNA, like adriamycin, are relatively toxic to both types of cells. However, adriamycin, which was found to be more toxic to rapidly proliferating tumor cells than to most normal cells with slower turnover rates, shows significant selectivity against tumor cells to find clinical application as an antitumor agent. Most of the antibiotics introduced since the 1970s have been derived from synthetic modifications of the -lactam antibiotics. Chemical classification of some commercial antibiotic families includes aminoglycosides, which characteristically contain amino sugars and deoxystreptamine or streptamine and are referred to as aminocyclitol aminoglycosides, eg, streptomycin, neomycin, kanamycin, gentamicin, tobramycin, and amikacin; ansamacrolides, also referred to as ansamycins, benzenoid, or naphthalenoid aromatic compounds in which nonadjacent positions are bridged by an aliphatic chain to form a cyclic structure, of which rifampin is a semisynthetically derived member of clinical importance; -lactams, chemically characterized by a -lactam ring, with the substructure groups penicillins, cephalosporins, carbapenems, monobactams, nocardicins, and clavulanic acid (commercially this family is the most important group of antibiotics used to control bacterial infections); chloramphenicol, a nitro benzene derivative of dichloroacetic acid, which inhibits protein biosynthesis; glycopeptides, eg, vancomycin, avoparcin, and teicoplanin, which have cyclic peptide structures and biphenyl containing amino acids and inhibit bacterial cell-wall biosynthesis by binding to d-alanyl-d-alanine units found in the cell walls; lincomycins and celesticetins, a small family that have carbohydrate-type structures, among which clindamycin, a chemical modification of lincomycin, is clinically superior, and that inhibit gram-positive aerobic and anaerobic bacteria by interfering with protein biosynthesis; macrolides macrocyclic lactones in two main subgroups ie, polyene macrolides, that are antifungal agents and include compounds like nystatin and amphotericin B, and antibacterial antibiotics represented by erythromycin and tylosin; polyethers, which contain a number of cyclic ether and ketal units and have a carboxylic acid group, and interact with bacterial cell membranes and allow cations to pass through the membranes causing cell death; and tetracyclines, a small group of antibiotics, eg, tetracycline, minocycline, and doxycycline, characterized as containing a polyhydronaphthacene nucleus that are commercially important and have been used clinically against gram-positive and -negative bacteria, spirochete, mycoplasmas, and rickettsiae and have veterinary applications in promoting growth and feed efficiency. Most of the microorganisms used to produce antibiotics were isolated from soil samples and generally inhabit the top few centimeters of soil. They include actinomycetes, bacteria, and fungi. Actinomycetes are the most productive for antibiotics. Once a pure culture has been found that produces a new antibiotic, improvement in antibiotic yield is accomplished by modification of the fermentation medium or strain selection and mutation of the producing organism. Production may take years to accomplish. Genetic engineering technology has begun to evolve, which allows modifications of a microorganism's DNA so that it will produce new antibiotics. Initial production levels at the discovery phase for a new antibiotic are very low and high levels up to 30 grams per liter or more for commercial production may take years to accomplish. Antibiotics are used as antibacterial agents, anticancer agents, antituberculin agents, antifungal agents, antiviral agents, and in veterinary products and animal feed supplements for growth promotion.