FREE ESSAY ON ANTIBIOTICS

ANTIBIOTICS

46. Antibiotics
Antibiotics are chemical compounds used to kill or inhibit the growth of infectious
organisms. Originally the term antibiotic referred only to organic compounds, produced by
bacteria or molds, that are toxic to other microorganisms. The term is now used loosely
to include synthetic and semisynthetic organic compounds. Antibiotic refers generally to
antibacterials; however, because the term is loosely defined, it is preferable to specify
compounds as being antimalarials, antivirals, or antiprotozoals. All antibiotics share
the property of selective toxicity: They are more toxic to an invading organism than they
are to an animal or human host. Penicillin is the most well-known antibiotic and has been
used to fight many infectious diseases, including syphilis, gonorrhea, tetanus, and
scarlet fever. Another antibiotic, streptomycin, has been used to combat tuberculosis.
Antibiotics can be classified in several ways. The most common method classifies them
according to their action against the infecting organism. Some antibiotics attack the
cell wall; some disrupt the cell membrane; and the majority inhibit the synthesis of
nucleic acids and proteins, the polymers that make up the bacterial cell. Another method
classifies antibiotics according to which bacterial strains they affect: staphylococcus,
streptococcus, or Escherichia coli, for example. Antibiotics are also classified on the
basis of chemical structure, as penicillins, cephalosporins, aminoglycosides,
tetracyclines, macrolides, or sulfonamides, among others.
Most antibiotics act by selectively interfering with the synthesis of one of the
large-molecule constituents of the cell-the cell wall or proteins or nucleic acids. Some,
however, act by disrupting the cell membrane . Some important and clinically useful drugs
interfere with the synthesis of peptidoglycan, the most important component of the cell
wall. These drugs include the B-lactam antibiotics, which are classified according to
chemical structure into penicillins, cephalosporins, and carbapenems. All these
antibiotics contain a B-lactam ring as a critical part of their chemical structure, and
they inhibit synthesis of peptidoglycan, an essential part of the cell wall. They do not
interfere with the synthesis of other intracellular components. The continuing buildup of
materials inside the cell exerts ever greater pressure on the membrane, which is no
longer properly supported by peptidoglycan. The membrane gives way, the cell contents
leak out, and the bacterium dies. These antibiotics do not affect human cells because
human cells do not have cell walls.
Many antibiotics operate by inhibiting the synthesis of various intracellular bacterial
molecules, including DNA, RNA, ribosomes, and proteins. The synthetic sulfonamides are
among the antibiotics that indirectly interfere with nucleic acid synthesis. Nucleic-acid
synthesis can also be stopped by antibiotics that inhibit the enzymes that assemble these
polymers-for example, DNA polymerase or RNA polymerase. Examples of such antibiotics are
actinomycin, rifamicin, and rifampicin, the last two being particularly valuable in the
treatment of tuberculosis. The quinolone antibiotics inhibit synthesis of an enzyme
responsible for the coiling and uncoiling of the chromosome, a process necessary for DNA
replication and for transcription to messenger RNA. Some antibacterials affect the
assembly of messenger RNA, thus causing its genetic message to be garbled. When these
faulty messages are translated, the protein products are nonfunctional. There are also
other mechanisms: The tetracyclines compete with incoming transfer-RNA molecules; the
aminoglycosides cause the genetic message to be misread and a defective protein to be
produced; chloramphenicol prevents the linking of amino acids to the growing protein; and
puromycin causes the protein chain to terminate prematurely, releasing an incomplete
protein.