Antibiotics Essay, Research Paper

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.