Hundreds of anti-bacterial drugs known as antibiotics are marketed in the United States. About 8000 antibiotics have been found in nature; and currently the most frequently 250 drugs are grouped into about 18 drug families. Drug companies often assign different names to the same antibiotic, so the study of antibiotics is often separated into a special class related to pharmacology. Early antibiotics were discovered in the late 1920s, and their use in controlling bacteria has expanded rapidly. Antibiotics do not inhibit viruses, but are sometimes prescribed during viral infections to reduce the chance of secondary bacterial infections. Probably less than 1% of the known antibiotics are of practical value in medicine. Antibiotics are used more effectively in inhibiting gram-positive bacteria; but some antibiotics have wider use and are called broad-spectrum antibiotics. Antibiotics can be grouped according to their molecular structure or method of action.

Some antibiotics interfere with protein synthesis and are produced during the metabolic activities of organisms like Streptomyces. Many of these antibiotics cause serious side effects and are being replaced by other antibiotics. Antibiotics that alter ribosome activity include tetracycline (used to treat Chlamydia and Rickettsial infections), streptomycin, chloramphenicol (treat meningitis), erythromycin (applied to eyes of newborn infants and used as a substitute for persons allergic to penicillin and to treat Legionella), and gentamycin. It is now recognized that numerous microbes have developed resistance to these antibiotics.

Some antibiotics like polymixin are produced by the bacterium Bacillus, prevent normal cell membrane activity and are used to treat severe infections.

Bacillus, Streptomyces, and molds like Penicillium and Cephalosporium produce antibiotics that interfere with bacterial cell wall formation. These antibiotics are often the "antibiotic of choice" for treating human infections. Do you know why? Our human cells do not have cell walls so the antibiotics are more selectively toxic on bacterial cells and cause less damage to our cells. However, one antibiotic called vancomycin, which is produced by Streptomyces, is very neurotoxic and its use if often limited to life-threatening Staphylococcuc infections in children. Another antibiotic, produced by Bacillus is called bacitracin and commonly found in antibiotic ointments.The antibiotic penicillin is produced by a mold. Can you name the mold? Penicillium. Penicillin is used to prevent secondary infections and inhibit the growth of gram positive bacteria. The natural form (penicillin G) is usually given by injection into muscle tissue because that form of penicillin is destroyed by stomach acids. Penicillin blocks the action of transpeptidase which is the enzyme that links the molecules in the cell walls of bacteria. Some bacteria have mutated and produce an enzyme called Cepenicillinase which inhibits the action of penicillin. If the bacteria involved in the infection is thought to have developed resistance to penicillin, clavulanic acid is usually given with the antibiotic to insure effectiveness of the penicillin.

Natural penicillin is destroyed by stomach acids, binds to foods, quite difficult to absorb, and may cause allergic reactions in about 5% of the population; therefore, several synthetic antibiotics that resemble penicillin have been produced. Synthetic penicillin-like antibiotics include ampicillin, amoxicillin, and methicillin. Some microbes are also developing resistance to methicillin. Penicillin is combined with streptomhycin (PenStrep) and used in dairy cows.A water mold that produces antibiotics is Cephalosporium. These antibiotics from this water mold are broader spectrum than the penicillins, have less side effects including reduced chances of blood clots, and lower allergic reactions. Antibiotics from this Cephalosporium include ceclor, duricef, keflex, and rocephine.

Some microbes develop the ability to resist a drug or have inherent resistance. For example, mycoplasmas lack the typical bacterial cell wall and are not inhibited by penicillin. Some microbes possess a cell membrane that does not permit the entry of the antibiotic. Other microbes alter their genetics and pass on the new alternative metabolic pathway and develop resistant populations of the microbe. This drug resistance (or drug fastness) must be kept to a minimum. We can check the sensitivity of a microbe to an antibiotic by measuring zone sizes using the Kirby-Bauer method. How is drug resistance kept at a minimum? Several actions that can be taken include (1) restricting antibiotics in animal feed, (2) requiring prescriptions for antibiotic use, (3) reduction of antibiotic use, and (4) withdaw antibiotics from purchase for various periods of time.

Since the widespread use of antibiotics began in the 1950s, many microbes have developed resistance. Penicillin has been replace by ceftriazone in the treatment of gonorrhea, and significantly higher doses of penicillin are needed to adequately inhibit Streptococcus pyogenes (the bacterium that causes strept throat). Several recent outbreaks of food poisoning have been related to the use of antibiotics in animal feeds to promote animal growth and prevent the occurrence of disease. Many children in Hungary were treated with penicillin for ear and sinus infections; by 1980 fifty percent of the infections were caused by penicillin-resistant Streptococcus . As Hungarian physicians reduced the use of penicillin, there was a corresponding reduction in the percent of resistant microbes. Careful monitoring of antibiotic use and education of the people is essential.

What is the ideal drug for the future? Pharmaceutical companies need to research and develop drugs that are selectively toxic to the pathogen, cidal and not static, able to function when diluted, reasonably priced and convenient, not harmful to normal flora, and produce few side effects. Antibiotics were once thought to be the ultimate cure-all; now and in the future we must use antibiotics carefully and appropriately.