27 November 2008
Mechanisms of genetic resistance to antimicrobial agents: Bacteria have developed, or will develop, genetic resistance to all known antimicrobial agents that are now in the marketplace. The five main mechanisms that bacteria use to resist antibacterial drugs are shown in the figure. a | The site of action (enzyme, ribosome or cell-wall precursor) can be altered. For example, acquiring a plasmid or transposon that codes for a resistant dihydrofolate reductase confers trimethoprim resistance to bacteria52. b | The inhibited steps can be by-passed. c | Bacteria can reduce the intracellular concentration of the antimicrobial agent, either by reducing membrane permeability, for example, as shown by Pseudomonas aeruginosa53, or by active efflux of the agent54. d | They can inactivate the drug. For example, some bacteria produce beta-lactamase, which destroys the penicillin beta-lactam ring50, 51 . e | The target enzyme can be overproduced by the bacteria. The invention: Sulfonamides and other drugs that have proved effective in combating many previously untreatable bacterial diseases. The people behind the invention: Gerhard Domagk (1895-1964), a German physician who was awarded the 1939 Nobel Prize in Physiology or Medicine Paul Ehrlich (1854-1915), a German chemist and bacteriologist who was the cowinner of the 1908 Nobel Prize in Physiology or Medicine. The Search for Magic Bullets Although quinine had been used to treat malaria long before the twentieth century, Paul Ehrlich, who discovered a large number of useful drugs, is usually considered the father of modern chemotherapy. Ehrlich was familiar with the technique of using dyes to stain microorganisms in order to make them visible under a microscope, and he suspected that some of these dyes might be used to poison the microorganisms responsible for certain diseases without hurting the patient. Ehrlich thus began to search for dyes that could act as “magic bullets” that would destroy microorganisms and cure diseases. From 1906 to 1910, Ehrlich tested numerous compounds that had been developed by the German dye industry. He eventually found that a number of complex trypan dyes would inhibit the protozoans that caused African sleeping sickness. Ehrlich and his coworkers also synthesized hundreds of organic compounds that contained arsenic. In 1910, he found that one of these compounds, salvarsan, was useful in curing syphilis, a sexually transmitted disease caused by the bacterium Treponema. This was an important discovery, because syphilis killed thousands of people each year. Salvarsan, however, was often toxic to patients, because it had to be taken in large doses for as long as two years to effect a cure. Ehrlich thus searched for and found a less toxic arsenic compound, neosalvarsan, which replaced salvarsan in 1912. In 1915, tartar emetic (a compound containing the metal antimony) was found to be useful in treating kala-azar, which was caused by a protozoan. Kala-azar affected millions of people in Africa, India, and Asia, causing much suffering and many deaths each year. Two years later, it was discovered that injection of tartar emetic into the blood of persons suffering from bilharziasis killed the flatworms infecting the bladder, liver, and spleen. In 1920, suramin, a colorless compound developed from trypan red, was introduced to treat African sleeping sickness. It was much less toxic to the patient than any of the drugs Ehrlich had developed, and a single dose would give protection for more than a month. From the dye methylene blue, chemists made mepacrine, a drug that was effective against the protozoans that cause malaria. This chemical was introduced in 1933 and used duringWorldWar II; its principal drawback was that it could cause a patient’s skin to become yellow. Well Worth the Effort Gerhard Domagk had been trained in medicine, but he turned to research in an attempt to discover chemicals that would inhibit or kill microorganisms. In 1927, he became director of experimental pathology and bacteriology at the Elberfeld laboratories of the German chemical firm I. G. Farbenindustrie. Ehrlich’s discovery that trypan dyes selectively poisoned microorganisms suggested to Domagk that he look for antimicrobials in a new group of chemicals known as azo dyes. A number of these dyes were synthesized from sulfonamides and purified by Fritz Mietzsch and Josef Klarer. Domagk found that many of these dyes protected mice infected with the bacteria Streptococcus pyogenes. In 1932, he discovered that one of these dyes was much more effective than any tested previously. This red azo dye containing a sulfonamide was named prontosil rubrum. From 1932 to 1935, Domagk began a rigorous testing program to determine the effectiveness and dangers of prontosil use at different doses in animals. Since all chemicals injected into animals or humans are potentially dangerous, Domagk determined the doses that harmed or killed. In addition, he worked out the lowest doses that would eliminate the pathogen. The firm supplied samples of the drug to physicians to carry out clinical trials on humans. (Animal experimentation can give only an indication of which chemicals might be useful in humans and which doses are required.) Domagk thus learned which doses were effective and safe. This knowledge saved his daughter’s life. One day while knitting, Domagk’s daughter punctured her finger with a needle and was infected with a virulent bacteria, which quickly multiplied and spread from the wound into neighboring tissues. In an attempt to alleviate the swelling, the infected area was lanced and allowed to drain, but this did not stop the infection from spreading. The child became critically ill with developing septicemia, or blood poisoning. In those days, more than 75 percent of those who acquired blood infections died. Domagk realized that the chances for his daughter’s survival were poor. In desperation, he obtained some of the powdered prontosil that had worked so well on infected animals. He extrapolated from his animal experiments how much to give his daughter so that the bacteria would be killed but his daughter would not be poisoned. Within hours of the first treatment, her fever dropped, and she recovered completely after repeated doses of prontosil. Impact Directly and indirectly, Ehrlich’s and Domagk’s work served to usher in a new medical age. Prior to the discovery that prontosil could be use to treat bacterial infection and the subsequent development of a series of sulfonamides, or “sulfa drugs,” there was no chemical defense against this type of disease; as a result, illnesses such as streptococcal infection, gonorrhea, and pneumonia held terrors of which they have largely been shorn.Asmall injury could easily lead to death. By following the clues presented by the synthetic sulfa drugs and how they worked to destroy bacteria, other scientists were able to develop an even more powerful type of drug, the antibiotic. When the American bacteriologist Rene Dubos discovered that natural organisms could also be used to fight bacteria, interest was renewed in an earlier discovery by the Scottish bacteriologist Sir Alexander: the development of penicillin. Antibiotics such as penicillin and streptomycin have become some of the most important tools in fighting disease. Antibiotics have replaced sulfa drugs for most uses, in part because they cause fewer side effects, but sulfa drugs are still used for a handful of purposes. Together, sulfonamides and antibiotics have offered the possibility of a cure to millions of people who previously would have had little chance of survival.