Antibiotic, pronounced AN tee by AHT ihk or pronounced an tih by AHT ihk, is a drug produced by certain microbes. Antibiotic substances are obtained from bacteria and fungi that live in the air, soil, and water. Most antibiotics are used by physicians to fight various diseases caused by harmful microbes. A few are used to treat certain cancers.

    Antibiotics are selectively toxic--that is, they damage certain types of cells, but do not damage others. Many antibiotics are harmful to the cells of pathogenic (disease-causing) microbes, but they do not normally damage human cells. Physicians use these types of antibiotics to treat a variety of bacterial diseases, including gonorrhea, syphilis, and tuberculosis, and infections caused by staphylococcal and streptococcal bacteria. A small number of antibiotics, however, were developed to attack human cells. Some of these are used to treat cancer. They are selectively toxic mostly because they only damage cells that are in the process of dividing.

    Antibiotics are sometimes called "wonder drugs" because they can cure many diseases that once were often fatal. The number of deaths that are caused by meningitis, pneumonia, and scarlet fever has declined drastically since people began using antibiotics.

    Antibiotics are also used to treat infectious diseases in animals and to control bacteria and fungi that damage fruits and grains. Farmers sometimes add small amounts of antibiotics to livestock feed to stimulate the animals' growth. Small quantities of antibiotics are also used as food preservatives.

Kinds of antibiotics

    There are more than 70 clinically useful antibiotics. Most of them are used to treat bacterial infections. Others fight harmful fungi and protozoa, and a few are used to treat cancer. Antibiotics are not effective against most viruses, and so they cannot be used for chickenpox, measles, and most other viral diseases.

    Antibacterial antibiotics. Many bacteria can be classified as either Gram positive (G+) or Gram negative (G-). This classification method was developed by Hans C. J. Gram, a Danish bacteriologist of the late 1800's. According to Gram's system, many bacterial infections are classified as either G+ or G-, depending on the type of bacteria that caused them. The bacteria in each group have certain characteristics that help determine the sensitivity of these microorganisms to antibiotics. Some antibiotics are most effective against G+ infections, and others work best for G- infections. Both these kinds of drugs are called limited-spectrum antibiotics. However, various broad-spectrum antibiotics fight G+ and G- infections, as well as other bacterial infections.

    Antibiotics used to treat chiefly G+ infections include clindamycin, erythromycin, and penicillin G. Those used for mainly G- infections include colistin and gentamicin. Such antibiotics as chloramphenicol and tetracycline fight both G+ and G- infections, as well as other types of bacterial infections. No limited-spectrum antibiotic works against all G+ infections or against all G- infections. Similarly, no broad-spectrum antibiotic is effective against all bacterial infections. Research has shown which antibiotics work best against certain infections, and physicians follow these guidelines when prescribing the drugs.

  Some widely used Antibiotics

    Other kinds of antibiotics. Antibiotics that fight pathogenic fungi include nystatin and griseofulvin. For example, griseofulvin is used to treat various fungus diseases, such as ringworm and other types of skin infections. Paromomycin is used to treat amebiasis, a disease caused by protozoa. Anticancer antibiotics include doxorubicin, which is used to treat acute leukemias, and bleomycin, which is used to treat Hodgkin's disease.

How antibiotics work

    Antibiotics fight pathogenic microbes and cancer cells by interfering with their normal cell processes. In most cases, this interference can occur in one of three ways: (1) prevention of cell wall formation, (2) disruption of the cell membrane, and (3) disruption of chemical processes.

    Prevention of cell wall formation. The contents of bacterial cells are enclosed in a membrane that is surrounded by a rigid wall that prevents the cells from splitting open. Penicillins and some other antibiotics destroy pathogenic microbes by hindering the formation of this wall. Human cells do not have nor need rigid cell walls and so are not damaged by these antibiotics.
 
    Disruption of the cell membrane. Some antibiotics, including amphotericin B and nystatin, disrupt the cell membrane of certain microbes. This membrane controls the movement of materials in and out of the cell. If the membrane is disrupted, vital nutrients may escape from the cell, or poisonous substances may enter and kill the cell. But the membranes of human cells are not affected because these antibiotics disrupt cell membranes that contain elements found only in microbial cells.

    Disruption of chemical processes. All cells produce proteins and nucleic acids, which are vital to the life of any organism. Some antibiotics fight disease by interfering with the chemical processes by which these substances are produced. For example, streptomycin and tetracycline prevent certain kinds of microbes from producing proteins, and rifampin interferes with the formation of nucleic acids.

    Human cells produce proteins and nucleic acids in much the same way that microbial cells do. But these processes differ enough so that some antibiotics interfere with chemical activities in microbial cells but not in human cells. However, the antibiotics that are used to treat cancer interact with DNA (deoxyribonucleic acid), thereby preventing human cancer cells from dividing.

Dangers and limitations of antibiotics

    Many antibiotics are recognized as among the safest drugs when properly used. But antibiotics can cause unpleasant or dangerous side effects. The three main dangers are (1) allergic reactions, (2) destruction of helpful microbes, and (3) damage to organs and tissues. The effectiveness of antibiotics is sometimes limited because pathogenic microbes can become resistant to them.

    Allergic reactions, in most cases, are mild and produce only a rash or fever. But a severe reaction to the drug may result in death. Although all antibiotics can produce allergic reactions, such reactions occur most frequently with penicillins. About 10 percent of the people of the United States are allergic to penicillins. Before prescribing an antibiotic, physicians usually ask if the patient has ever had an allergic reaction to the drug. Most people who are allergic to one antibiotic experience no such reaction to another one that has a significantly different chemical composition.
   
    Destruction of helpful microbes. Certain areas of the body commonly harbor both harmless and pathogenic microbes. These two types of microbes compete for food, and so the harmless microorganisms help restrain the growth of those that cause disease.

    Many antibiotics--especially broad-spectrum drugs--do not always distinguish between harmless and dangerous microbes. If a drug destroys too many harmless microorganisms, the pathogenic ones will have a greater chance to multiply. This situation often leads to the development of a new infection called a suprainfection. In most instances, physicians prescribe a secondary drug to combat the suprainfection.

    Damage to organs and tissues is rare in people using antibiotics that act only against the cells of pathogenic microbes. However, extensive use of some of these antibiotics may damage tissues and organs. For example, streptomycin, which is used to treat tuberculosis, has caused kidney damage and deafness. Physicians only take such risks if there is no other effective drug.

    Anticancer antibiotics are most toxic to groups of cells that are constantly dividing, such as cancer cells. However, some normal cells, especially cells in the bone marrow, stomach, and intestines, are also constantly dividing. Healthy tissues made up of such cells can be damaged by use of anticancer antibiotics.
   
    Resistance to antibiotics may be acquired by pathogenic microbes. Such resistance develops through changes in the genetic information of microbial cells. In some cases, it develops when a spontaneous genetic change called a mutation occurs. In other cases, resistant microbes transfer genetic material to nonresistant microbes and cause them to become resistant. During antibiotic treatment, nonresistant microbes are destroyed, but resistant types survive and multiply. Thus, extensive use of antibiotics can encourage the growth of resistant species. Mutations may also occur in cancer cells, making them resistant to anticancer antibiotics.
  
Testing and producing antibiotics

    Testing. Every year, scientists test thousands of natural and chemically modified microbial substances for potential use as antibiotics. First, these substances are tested against harmful microbes or cancer cells that have been grown either in test tubes or on laboratory plates.

    A substance that shows strong antibiotic activity against pathogenic microbes or cancer cells is tested extensively in laboratory animals. Then, if it produces no harmful effects in the animals, the antibiotic is tested in human beings. In the United States, human testing must be approved by the Food and Drug Administration (FDA). If the drug proves to be safer and more effective than antibiotics already being used, it is submitted for approval to the FDA. Finally, if the FDA approves the antibiotic, drug firms begin to produce it commercially.

     Production of antibiotics involves several steps. First, cultures of the antibiotic-producing microbes are grown in flasks and then transferred to huge fermentation vats (see FERMENTATION). The microbes multiply rapidly in the vats because the environment is controlled to stimulate their growth. After fermentation, the antibiotic substance is extracted from the culture and purified.

    Some natural antibiotic substances are modified chemically to produce semisynthetic antibiotics. Many such drugs are more effective than the natural antibiotics from which they came.

    Drug companies conduct special tests on antibiotics during and after production to ensure their quality. Finally, the purified antibiotic substances are made into pills, liquids, and ointments for medical use.

History

    For more than 2,500 years, people have treated certain skin infections with molds that form antibiotics. However, modern scientific study of these substances did not begin until the late 1800's. At that time, the great French chemist Louis Pasteur discovered that bacteria spread infectious diseases. Then Robert Koch, a German bacteriologist, developed methods of isolating and growing various kinds of bacteria. Koch also identified specific bacteria that cause certain diseases.

    Scientists then began working to develop drugs that could destroy pathogenic microbes, but the substances they produced proved either ineffective or dangerous. A historic breakthrough came in 1928, when the British bacteriologist Alexander Fleming observed that a mold of the genus Penicillium produced a substance that destroyed bacteria. He called the substance penicillin. Fleming recognized the potential use of penicillin in treating disease, but difficulty in extracting it from the mold hindered further experimentation.

    In the late 1930's, two British scientists, Ernst B. Chain and Howard W. Florey, developed a method of extracting and purifying small amounts of penicillin. The first successful medical treatment with penicillin occurred in 1941, when a British policeman suffering from bacterial blood poisoning received the drug. But the small supply of penicillin prevented physicians from using the drug extensively. A high-yielding type of Penicillium mold was discovered in 1943, and penicillin production increased greatly.

    In the early 1940's, the American bacteriologist Selman A. Waksman tested about 10,000 types of soil bacteria for antibiotic activity. In 1943, he discovered that some Streptomyces, a type of fungi, produced a substance that had potent antibiotic properties. A new antibiotic called streptomycin resulted from Waksman's research.

    Thousands of antibiotic substances have been found in nature or have been produced chemically. However, relatively few antibiotic substances have proven to be safe and effective. In addition, certain types of pathogenic microbes have acquired resistance to some antibiotics.

    Contributor: N. E. Sladek, Ph.D., Prof. of Pharmacology, Univ. of Minnesota, Twin Cities Campus.

 

Copyright © 1999 [Toxicology Associates, Inc.]. All rights reserved.
Revised: January 13, 2010