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 1800s. 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 drugs.

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 Hodg-kin's disease.

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Read the text and say how antibiotics work according to the following plan:

1. Prevention of cell wall formation.

2. Disruption of the cell membrane.

3. Disruption of chemical processes.

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 cells from splitting open. Penicillins and some other antibiotics destroy pathogenic microbes by hindering the formation of this wall. Human cells do not have rigid cell walls and so are not damaged by these antibiotics.

Disruption of the cell membrane. Some antibiotics, including ampho­tericin В 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 diseases 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 nucleic acids formation.

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.

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Read the text and tell as much as you can about the three main dangers of antibiotics:

— allergic reactions;

— destruction of helpful microbes;

— damage to organs and tissues.

Dangers and Limitations of Antibiotics

Many antibiotics are recognized as safe 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 per cent 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 the 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.

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