Bacteria also cause disease by destroying body tissues. As bacteria stick to body cells, they secrete digestive enzymes that break down tissue for its nutritional value, which allows further bacterial invasion. For example, some species of the genus Streptococcus make a blood clot-dissolving enzyme that allows bacteria to spread easily from tissue to tissue.
Antibiotics affect bacteria by interfering with certain cellular activities. Penicillin (PEN-i-SIL-in), for example, blocks the ability to build new cell wall material, while tetracycline (TE-truh-SIE-klin) blocks protein synthesis. Antibiotics are made naturally by some fungi and bacteria, and they kill neighboring bacteria or fungi that compete for resources. Scientists have worked to improve the effectiveness of these and other chemicals. Antibiotics that can kill more than one kind of organism are called broad-spectrum antibiotics.
A big worry for modern medicine is antibiotic resistance, the evolution of populations of pathogenic bacteria that antibiotics are unable to kill. Antibiotic resistance may develop in a population of bacteria in several ways. In one case, mutations in bacterial DNA give a bacterium resistance to antibiotics. A cell with such a mutation has a selective advantage when antibiotics are present. Mutant bacteria multiply and take over the population and thus stop the antibiotic's curing power. Figure 23-12 shows how resistance can evolve in a population of bacteria.
Unfortunately, antibiotics have been misused by doctors who overprescribe them and patients who do not finish their course of prescribed antibiotics. Many resistance genes are now present on plasmids called R-plasmids, which can pass easily between many bacteria by transformation. R-plasmids can carry resistance genes to many antibiotics. In this way, they give the bacterium multiple resistances to antibiotics. Many diseases that were once easy to treat with antibiotics, such as TB and staphylococcal infections, are now becoming more difficult to treat because of multiple resistances.
O Individual bacteria in a large population may have mutated genes that give the bacteria resistance to an antibiotic. © If the antibiotic is absent, these resistant cells usually grow very slowly. O The nonmutated cells eventually take over the population. O If the antibiotic is present, however, it kills off the normal cells and leaves the mutant, antibiotic-resistant cells. Q The mutant cells continue to grow and to take over the population. Antibiotics provide a selective advantage to antibiotic-resistant bacteria in this way.
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Modeling the Spread of Disease
Materials disposable gloves, lab apron, and safety goggles; clear plastic cup with liquid
1. Put on your disposable gloves, lab apron, and safety goggles.
3. Pour the contents of your cup into a classmate's cup. Have the classmate pour the liquid back into your cup. Repeat this step with two other classmates. Do not touch anyone else's hands.
4. Predict the number of classmates that have been "exposed" to the "disease." Your teacher will test the liquid in each student's cup to see who has caught the "disease." Count the number of "infected" students.
Analysis How did the actual results compare with your prediction? Was direct contact needed?
Physicians and scientists have become worried by emerging infectious diseases. Diseases such as TB, which is caused by species of the genus Mycobacteria, and staphylococcal post-surgical infections, which are caused by Staphylococcus aureus, are becoming more difficult to treat because of the increased numbers of antibiotic-resistant bacteria. Other diseases, such Lyme disease, are becoming more common than they were in the past. Lyme disease is caused by the bacterium Borrelia burgdorferi. If a tick infected with the bacterium bites a person, the bacterium can enter the person's bloodstream and cause flulike symptoms, arthritis, and sometimes heart inflammation.
Most emerging diseases develop when infectious agents, such as bacteria, pass from wild animals to humans. A disease that can pass from animals to humans is called a zoonosis (zo-uh-NOH-sis). One reason for the emergence of Lyme disease is the flow of human populations into previously untouched natural areas. As people move into these areas, the chance of contact with infected ticks increases. The increase in global travel has also added to the spread of new and difficult-to-treat infectious diseases around the world.
Foodborne illnesses can be a major public health hazard, especially if an outbreak happens in a place such as a restaurant, where food is served to large numbers of people. However, the improper preparation, handling, or storage of food at home may cause most cases of foodborne illness in the United States. Many illnesses, such as campylobacteriosis (caused by Campylobacter jejuni), are relatively mild. Usual symptoms are nausea, vomiting, and intestinal cramps that last less than a week. However, certain foodborne pathogens, such as the E. coli O157:H7 strain, can cause serious illness and complications such as kidney damage in children and older adults. E. coli O157:H7-related illness is usually due to eating undercooked, contaminated hamburger meat.
Many foodborne illnesses can be avoided by selecting, storing, cooking, and handling food properly. For example, choose only fresh, clean foods, and wash raw fruit and vegetables well in clean water before eating them. Wash hands, kitchen towels, and kitchen utensils often in hot, soapy water to avoid cross-contamination during food preparation. Also, avoid cross-contamination of raw and cooked foods by keeping these foods separated during preparation and storage. Always refrigerate raw foods (including eggs) and deli meats at 4°C (39°F). Cook foods, especially meats, thoroughly and heat them to recommended temperatures to kill possible pathogens. Also, refrigerate leftovers promptly, and then reheat them thoroughly before eating them. Wash your hands well with soap and water for 10 to 15 seconds. Always wash your hands after using the bathroom, changing a diaper, handling animals, working outdoors, and coughing or sneezing into your hands.
Timeline i677 Anton van
Leeuwenhoek first observes living bacteria.
1861 Louis Pasteur invents sterilization.
1876 Joseph Lister uses sterile surgical techniques.
1876 Robert Koch proves Bacillus anthracis causes anthrax.
1910 Paul Ehrlich invents Salversan, a drug for syphilis.
1928 Alexander Fleming discovers antibiotic effects of penicillium mold.
1939 Howard Florey and Ernst Chain isolate penicillin and create first antibiotic drug.
1950s Faster antibiotic development and successful prevention and treatment of disease occur.
1980 Molecular techniques aid the study of pathogens.
Present Development of antibiotic resistance spurs more research into antibiotic treatment.
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