Bacterial diseases refer to a large variety of diseases caused by bacteria or bacterial components that affect humans, domesticated animals, wildlife, fish, and birds. Most of these diseases are contagious—that is, they can be passed from one member of a species to another member, or, in a smaller number of instances, from one species to a different species. Depending on the organism, bacterial disease can be spread in different ways. Examples include contaminated food or water, air currents, infection of an environment that is not normally inhabited by the particular bacterium, and the possession or release of toxins by the bacteria.
The history and characteristics of bacterial diseases are as varied as the diseases caused. Bacillus anthracis, the cause of anthrax, and Yersinia pestis, the cause of plague, have been present for millennia. Indeed, references to these diseases can be found in chapters of the Old Testament. Other bacterial infections have arisen only very recently. One example is the severe diarrheal and potentially kidneydestroying infection caused by the consumption of water or food that is contaminated by Escherichia coli strain O157:H7. The effects of O157:H7 are due to a celldamaging toxin that can be released by the bacteria. Scientists who have studied this bacterium argue that this strain arose in the 1970s when E. coli residing in the intestinal tract (their normal environment) acquired genetic material that coded for the production of a destructive toxin from a related bacteria, Shigella.
Some bacterial diseases depend on the number of infecting bacteria present, and so are related to the growth of the bacteria. One example is the intestinal upset, diarrhea, and vomiting that results from the growth of Campylobacter following the ingestion of contaminated food or water. Poultry is a particularly important source of this infection, since the bacterium is a normal inhabitant of the intestinal tract of poultry. Release of intestinal contents during slaughter contaminates over 50% of the poultry sold each year in the United States, according to the U.S. Food and Drug Administration. The symptoms of Campylobacter can take a few days to develop, since the bacteria need time to reach sufficient numbers in the intestinal tract.
Other bacterial infections, particularly those involving toxins, require the presence of only a few bacteria, and growth of the bacteria is not necessary to produce the disease.
Bacterial diseases also vary in their methods of establishing infection. Some bacteria readily cause infections, since they are contagious—they can be easily passed from person-to-person, or can be easily spread to humans via a vector (another organism that transmits the bacteria from their normal host to a susceptible recipient). An example of a contagious bacterial disease is plague, which is caused by Yesinia pestis, and which is passed to people via the bite of an infected flea. Throughout history, plague has claimed millions of lives. In contrast, other bacteria cause infections opportunistically—that is, they are not normally infectious but can cause disease under certain circumstances. An example is Pseudomonas aeruginosa, abacterium normally found in soil which is normally of little consequence to humans. However, in burn victims, the organism can infect the damaged skin. In addition, people who have cystic fibrosis and whose lungs can contain deposits of a thick mucus can be susceptible to recurring P. aeruginosa infections that can progressively compromise lung function.
Another means by which a few types of bacteria are able to cause infection is via their production of an environmentally hardy structure known as a spore. Similar to plant spores, bacterial spores are designed to help a bacterium survive tough environmental challenges, which can include temperatures that are too high or low for growth and lack of moisture. In a more hospitable environment, the spore can germinate and bacterial growth and division will resume. Bacteria in the genus Bacillus can form spores and, when they germinate, cause disease. A well known example is B. anthracis, which causes anthrax. Inhalation of only about 10 spores can be sufficient to cause pulmonary anthrax.
Bacterial disease occurs virtually worldwide, with the exceptions of the far North and Antarctica and at very high altitudes. Even temperate waters can harbor diseasecausing (pathogenic) bacteria, such as Vibrio cholerae, the cause of cholera.
Antibiotics are the standard treatment for bacterial infections caused by organisms sensitive to their actions. The type of antibiotic and the concentration required to kill the target bacteria depend on the organism. Frequently, bacteria develop resistance to a variety of antibiotics. Vaccines continue to be a valuable means of preventing bacterial diseases, such as diphtheria, meningococcal disease, and pertussis (whooping cough).
Bacterial diseases can be prevented in a variety of ways. Avoiding the source of the organism (for example, not drinking contaminated water), practicing good hygiene, such as regular handwashing with an antibacterial soap, and maintaining a balanced and healthy diet to keep the body's immune system efficient, are a few examples of good preventive measures.
Bacterial diseases have been responsible for countless millions of deaths and continue to be a significant problem. Only a few decades ago, it was thought that many bacterial infections had been brought under control with the discovery or synthesis of a variety of antibiotics that were tremendously effective. However, this optimism has been short-lived. Antibiotic resistance is looming as one of the great medical challenges of the twenty-first century.
As of 2007, a number of vaccines that can be given orally are under development for several bacterial diseases. Vaccines against the intestinally damaging types of Escherichia coli, Shigella, and Campylobacter will hopefully lessen the occurrence of the diarrhea caused by these organisms. These and some other vaccines under development have the advantage of being stable at nonrefrigeration temperatures, which would make them suitable for rural areas of underdeveloped and developing countries.
While progress is being made to lessen the occurrence of some bacterial diseases, the threat posed by the deliberate malicious use of disease-causing bacteria remains. Biological warfare has been practiced for centuries. In the twentieth century, a number of countries, including the United States, experimented with the use of bacteria as a weapon. Now, this threat has moved from governments to organizations and individuals. The use of bacteria as a biological weapon has become part of the nightly news. As exemplified by the deliberate contamination of letters with Bacillus anthracis in Washington, D.C., in 2001, the danger posed by bioterrorism is real and difficult to prevent.
WORDS TO KNOW
ANTIBIOTIC RESISTANCE: The ability of bacteria to resist the actions of antibiotic drugs.
RE-EMERGING DISEASE: Many diseases once thought to be controlled are reappearing to infect humans again. These are known as re-emerging diseases because they have not been common for a long period of time and are starting to appear again among large population groups.
SPORE: A dormant form assumed by some bacteria, such as anthrax, that enables the bacterium to survive high temperatures, dryness, and lack of nourishment for long periods of time. Under proper conditions, the spore may revert to the actively multiplying form of the bacteria.
The specter of bioterrorism and the often frenzied reporting of bacterial disease outbreaks has spawned a growing apprehension in many people about diseases that are, in fact, not common. For example, the fear of the bacteria that cause necrotizing fasciitis, termed the “flesh-eating bacteria” by the media, is out of proportion to the handful of cases that occur in North America each year.
What is a more realistic concern is the emergence or re-emergence of bacterial diseases that do pose a health threat. One example is the re-emergence of tuberculosis. The emerging strains are also more antibiotic resistant than their predecessors. In developing nations, multidrug resistant tuberculosis is considered an emergency by agencies such as the World Health Organization (WHO). WHO and other agencies, including the U.S. Centers for Disease Control and Prevention, have spearheaded surveillance and notification campaigns designed to detect and respond rapidly to such outbreaks.
Brunelle, Lynn, and Barbara Ravage. Bacteria. Milwaukee: Gareth Stevens, 2003.
Roemmele, Jacqueline A., and Donna Batdorff. Surviving the Flesh-eating Bacteria: Understanding, Preventing, Treating, and Living with Necrotizing Fasciitis. New York: Avery, 2003.
Centers for Disease Control and Prevention. “Division of Bacterial and Mycotic Diseases Home Page.” May 12, 2006. <http://www.cdc.gov/ncidod/dbmd/index.htm> (accessed March 27, 2007).