Aids

Chicken pox/Shingles Common colds Diabetes (some) Encephalitis Genital herpes Infectious hepatitis Influenza Measles

Meningitis (some)

Mononucleosis

Mumps

Pneumonia (some)

Poliomyelitis

Rabies

Rubella

Smallpox

Yellow fever

Stop Action Sneeze
Figure 17.6 Stop-action photograph of a sneeze. (Reprinted with the permission of Marshall W. Jennison, Syracuse University)

resistance is not there, however, a number of diseases, including diphtheria, whooping cough, some forms of meningitis and pneumonia, and strep throat, can develop. Some bacteria such as anthrax require inhalation of a minimum of about 8,000 spores for the disease to develop internally in humans.

Psittacosis, a disease carried by birds, is caused by the inhalation of chlamydias, which are exceptionally minute organisms unable to manufacture their own ATP molecules. They are apparently energy parasites, depending on their host cells for the energy needed to carry on their own functions. Some chlamydias are transmitted sexually and in recent years have become a widespread human problem.

Access Through Contamination of Food and Drink

Food Poisoning and Diseases Associated with Natural Disasters. In the past, open sewers and unsanitary conditions for food preparation caused a number of bacterial diseases, including cholera, dysentery, Staphylococcus and Salmonella food poisoning, and typhoid and paratyphoid fevers, to reach epidemic proportions all over the world. Although Staphylococcus food poisoning, which is seldom fatal, is still fairly common today, developed countries now rarely see epidemics caused by other bacteria unless a natural disaster, such as a flood or a typhoon, disrupts normal sewage disposal. The diseases are more often spread by carriers who handle food, or by houseflies.

The United States and other countries with ocean shorelines ban the harvesting of shellfish at certain times because Salmonella bacteria may multiply enough in clams and mussels to cause illness in humans who eat them—particularly if they are not well cooked. The Salmonella bacteria multiply in the intestinal tract or spread from there to other parts of the body.

Legionnaire Disease. Some bacteria are very common on algae in freshwater streams, lakes, and reservoirs. One such bacterium, Legionella pneumophila, causes Legionnaire disease, which killed 34 members of the American Legion attending a convention in a Philadelphia hotel in 1976. An estimated 50,000 Americans are infected annually by Legionnaire disease bacteria, which nearly always pass through the human digestive tract harmlessly without multiplying. On rare occasions, however, something unknown triggers their reproduction, mostly in older males who are heavy smokers or alcoholics. The results are often fatal.

Botulism. The most deadly of all known biological toxins is produced by a bacterium with the scientific name of Clostridium botulinum (Fig. 17.7). The name comes from botulus, the Latin word for sausage, since the bacterium was first discovered after people had died from eating some contaminated sausage at a picnic.

Kingdom Bacteria, Kingdom Archaea, and Viruses 307

Papaver Plant Disease
Figure 17.7 Botulism bacteria, x1500. (Courtesy Robert McNulty)

Unlike Salmonella food poisoning, botulism is not an infection but is poisoning from a substance produced by bacteria that can grow and multiply anaerobically (in the absence of oxygen) in improperly processed or stored foods. Home-canned beans, beets, corn, and asparagus, in particular, have been known to permit the development of botulism bacteria. Just 1 gram (0.035 ounce) of the toxin is enough to kill 14 million adults, and a little more than half a kilogram (1.1 pound) could eliminate the entire human race.

The bacteria, which are present in most soils, produce unusually heat-resistant spores and are likely to be present on any soil-contaminated foods. They may not be destroyed during canning unless the food is heated for 30 minutes at 80°C (176°F) or boiled for 10 to 15 minutes. They ordinarily will not grow in foods that have been preserved in brines containing at least 10% salt (sodium chloride) or in fairly acid media, such as the juices produced by most stone fruits. The toxin is absorbed directly from the stomach and the intestines, affecting nerves and muscles and causing paralysis. While some antidotes are available, they are ineffective after symptoms have become advanced.

Deaths reported from botulism in the United States reached a peak of about 25 per year in the 1930s but have declined to 5 or 6 per year since then. There is evidence that some deaths of infants due to unknown causes are actually due to botulism. For reasons that are not clear, botulism bacteria, which pass harmlessly through the digestive tracts of humans over the age of 1 year, may germinate in the intestines of infants less than a year old. About 100 cases of sudden infant death reported worldwide each year appear to have been due to botulism, and the evidence now suggests that botulism is responsible for about 5% of the 8,000 cases of sudden infant death that occur each year in the United States alone.

Access Through Direct Contact

Syphilis and Gonorrhea. Bacteria responsible for diseases such as syphilis, gonorrhea, anthrax, and brucellosis enter the body through the skin or mucous membranes (i.e., those membranes lining tracts with openings to the exterior of the body). Both syphilis and gonorrhea are transmitted through sexual intercourse or other forms of direct contact, rarely through the use of public washroom towels or toilets. In recent years, gonorrhea has accounted for the largest number of reported cases of any communicable disease in the United States, and currently, despite increasing precautions taken by the public since the escalation of AIDS, more than 1 million Americans become infected every year. The symptoms of both syphilis and gonorrhea, which include persistent sores or discharges from the genitalia, sometimes disappear after a few weeks, leading victims to believe the body has healed itself. It is estimated that about 50% of females infected with gonorrhea at first exhibit no symptoms at all. More often than not, however, symptoms appear or reappear in different parts of the body at a later date. Because gonorrheal eye infections often occur in newborns as they pass through an infected birth tract, the United States, by law, requires a drop of antibiotic or silver nitrate solution be placed in the eyes of infants at birth. Both syphilis and gonorrhea are curable when treated promptly, but it is very important that such treatment be sought, since failure to do so can lead to sterility, blindness, and even death.

Anthrax. Anthrax, which is primarily a disease of cattle and other farm animals in addition to wild animals, is sometimes transmitted to humans, particularly workers in the wool and hide industries. Since September 2001, anthrax has received much publicity as an agent of bioterrorism. Like syphilis and gonorrhea, it can be effectively eliminated if treated early enough but may be fatal if allowed to progress. Brucellosis, another disease of farm animals, is occasionally transmitted to humans through direct contact or through the consumption of contaminated milk. It is sometimes called undulant fever because of a daily rise and fall of temperature apparently associated with the release of toxins by the bacteria.

Access Through Wounds

Tetanus and Gas Gangrene. When one steps on a dirty nail or is wounded in such a way that dirt is forced into body tissues, tetanus ("lockjaw") bacteria, which are common soil organisms, may gain access to dead or damaged cells. There they can multiply and produce a deadly toxin so powerful that 0.00025 gram (0.00000088175 ounce) is enough to kill an adult. In contrast, about 150 times that amount of strychnine is needed to achieve the same result. Control of tetanus through immunization is very effective and has become widespread. About 100 cases of tetanus are reported in the United States each year, most of the victims being intravenous drug users. Several related bacteria that gain access to the body in the same way are responsible for potentially fatal gas gangrene, which used to be feared on the battlefields in the past but is now controlled through the use of antibiotics and aseptic techniques.

Access Through Bites of Insects and Other Organisms

Bubonic Plague. Bubonic plague (the "Black Death") and tularemia are two bacterial diseases transmitted by fleas, deerflies, ticks, or lice that have been parasitizing infected animals, particularly rodents.

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Bubonic Plague Organism
Figure 17.8 A rat climbing over a barrier on a ship's mooring line. (Courtesy U.S. Public Health Service)

Rat fleas, found on infected rats that inhabit dumps, sewers, barnyards, and ships (Fig. 17.8), acquire the bacteria for plague and then pass them on to humans through their bites. The disease has been found in ground squirrels and other rodents in the United States, particularly in the West, since 1900.

In the past, bubonic plague spread with great speed and reached devastating epidemic proportions. In 1665, in London, hundreds of thousands perished from the disease, and between 1347 and 1349, it is believed to have killed one-fourth of the entire population of Europe (about 25 million persons). Today, plague is rare in North America, but it still occasionally manifests itself in port cities of Asia, Europe, and South America. Control depends on control of rats and fleas, which are virtually impossible to eradicate entirely, and the use of vaccines, which produce immunity for about 6 to 12 months.

Tularemia, Rickettsias, and Mycoplasmas. Tularemia is primarily a disease of animals, but infected ticks or deerflies may transmit the disease to humans through bites. It is an occupational disease of meat handlers and is fatal in 5% to 8% of the cases. Ticks, lice, and fleas may also transmit rick-ettsias, which cause typhus and spotted fevers. Rickettsias are extremely tiny bacteria that live within eukaryotic cells.

Mycoplasmas, referred to in the past as pleuropneumo-nialike organisms (PPLOs), are also minute bacteria that may be transmitted in various ways. These have no cell walls and therefore are quite plastic. They are found in many plants, in hot springs, and in the moist surfaces of the respiratory and intestinal tracts of animals and humans. They are responsible for a form of human pneumonia and for numerous plant diseases. Like an increasing number of bacteria, they are resistant to penicillin.

Lyme Disease. Lyme disease, which was known in Europe before 1900, has spread rapidly throughout the United States since 1975 when an outbreak occurred at Lyme, Connecticut. Its arthritis-like effects are caused by a bacterium (Borrelia burgdorferi) hosted by deer and field mice. It is injected into the human bloodstream by deer ticks.

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