"Daddy, the garbage man is here! Tell him we don't need any." -- Edward Abbey

I. Bacteria -- classified in the Kingdom Monera.
Recommended supplementary Web site: Introduction to the Bacteria

1. Most bacteria are quite small, only a few microns (millionths of a meter) in length. (Exception: cyanobacteria, a.k.a. "blue-green bacteria")
2. Bacteria are prokayotes. . .
   1. There's no true nucleus, but the DNA (genetic material) is typically clumped in the center of the cell.
   2. Almost all bacteria have a cell wall -- quite different in structure from that found in plants and fungi.
   3. Often there are long, fine threads extending from the cell wall, whose movement propels the bacteria. These are flagella.
   4. Some bacteria can produce endospores -- very thick-walled capsules that allow the bacterium to survive harsh conditions in a dormant form
   5. Most bacteria have one of three common shapes:
      1. Bacillus (rod-shaped)
      2. Coccus (sphere-shaped)
      3. Spirillum (corkscrew-shaped) or vibrio (comma-shaped)
3. Reproduction: Usually fission: the DNA is duplicated and then the cell divides into two down the middle.
   1. Some bacteria practice a type of sex, but not one that you or I would recognize. . . .
   2. Bacterial sex involves two bacteria joining together by means of a hollow tubelike structure called a pilus.
   3. One bacterium then sends some of its DNA through the pilus to the other bacterium.
   4. Practical significance: Bacteria that can resist antibiotics, for instance, can transfer their ability to other bacteria -- which means that bacterial populations may develop antibiotic resistance extremely fast. Also a problem for genetic engineering: what if a genetically engineered bacterium gets loose and passes genes on to wild bacteria? (More on this will follow after we've talked more about genetics.)

1. The best-known bacteria are the ones that cause human disease.
   1. EXAMPLES: Syphilis, gonorrhea, tooth decay, stomach ulcers, cholera, bacterial pneumonia, botulism, bubonic plague, tetanus, Lyme disease, legionnaire's disease, tuberculosis, strep throat, diphtheria, impetigo, boils, leprosy. . .
   2. Often, it's not so much the bacteria themselves that cause illness, but toxins that the bacteria produce.
      1. Exotoxins are released by bacterial cells (examples: tetanus, botulism, staph, diphtheria)
      2. Endotoxins are certain components of bacterial cell walls (example: salmonella)
   3. 19th-century German bacteriologist Robert Koch came up with rigorous experimental procedures for determining whether a bacterium causes a particular disease. These are Koch's postulates.
      1. The same bacteria must be present in every human or animal with a particular disease.
      2. The bacterium must be grown in the laboratory in a pure culture -- one with no other cells present.
      3. The disease must appear in experimental animals when the cultured bacteria are injected into them.
      4. The same bacteria must be isolated from the experimental animals after they get the disease.

1. The vast majority of bacteria live in soils and waters, and do no harm to anyone at all. In fact, they're necessary for all other life on Earth.
   1. Many bacteria require oxygen to live; these are called aerobes.
   2. Other bacteria do not need oxygen, or may even be poisoned by oxygen. These are anaerobes.
      1. EXAMPLE: Clostridium botulinum, which can survive the food canning process (because it makes endospores), and then grow in canned food and cause the cans to become dented or bulged. These cause very severe food poisoning. (That's why you should never eat food out of dented or bulging cans.)
   3. Some can use oxygen but can also get by without it; these are facultative aerobes.
   4. There are photosynthetic, autotrophic bacteria -- such as the blue-green bacteria, or cyanobacteria -- as well as heterotrophic bacteria.
   5. Heterotrophic bacteria are often saprobic, or saprotrophic -- that is, they feed on dead organisms and cause decay.
   6. A group of prokaryotes known as Archaea has many members that live in extreme environments: highly acidic waters (acidophiles), or temperatures over 100 degrees Celsius (thermophiles), or water many times saltier than sea water (halophiles).
2. Many bacteria are quite beneficial:
   1. Escherischia coli bacteria in your intestines help you digest food. They also make vitamin K and vitamin B₁₂.
   2. Lactobacillus bacteria in the human vagina help maintain the proper pH and help protect against infections.
   3. . Other bacteria, living inside the roots of plants such as alfalfa, take up nitrogen gas from the air (N₂) and convert it into a form the plant can use (ammonia, or NH₃).
   4. A few produce antibiotic drugs such as streptomycin and nocardicin.
   5. Bacteria used in the food industry convert milk to buttermilk and yogurt, and wine to vinegar. They put the tangy taste in sourdough bread, and the flavor (and holes) in Swiss cheese.
   6. Bacteria are used in sewage treatment to break down wastes.
   7. Strains of bacteria exist that can feed on petroleum. These have been used to clean up oil spills. Others can extract metals from mining waste, and are becoming used in mining and in environmental clean-up.
   8. As a byproduct of photosynthesis, blue-green bacteria, or cyanobacteria, produce much of the oxygen that we and other organisms breathe.