Bacteria are single-celled microorganisms, known for their tremendous ability to adapt and multiply as well as for their ancient history. Some of the oldest known fossils--nearly 3.5 billion years old--are those of bacteria-like organisms. While some bacteria bring disease and death, others are benign or even beneficial, breaking down dead organic matter or producing antibiotics. Bacteria are usually grouped into three categories, classified by shape. According to the Merck Manuals Online Medical Library, the three shapes are spherical, cylindrical and spiral.
The coccus bacteria are spherical or oval in shape, like a berry. In fact, the name is derived from the Greek word "kokkos," which means berry. These are some of the smallest and simplest bacteria, with an average size of about 0.5 to 1.0 micrometers in diameter. (A micrometer is about 1/1,000,000 of a meter.) A number of pathogenic (disease-causing) bacteria belong to this category. Some examples of cocci are streptococcus, which can cause strep throat and scarlet fever; staphylococcus, specifically Staphylococcus aureus, which can cause food poisoning and toxic shock syndrome; and meningococcus, which can cause a number of meningococcal diseases, including epidemic bacterial meningitis.
The bacillus bacteria are rod-like in shape. These bacteria are a bit more complex than the coccus family and, on average, are 0.5 to 1.0 microns wide by 1.0 to 4.0 microns long. A number of these bacteria are pathogenic, like Yersinia pestis, which can cause bubonic and pneumonic plague, or Bacillus anthracis, which is the cause of anthrax. But beneficial bacteria also belong to this family, such as those used to make antibiotics as well as those that colonize the human intestinal tract, aiding in digestion.
The spirochete bacteria are spiral in shape. When viewed under a microscope, they appear almost worm-like, wiggling wildly and moving about. Two of the more well-known members of the spirochete family are Treponema pallidum, the bacteria that causes syphilis, and Leptospira, which causes the illness leptospirosis. Beneficial spirochetes include symbiotic spirochetes, which inhabit the stomachs of ruminants such as sheep, cattle and goats where they convert cellulose and other difficult-to-digest plant polysaccharides into nourishing food and fiber for their host. Beneficial spirochetes also live in the intestines of termites and help in the digestion of wood and plant fiber. This allows termites to contribute to the removal of rotted and diseased wood and release organic matter into the soil, enriching its quality.