A. The Oldest Organisms — Bacteria
For the first two billion years of Earth’s existence, bacteria were its major if not only tenants. Structurally the simplest of life forms, bacteria are single-celled prokaryotic organisms. Although capable of carrying out all of the cellular life functions, bacteria lack the internal structures, such as a nucleus and mitochondria, found in eukaryotic cells. Their genetic information is found on a single chromosome within the cell. Bacteria are also characterized by a cell wall made up of polysaccharides that surrounds the cell membrane.
Bacteria constitute a large and diversified group of organisms. Capable of growing in a remarkably wide range of habitats and conditions, bacteria can be found just about anywhere—in the saltiest sea, in the hottest hot spring, and in the most acidic or alkaline conditions. They make the soil fertile: in every gram of fertile soil there exist about 100,000,000 living bacteria; this amounts to about 90–250 kg (200–550 lb) of bacteria for every acre of soil. Bacteria decompose dead organic matter, help plants obtain vital nitrogen from the air, help humans synthesize vitamins and fend off undesirable microbes, and provide us with some of life’s pleasures, such as yogurt and cheese. Most mammals are walking apartment complexes for a wide variety of bacteria, some of which are essential to the well-being of the animal, but most of which are just along for the ride. Bacteria make up about 10% of the dry weight of a human; that is, a 150-lb. person comprises about 4 lbs. of bacteria.
Though most people are scarcely aware of the bacteria around (and within) them, life would be very difficult—if not impossible—without bacteria. Despite all the important processes and products bacteria provide, people generally only recognize the existence of bacteria when they become ill. For this reason, bacteria are generally viewed as “bad.” Like all living things, however, bacteria are only carrying out the processes of life, which include taking nutrients from their environment so that they can grow and reproduce. For the majority of bacteria, this environment is the soil or water, but for others, it is another organism that may become sick as a result. These troublesome, pathogenic bacteria, however, represent only a small proportion of the total bacterial world.
Despite their abundance and diversity of species, bacteria are remarkably lacking in variety when it comes to shape and distinguishing structural features (see Table 2). They can be spherical, as are the bacteria Streptococcus pyogenes (the causative agent of sore throats); rod-shaped, as are Salmonella typhi (the cause of typhoid fever), Vibrio cholerae (the causative agent of cholera), and Pseudomonas aeruginosa (bacteria commonly found in soil); or helical or spiral-shaped, as are Treponema pallidum (the cause of syphilis) and Spirochaeta picatilis (large and harmless spirochetes commonly found in water).
Table 2. Examples of bacteria
B. The Bacterial Way of Life
When bacteria do cause disease they can do it in a variety of ways. Living in blood, on skin, on mucous membranes, and sometimes within cells, these tiny invaders may secrete toxic substances that damage vital tissues, feast on nutrients intended for the cell, or form colonies that disrupt normal functions in the host’s body. Directly or indirectly, their actions can cause extensive damage to the host.