Bacteria may have been the first form of life to exist on earth. Early bacteria may have taken their energy from iron, sulfur and nitrogen rather than from the sun. Such bacteria still exist today. There is no doubt that bacteria are the most common form of life on earth. They cover our skin. They live inside of our bodies. They exist in the air, water, soil, in our foods, and on plants. They exist in the depths of the ocean and in hot springs. There are bacteria which thrive at freezing temperatures and others which thrive in temperatures as high as 185-190 F degrees. Bacteria are ubiquitous--a word which means they are everywhere.
Bacteria exist as a single cell which is so small that they cannot be seen without a microscope. With a 400 power microscope, large bacteria appears as specks. With a 1,000 power microscope they can be distinctly seen. To examine them closely requires an electron microscope.
Like viruses, bacteria are measured by the millimicron. Bacteria range in size from 500 to 10,000 millimicron in length. Several thousand bacteria could occupy the space of the period at the end of this sentence.
Some bacteria live as single cells. Others remain attached after cell division to form colonies. Bacteria are classed according to their shape, as well as the shape of their colony.
1. Coccus (cocci, plural) are sphere-shaped (round) bacterial cells.
a. diplococcus-spherical bacterial cells joined in pairs.
b. staphylococcus--spherical shaped bacterial cells which form colonies in clusters like a cluster of grapes.
c. streptococcus--spherical shaped bacterial cells which form colonies of filaments, or strings of cells.
2. Bacillus (bacilli, plural) are rod shaped bacteria.
a. diplobacillus--rod shaped bacteria in colonies of pairs.
b. streptobacillus--rod shaped bacteria joined end to end to form a colony which resembles a string or filament.
3. Spirillum (spirilla, plural) are cells shaped like bent rods or cork screws.
Bacteria have a cell wall which gives it shape. Within this cell wall there is a thin plasma membrane which surrounds the cytoplasm. There is no nuclear membrane, but a nuclear area near the center of the cell. Surrounding the cell wall is a slime layer which protects the bacteria and helps the cell stick on to things.
This slime layer is sometimes called a capsule. The capsule for many bacteria contains and secretes a poison that protects the bacteria from predators. It is the poisonous slime layer (capsule) that can frequently cause human illness.
Some bacteria can move. Movement is referred to scientifically as motility. Most of the bacteria that are motile move by way of flagella. Flagella is a whip-like structure which whips about allowing the bacterium to move in fluids.
Many bacteria have the ability to go dormant when temperature, moisture, light conditions and food supply do not allow them to actively live. These bacteria transform themselves into endospores which are capable of remaining dormant for thousands of years. When conditions become right for living, they spring back to active life. An endospore is able to tolerate great coldness, as well as great heat. Some endospores can survive boiling water. Endospores have been found in deep layers of ice in the antarctic, trapped there for thousands of years. When placed in a better environment they become active cells.
Most bacteria grow best in the dark. In fact many bacteria are killed by the ultraviolet radiation of the sun. Some bacteria are aerobic and use oxygen for respiration. However, most are anaerobic and are killed by oxygen. Still other bacteria can either take or leave oxygen, but do best either with or without oxygen.
1. Those which must have oxygen are called obligate aerobes.
2. Those which are killed by oxygen are called obligate anaerobes. 3. Those which grow best without oxygen, but are not killed by oxygen are called facultative anaerobes.
4. Those which grow best with oxygen, but are not killed by a lack of it are called facultative aerobes.
The anaerobic respiration of bacteria can produce several different by-products, such as ethanol, lactic acid and methane.
Some bacteria are autotrophic and make their own food. Some have chlorophyll for photosynthesis, others use iron, sulfur or nitrogen and make their food through chemosynthesis. However, most bacteria are heterotrophic and feed on other organisms.
Some heterotrophic bacteria are saprophytes. Saprophytes feed on dead organic matter. By doing so the bacteria cause this dead organic matter to decompose. These bacteria also feed on the dead organic matter which is intended to be human food. Other heterotrophic bacteria are parasites. These invade the bodies of plants and animals and get food directly from the living tissue of these organisms. The organism invaded by these bacteria is called the host.
Most bacteria reproduce by binary fission. In binary fission everything in the cell is duplicated and then the cell splits into two cells. Under good conditions, a new cell may be formed by division, mature, and begin its own division within as little as 20 minutes. Starting with a single bacteria, after 7 hours there would be more than 2 million bacteria. A few bacteria, such as E. coli which inhabits human large intestines, reproduces sexually.
Not all bacteria are disease causing (pathogenic). In fact, most bacteria are harmless, and others are very helpful. Soil bacteria decomposes dead plants and animals so that their nutrients are added to the soil in a useable form for growing plants. Without these soil bacteria, the soil would lack usable fertilizer. Bacteria fix nitrogen so that plants can use it. Although 70% of earth's atmosphere is nitrogen, it is not in a useable form for plants. Bacteria transform nitrogen into a useable form for plants. Bacteria are the main decomposers. If there were no bacteria, dead bodies would lie around for thousands of years unchanged.
Bacteria is also used to make alcoholic beverages, vinegar, buttermilk, cheese, and yogurt. Other bacteria eat crude oil and are used to clean up oil spills.
Some bacteria cause disease. The things that we describe as germs are usually either bacteria or viruses. In the late 1800's Louis Pasteur formulated germ theory, which changed the course of modern medicine. Germ theory is that bacteria can cause disease. Some of these diseases represent bacteria feeding on human tissue. Other bacterial disease occurs because bacteria produce and secrete poisons called endotoxins. Some of the diseases caused by bacteria are: bubonic plague, pneumonia, whooping cough, strep throat, syphilis, gonorrhea, botulism, salmonella, and leprosy. However, because bacteria have a cell wall and a plasma membrane, drugs have been developed that will kill bacteria by destroying the wall or membrane. Many years ago it was noted that many bacteria and molds produced chemicals which made it impossible for other bacteria to grow anywhere near them. If they did attempt to grow near them, they were killed. These chemicals were extracted from these bacteria and molds and became antibiotic drugs, such as penicillin. Antibiotic drugs come directly from bacteria and molds. Abiotic drugs are synthetic versions of the chemicals which kill bacteria. However, the primary defense against bacterial invasion is the immune system. We have white blood cells which recognize that bacteria do not belong in the human body, and they eat them through phagocytosis.
If we should lack the white blood cell which is capable of killing a certain bacteria, cells in the immune system can get information about the bacteria and take it to the thymus gland, so that a white blood cell which will kill it can be made. This, however, takes time, and during this time the bacteria are multiplying and eating human tissue or releasing endotoxin. This is why vaccination is so important.
Like with viruses, bacteria can be killed or weakened and injected into the human blood stream. The white blood cells do not know that they are weakened or dead, so they take them to the thymus to make a white blood cell which will kill them. Then when the actual bacteria do invade the body, because of vaccination, we have the white blood cell which can kill the invader quickly.
Some bacteria have in addition to their DNA an extra circular chromosome called a plasmid. The function of the plasmid is to produce substances which make the bacteria resist the chemicals in the slime layer of other bacteria and molds. It also functions to make them resistant to antibiotic drugs. This plasmid is used by scientists to turn bacteria into biological factories.
Scientists can replace the plasmid with the gene for human insulin, for example, and the bacteria will reproduce the gene and develop colonies of bacteria that now produce human insulin. Scientists can replace the plasmid with the gene for the human growth hormone, and it will be produced in mass quantities. This process called genetic engineering or recombinant DNA, has made it possible for scientist to mass produce human chemicals which are used medically for people who need these chemicals.
Bacteria like to eat the same foods that we eat, so people have
developed techniques to kill the bacteria in foods. Pasteurization
is a process of heating certain liquid food, like milk for example,
to kill most of the bacteria. Canning prevents bacterial invasion,
as does refrigeration, freezing and dehydration. Salt and sugar
can be used to kill bacteria. Filters with very small holes can
filter bacteria from liquids. Chemical can also preserve food.