Bacteria

Bacteria are single-cell microbial organisms belonging to the prokaryote domain. Unlike other organisms, prokaryotes are characterized by the absence of a cell nucleus and other membrane-bound organelles. Although bacteria are known to be responsible for many infectious diseases, they also serve many beneficial functions such as the decomposition of organic matter and have formed a number of complex symbiotic relationships with other organisms.

Life in any form on Earth could not exist without these tiny cells. Scientists have discovered fossilized remains of bacteria that were radiometrically dated to more than 3.5 billion years old. Although many creationists take issue with these dating techniques, such discoveries illustrate that life has existed on Earth much closer to its formation than previously thought.

Ecology
Like humans, some bacteria (aerobic bacteria) need oxygen to survive. Others (anaerobic bacteria), however, do not. Amazingly, some can adapt to new environments by learning to survive with or without oxygen.

Many bacteria prefer the milder temperature of the healthy human body. However, they can inhabit a variety of environments, including extremely hot and cold areas.
 * Psychrophiles, or cold-loving bacteria, can live in the subfreezing temperature of the Arctic.
 * Thermophiles are heat-loving bacteria that can live in extreme heat, such as in the hot springs in Yellowstone National Park.
 * Extreme thermophiles, or hyperthermophiles, thrive at 235 degrees Fahrenheit near volcanic vents on the ocean floor.

Like all living cells, each bacterium requires food for energy and building materials. There are countless numbers of bacteria on Earth—most are harmless and many are even beneficial to humans. In fact, less than 1 percent of bacteria cause diseases in humans. For example, harmless anaerobic bacteria, such as Lactobacilli acidophilus, live in our intestines, where they help to digest food, destroy disease-causing microbes, fight cancer cells, and give the body needed vitamins. Healthy food products, such as yogurt, sauerkraut, and cheese, are made using bacteria.

Anatomy
Under a microscope, bacteria look like balls, rods, or spirals. Bacteria are so small that a line of 1,000 could fit across the eraser of a pencil.

Infectious disease

 * Main Article: infectious disease

Some bacteria produce poisons called toxins, which also can make us sick. Certain bacteria give off toxins that can seriously affect your health. Botulism, a severe form of food poisoning, affects the nerves and is caused by toxins from Clostridium botulinum bacteria. Under certain circumstances, however, bacterial toxins can be helpful. Several vaccines that protect us from getting sick are made from bacterial toxins. One type of pertussis vaccine, which protects infants and children from whooping cough, contains toxins from Bordetella pertussis bacteria. This vaccine is safe and effective and causes fewer reactions than other types of pertussis vaccine.

Selected bacterial diseases:

Reproduction

 * Main Article: Binary fission

Bacteria reproduce through a fairly simple process called binary fission, or the reproduction of a living cell by division into two equal, or near equal, parts. As just noted, this type of asexual reproduction theoretically results in two identical cells. However, bacterial DNA has a relatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of developing resistance to antibiotics and helps them exploit invasion into a wide range of environments.

Similar to more complex organisms, bacteria also have mechanisms for exchanging genetic material. Although not equivalent to sexual reproduction, the end result is a bacterium that contains a combination of traits from two different parental cells. Three different modes of exchange have thus far been identified in bacteria.

Conjunction involves the direct joining of two bacteria, which allows their circular DNAs to undergo recombination. Bacteria can also undergo transformation by absorbing remnants of DNA from dead bacteria and integrating these fragments into their own DNA. Lastly, bacteria can exchange genetic material through a process called transduction, in which genes are transported into and out of the cell by bacterial viruses, called bacteriophages, or by plasmids, an autonomous self-replicating extrachromosomal circular DNA.

Antibiotic resistance

 * Main Article: Antibiotic resistance

Research from the Proceedings of the National Academy of Sciences suggests that immunity and resistance play less of a role than previously thought. Instead, the spread and variation of bacteria depends more on chance.