Bacteria are among the oldest types of life on the planet. Unlike most larger organisms, bacteria are single-celled, and have significant differences in cellular morphology from more complex life forms. As a kingdom of life, bacteria are ubiquitous. The most common forms are found everywhere from surfaces to in foods, on human skin and in the digestive tract.
Function
Bacteria serve many purposes. In addition to maintaining their own lives and reproducing, many common bacteria also serve useful symbiotic purposes through their interactions with other living organisms. For instance, all living things require a source of reactive nitrogen, but atmospheric nitrogen is inert and most organisms can't use it. Bacteria found in soil and in the root nodules of leguminous plants can "fix" nitrogen, meaning they convert unreactive nitrogen to a reactive form, notes Dr. Jacquelyn Black in her book, "Microbiology."
Features
Common bacterial species share many features that distinguish them from larger organisms. Unlike humans and animals, bacteria have cell walls that surround them and protect them from the environment, notes Dr. Black. They also lack many of the cellular organelles that larger organisms have, including true nuclei and mitochondria. Some very common species, including Escherichia coli, have organelles called flagella that help them move through their environments. Very few cells in larger organisms have flagella, sperm cells being a notable exception.
Considerations
While some bacterial species can survive in extreme environments, including those in which temperatures are very high or low and those in which oxygen is unavailable, most common bacterial species either use or at least tolerate oxygen. Many common bacteria are, therefore, classified as "aerobic." This, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry," means the bacteria process sugar and other nutrients by chemically burning the molecules in the presence of atmospheric oxygen.
Misconceptions
It's common to think that all bacteria cause infection, but humans live in harmony with many bacterial species. Staphylococcus aureus, for instance, is a nearly ubiquitous bacterial species that humans carry on the surface of the skin. In most cases, these bacteria cause no problems whatsoever, notes Dr. Black. In certain individuals or the immunocompromised, Staph infections can result from the otherwise harmless bacteria. Common bacteria like E. coli also live in the human intestine, harmlessly digesting food components that humans can't, notes Dr. Lauralee Sherwood in her book, "Human Physiology."
Expert Insight
Humans use commonly found bacterial species to their benefit in many instances. The species Lactobacillus acidophilus is one that consumes the sugars in milk. As a result of the bacteria's mechanism of converting food into energy, notes Dr. Black, Lactobacillus curdles milk, producing a creamy or curdy texture and sour flavor. Humans purposely seed milk with Lactobacillus to produce foods like yogurt and cheese, and also to benefit from the effects of consumption of Lactobacillus, which benefit the digestive tract.
References
- "Microbiology: Principles and Explorations"; Jacquelyn Black; 2008
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
