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|Bacteria were among the first life forms on Earth. They are very small one-celled organisms that lack a nucleus (size ~ 10-4 cm). Despite their small size, bacteria are the most abundant of any organism on Earth. They are highly adaptable. Their normally rapid reproduction rate (by asexual binary fission) and high capacity for spontaneous mutation allows them to respond to changing environments readily.|
The cells of all bacteria are classified as "prokaryotic", the simplest and most ancient of the cell types. Prokaryotes lack many of the structures found in the more complex, eukaryotic cells. Bacteria occur in 3 basic shapes (Figure 11-40): rod (bacillus), spherical or round (coccus), and spiral (spirillum). The bacilli and the cooci may form chains of a length typical of the particular bacterium. When faced with unfavorable environmental conditions, some bacteria form endospores. During the formation process, the cell shrinks, rounds up within the former cell membrane, and secretes a new, thicker wall inside the old one. Endospores are amazingly resistant to extreme temperatures, drying out, and harsh chemicals, including acids and bases. When conditions are suitable for growth, the spore absorbs water, breaks out of the inner shell, and becomes a typical bacterial cell again.
Some bacteria are obligate anaerobes and are unable to grow in the presence of oxygen. Some other bacteria are able to grow in either the presence or absence of oxygen. Most bacteria, however, are aerobic and like animals require a constant supply of oxygen to carry out cellular respiration.
Every type of nutrition is found among bacteria except holozoism (eating whole food). Some autotrophic bacteria are photosynthetic. Some are chemosynthetic bacteria, which oxidize inorganic compounds to obtain necessary energy to produce their own food. The majority of bacteria are free-living aerobic heterotrophs and feed on dead organic matter by secreting digestive enzymes and absorbing the products of digestion. They are needed to complete the elementary cycles of nature (the carbon cycle, the nitrogen cycle, the phosphate cycle, and the sulphur cycle) by degrading the wastes and the corpses from higher organisms back to inorganic and mineral compounds.
Bacteria are often symbiotic; they live in association with other organisms. The nitrogen-fixing bacteria in the nodules of legumes are mutualistic, as are the bacteria that live within our own intestinal tract. We provide the bacteria with a home, and they provide us with certain vitamins. The interaction between the intestine and its bacterial flora also adjust metabolism and promote immunization. Although there are more than 1000 different species of bacteria existing in the human intestine, a study in 2011 discovered that similar to the ABO blood types, every one of us has one of the three types (of bacteria). Each of the three types is dominated by one bacterium as shown in Table 11-05 below.
|Bacteroides||Breaks down carbohydrates and proteins||Vitamins B2, B5, B7 and C||Foul-smelling gases, such as H2S|
|Prevotella||Breaks down intestinal mucin protein||Vitamins B1 and B9||Possibly acetone|
|Ruminococcus||Adjust intestinal content of carbohydrates||Iron in the blood's hemoglobin||Odorless methane|
Cyanobacteria, formerly called blue-green algae, are the most prevalent of the photosynthetic bacteria. They are believed to be responsible for first introducing oxygen into the primitive atmosphere. Cyanobacteria can be unicellular, filamentous (see Figure 11-41), or colonial. The filaments and colonies are not considered multicellular because each cell is independent of the others. Cyanobacteria lack any visible means of locomotion. They are common in fresh water, in soil, and on moist surfaces but also are found in inhospitable habitats, such as hot springs. They also form symbiotic relationships with a number of organisms, such as ferns and even at times invertebrates, like corals. In association with fungi, they form lichens, which can grow on rock. Therefore, cyanobacteria may have been among the first organisms to colonize land.