Bacteria

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Gram-Positive and Gram-Negative Bacteria

Another system of classifying bacteria makes use of differences in the composition of cell walls. The difference becomes clear by means of a technique called Gram’s stain, which identifies bacteria as either gram-positive or gram-negative. After staining, gram-positive bacteria hold the dye and appear purple, while gram-negative bacteria release the dye and appear red. Gram-positive bacteria have thicker cell walls than gram-negative bacteria. Knowing whether a disease-causing bacterium is gram-positive or gram-negative helps a physician to prescribe the appropriate antibiotic. The stain is named for H. C. J. Gram, a Danish physician who invented it in 1884.

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The Cell and Its Structure

The cell wall generally determines the shape of the bacterial cell. The wall is a tough but resilient shell that keeps bacterial cells from drying out and helps them resist environmental stress. In some cases the cell wall protects the bacterium from attack by the body’s disease-fighting immune system cells. Some bacteria do not have much of a cell wall, while others have quite thick structures. Many species of bacteria move about by means of flagella, hairlike structures that project through the cell wall. The flagellum’s rotating motion propels the bacterial cell toward nutrients and away from harmful substances.

Like all cells bacteria contain the genetic material DNA. But bacterial DNA is not contained within a nucleus, as is DNA in plant and animal cells. Most bacteria have a single coil of DNA, although some bacteria have multiple pieces. Bacterial cells often have extra pieces of DNA called plasmids, which the cell may gain or lose without dying. Surrounding the DNA in a bacterial cell is cytoplasm, a watery fluid that is rich in proteins and other chemicals. A cell membrane inside the wall holds together the DNA and the constituents of the cytoplasm. Most activities of the bacterial cell are carried out within the cytoplasm, including nutrition, reproduction, and the manufacture of proteins.

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How Bacteria Function

Bacterial cells, like all cells, require nutrients to carry out their work. These nutrients must be water soluble to enter through pores in the cell wall and pass through the cell membrane into the cytoplasm. Many bacteria, however, can digest solid food by secreting chemicals called exoenzymes into the surrounding environment. The exoenzymes help break down the solid food outside the bacteria into water-soluble pieces that the cell wall can absorb. Bacterial cells use nutrients for a variety of life-sustaining biochemical activities known collectively as metabolism.

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Anabolism and Catabolism

The metabolic activities that enable the cell to function occur in two ways: anabolism and catabolism. Simply put, anabolism is the manufacture of complex molecules from simple ones, and catabolism is the breakdown of complex molecules into simple ones. Cells use the energy from catabolism for all their other tasks, including growth, repair, and reproduction.

A single bacterial cell takes up small molecules from the environment by means of specific transport proteins in the cell membrane. In the case of more complex molecules, such as proteins or complex carbohydrates, bacteria first secrete digestive enzymes into the environment to break the nutrients down into smaller molecules, which are transported across the membrane. Enzymes (proteins that speed chemical reactions) within the cytoplasm then digest the molecules further. This breakdown, called catabolism, results in energy transfer through the processes of respiration and fermentation. During metabolism, some of the small molecules are converted into the molecules the cell needs to synthesize (manufacture) its own proteins, nucleic acids (building blocks of DNA), lipids (fatty substances), and polysaccharides (sugars and starches). The metabolic processes for synthesis of these complex cells are anabolism.

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Adaptation to Environmental Stress

All organisms have some capacity to adapt to environmental stress, but the extent of this adaptive capacity varies widely. Heat, cold, high pressure, and acid or alkaline conditions can all produce stress. Bacteria easily adapt to environmental stress, usually through changes in the enzymes and other proteins they produce. These adaptations enable bacteria to grow in a variety of conditions. Gradual exposure to the stress, for example, may enable bacteria to synthesize new enzymes that allow them to continue functioning under the stressing conditions or that enhance their capacity to deal with the stressing agent. Or they may resist environmental stress in other ways. Some bacteria that live in extremely acidic conditions can pump out acid from their cell.

Extremophiles are organisms that can grow in conditions considered harsh by humans. Some kinds of bacteria thrive in hydrothermal vents on the ocean floor or in oil reservoirs within Earth, at high pressures and temperatures as high as 120^oC (250^oF). Other kinds can live at temperatures as low as –12^oC (10^oF) in Antarctic brine pools. Other bacteria have adapted to grow in extremely acid conditions, where mines drain or minerals are leached from ores and sulfuric acid is produced. Others grow at extremely alkaline or extremely salty conditions. Still others can grow in the total absence of oxygen. Bacteria able to function in these extreme conditions generally cannot function under conditions we consider normal.

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