III.

Characteristics of Bacteria

Bacteria are so small that they can be seen only under a microscope that magnifies them at least 500 times their actual size. Some become visible only at magnifications of 1,000 times. They are measured in micrometers (µm) and average about 1 to 2 µm in length. One micrometer equals one-millionth of a meter (0.0000001 m or about 0.000039 in).

Bacteria not only have many uses, they also occur in diverse shapes and types. As a group they carry out a broad range of activities and have different nutritional needs. They thrive in a variety of environments.

A.

Types of Bacteria

Scientists use various systems for classifying bacteria into different types. One of the simplest systems is by shape. Other systems depend on oxygen use, source of carbon, and response to a particular dye.

A.1.

Classification by shape

Most bacteria come in one of three shapes: rod, sphere, or spiral. Rod-shaped bacteria are called bacilli. Spherical bacteria are called cocci, and spiral or corkscrew-shaped bacteria are called spirilla. Some bacteria come in more complex shapes. A hairlike form of spiral bacteria is called spirochete (see Spirochetes). Streptococci and staphylococci are well-known disease-causing bacteria among the cocci.

A.2.

Aerobic and Anaerobic Bacteria

Scientists also classify bacteria according to whether they need oxygen to survive or not. Aerobic bacteria require oxygen. Anaerobic bacteria cannot tolerate oxygen. Bacteria that live in deep ocean vents or within Earth are anaerobic. So are many of the bacteria that cause food poisoning.

A.3.

Autotrophic and Heterotrophic Bacteria

All bacteria require carbon for growth and reproduction. Bacteria called autotrophs (“self-feeders”) get their carbon from CO₂. Most bacteria, however, are heterotrophs (“other feeders”) and derive carbon from organic nutrients such as sugar. Some heterotrophic bacteria survive as parasites, growing within another living cell and using the nutrients and cell machinery of their host cells. Some autotrophic bacteria, such as cyanobacteria, use sunlight to produce sugars from CO₂. Others depend instead on energy from the breakdown of inorganic chemical compounds, such as nitrates and forms of sulfur.

A.4.

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.

A.5.

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.

B.

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.

B.1.

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.

B.2.

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.

B.3.

Reproduction and Survival

Bacteria reproduce very rapidly. Replication in some kinds of bacteria takes only about 15 minutes under optimal conditions. One bacterial cell can become two in 15 minutes, four in 30 minutes, eight in 45 minutes, and so on. Bacteria would quickly cover the entire face of the globe if their supply of nutrients was unlimited. Fortunately for us, competition for nutrients limits their spread. In the absence of sufficient nutrients, however, many bacteria form dormant spores that survive until nutrients become available again. Spore formation also enables these bacteria to survive other harsh conditions.

B.3.a.

Binary Fission

The simplest sort of bacterial reproduction is by binary fission (splitting in two). The bacterial cell first grows to about twice its initial size. Toward the end of that growth, the cell membrane forms a new membrane that extends inward toward the center of the cell. The cell wall follows closely behind, bisecting the cell. The membrane then seals to divide the enlarged cell into two small cells of equal or nearly equal size, and a new wall forms between the membranes.

The growth and division of a bacterial cell has two main phases. In one phase, the cell replicates its DNA and makes all the other molecules needed for the new cell. The second phase—cell division—occurs when DNA replication stops. In the bacterium Escherichia coli replication takes about 40 minutes and cell division lasts about 20 minutes. The entire cycle takes about an hour. Yet the time for one cell to become two cells still takes only about 20 minutes. How is this possible? The cell does not wait for one cycle of replication to end before it starts another. Thus, a rapidly growing bacterial cell is carrying out multiple rounds of replication at the same time.

B.3.b.

Spore Formation

In response to limited nutrients or other harsh conditions, many bacteria survive by forming spores that resist the environmental stress. Spores preserve the bacterial DNA and remain alive but inactive. When conditions improve, the spore germinates (starts growing) and the bacterium becomes active again.

The best-studied spores form within the bodies of Bacillus and Clostridium bacteria, and are known as endospores. Clostridium botulinum spores cause deadly botulism poisoning. Endospores have thick coverings and can resist environmental stress, especially heat. Even boiling in water does not readily kill them. But they can be killed by heating in a steel vessel filled with steam at high temperature and high pressure. Endospores can live for centuries in their dormant state.

Some bacteria form other types of spores. These spores are usually dormant but not as heat resistant or long-lived as endospores. Some aquatic bacteria, for example, attach to surfaces and produce swarmer cells during division. The swarmer cell swims away to attach to another surface and give rise to still more swarmer cells. Still other bacteria survive by forming colonies made up of millions of cells that act in a coordinated way to keep the organism alive.

B.3.c.

Genetic Exchange

Bacterial cells often can survive by exchanging DNA with other organisms and acquiring new capacities, such as resistance to an antibiotic intended to kill them. The simplest method of DNA exchange is genetic transformation, a process by which bacterial cells take up foreign DNA from the environment and incorporate it into their own DNA. The DNA in the environment may come from dead cells. The more the DNA resembles the cell’s own DNA, the more readily it is incorporated.

Another means of genetic exchange is through incorporation of the DNA into a virus. When the virus infects a bacterial cell, it picks up part of the bacterial DNA. If the virus infects another cell, it carries with it DNA from the first organism. This method of DNA exchange is called transduction.

Transformation and transduction generally transfer only small amounts of DNA, although bacterial geneticists have worked to increase these amounts. Many bacteria are also capable of transferring large amounts of DNA, even the entire genome (set of genes), through physical contact. The donor cell generally makes a copy of the DNA during the transfer process so it is not killed. This method of exchange is called conjugation. DNA exchange enables bacteria that have developed antibiotic-resistant genes to rapidly spread their resistance to other bacteria.