Immunodeficiency virus (hiv) and other disease-causing microorganisms.

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.

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.

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.

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.

All bacteria require carbon for growth and reproduction. Bacteria called autotrophs (“selffeeders”)

get their carbon from CO2 and bacteria called heterotrophs (“other feeders”) derive carbon

from organic nutrients such as sugar.

Another system of classifying bacteria makes use of differences in the composition of cell

walls.

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.

Bacteria contain the genetic material DNA. But bacterial DNA is not contained within a

nucleus. 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.

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.

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.

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.

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 120oC (250oF). Other kinds can live at

temperatures as low as –12oC (10oF) 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.

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.

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.

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.

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.

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.

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