On average, bacteria are about 1 micro metre (0.000039 inch) long and 0.5
micro metre in diameter. All bacteria are surrounded by a lipid membrane that
regulates the flow of materials in and out of the cell. A rigid cell wall
completely surrounds the bacterium and lies outside the membrane. Gram-positive
bacteria are stained blue by the gram stain, because their cell walls have a
relatively thick and mesh like structure that traps the dye. In gram-negative
bacteria, the cell wall is thin and releases the dye readily when washed with
an alcohol solution.
Outside the cell wall, some species of bacteria also have a capsule made up of
polysaccharides. Such capsules have many functions, including protecting the
bacterium from phagocytes and from desiccation (drying). Many species of
bacteria swim by means of flagellae, i.e., hairlike structures whose whip like
lashing provides propulsion.
The DNA of most bacteria is found in a single circular chromosome and is
distributed throughout the cytoplasm rather than in a membrane-bound nucleus.
Smaller circular auxiliary DNA strands, called plasmids, are also found in the
cytoplasm. A number of other structures are distributed about the bacterial
cytoplasm, including ribosomes.
When applied to bacteria, the term growth refers to an increase in the number
of bacteria in a population rather than in the size of an individual
microorganism. Bacteria usually reproduce through binary fission, an asexual
process in which the mother cell increases in size until it divides into two
identical daughter cells. There are also bacteria that reproduce through
budding, through chains of spores, and through the segmentation of elementary
units. Bacteria do not reproduce sexually, but there are several mechanisms by
which DNA is exchanged in a one-way transfer between them.
All bacteria require carbon, nitrogen, sulfur, phosphorus, inorganic salts, and
micro nutrients. Bacteria that use an organic compound as their source of carbon
are called heterotrophs, while those that use an inorganic source are called
autotrophs. In addition, some bacteria use photosynthesis to generate energy in
the form of the compound ATP; these are called phototrophs. Some species of
bacteria are parasitic and can grow only within a living host cell; examples
include the genera Rickettsia and Chlamydia, both of which are parasites in
eukaryotic cells. Those bacteria that require oxygen, such as Bacillus, are
called aerobes; anaerobes, such as Clostridium, cannot survive in the presence
Various types of bacteria that are present in water can cause disease in
humans, and water-purification plants are designed to destroy these
microorganisms. Bacteria from industrial wastes may also act as pathogens, or
agents of disease. Conversely, some types of bacteria act as cleansing agents
in water, and water-treatment facilities utilize some such bacteria to break
down the organic matter that is present in sewage.
Various types of bacteria contaminate foods and can cause food poisoning in
humans. Pasteurization is routinely used to neutralize bacteria that may be
present in milk, for example. Other sterilization techniques include high
temperature, radiation, ethylene oxide, and other antiseptics and germicides.
A number of bacteria cause diseases in humans. Some, such as meningococcal
bacteria which infect the brain membranes, have a specificity for a particular
part of the body; others, such as staphylococcal bacteria, can affect various
parts of the body.
Although human interest in bacteria frequently focuses on their harmful
effects, most bacteria are harmless to human beings, and many of them are
actually beneficial. Saprophytic bacteria, for example, perform an ecologically
indispensable role in the breakdown of dead organisms and organic wastes;
without such agents of decomposition, the cycling of various elements vital to
living organisms would cease in the biosphere.
Bacteria also form highly beneficial associations with animals. For instance,
the bacterial inhabitants of the ruminant stomach break down cellulose; this
enables cows, sheep, and other ruminants to digest grass. Humans also harbor
beneficial bacteria, such as those in the lower intestine that synthesize
vitamin K. Bacteria are also used in various industrial processes, especially
in the food industry; the production of buttermilk, yogurt, cheeses, pickles,
and sauerkraut are all dependent upon bacterial action.
The scientific classification of bacteria is in transition, particularly at the
higher taxonomic levels. The bacteria form the only prokaryotic kingdom, that
of the Monera. Within this kingdom, at least two groups have been
distinguished, the eubacteria and archaebacteria. DNA hybridization studies of
ribosomal RNA have proved useful in defining these groups.