Bacteria are one of the five types of microbes1. Many bacteria are helpful to humans.
They aid in our digestion, producing vitamin K and helping to break down food. They are used to make products such as cheese, butter, and yogurt. They are used to make many kinds of beneficial drugs, including insulin and antibiotics such as tetracycline and streptomycin.
Nitrogen-fixing bacteria such as rhizobium in the soil are essential to life on Earth. Bacteria are used in sewage treatment plants, and tailored bacteria have been made that can clean up chemical and oil spills, and remove environmental pollutants from coal2.
Not all bacteria are benign, though. Bacteria cause many kinds of disease. Bacteria that cause disease are called pathogens. We sometimes refer to them as "germs" in everyday speech. Some common forms are listed under Diseases, below.
Bacteria are a type of single-celled organism called prokaryotes. They belong to the domain Eubacteria, and also the kingdom by the same name.
Bacteria were formerly classified in the kingdom Monera, but recently most scientists agreed to
divide the prokaryotes into two groups. The 'true' bacteria, called eubacteria, are now distinguished from the Archaea. Archaea mix traits of prokaryotes with those of eukaryotes, and are generally considered to be in a separate kingdom.
Structure and form
Prokaryotes do not have their genetic material contained inside a discrete nucleus. This does not mean bacteria have no structure at all! The structure of bacteria varies widely due to the many types and adaptations they have developed, but a bacterial cell has three main structural areas:
- the interior, called the cytoplasmic region,
- the surface, called the cell envelope, and
- the appendages.
The cytoplasmic region contains the cell genome (DNA) and ribosomes and other organic compounds necessary for life. Bacterial DNA is arranged in a single, closed loop.
The cell envelope consisting of a capsule, cell wall and plasma membrane. The capsule is composed of lipids and sugars which protect the bacteria from damage by the body's white blood cells which would otherwise consume it. The cell wall protects the cell and gives it its shape. The membrane transports material into and out of the cell and regulates cellular respiration.
Some kinds of bacteria ("Gram-negative") possess an outer membrane that resists antibiotics by preventing
their entry into the cell.
The appendages (proteins attached to the cell surface) in the form of a flagellum and pili. The pili (singular pilus), also called fimbriae, are short, hair-like structures. They allow the bacteria to attach to various surfaces, and a special subtype of pilus is used to transfer genetic material. The flagellum, if present, moves the cell with a whiplike swimming action. Bacteria without a flagellum can move in other ways. Myxobacteria, for example, secrete and slide through a layer of slime.
Bacteria have one of three basic forms: the sphere (Coccus), the rod (Bacillus), and the spiral (Spirillum). When cocci occur in a chain, they are called streptococci, and in a cluster, staphylococci.
A few bacteria manufacture food directly. Some of these use photosynthesis, making food from carbon dioxide
and water. Some of these (the cyanobacteria) use chloropyhyll2 but most use other means.
Others use chemosynthesis, making food from inorganic substances and water.
However, most bacteria obtain their food from living and dead organisms. Some strains are saprotrophs,
which feed by secreting digestive enzymes into organic matter and absorbing the resulting products.
The bacteria that cause food spoilage are of this form. The bacteria leave toxins in the food that can
damage or kill human tissue.
Many bacteria are parasites, which feed and get protection from a host organism. Most disease-causing bacteria are parasites. Disease results when bacteria multiply rapidly and damage or kill human tissue, as in pneumonia and tuberculosis. Diseases can also produce toxins that damage or kill human tissue, as in food poisoning or cholera.
Some bacteria are beneficial to their host, such as the Escherichia coli bacteria in human intestines.
Note that this form of E. coli is harmless to humans, it is the mutant form E. Coli 0157:H7
which is deadly.
Bacteria reproduce by binary fission, a form of asexual reproduction.
The loop of DNA replicates, and then the cell divides into two cells, each taking one loop of DNA.
This results in two cells. Bacteria can replicate every 20 minutes under ideal conditions.
This results in identical copies of the parent cell.
Most bacteria need oxygen, warmth, and water to reproduce.
Bacteria can mutate by exchanging genes. DNA may be passed directly between two bacteria
through contact at the pili, or it can be transferred between two bacteria by a virus.
DNA can also be absorbed from dead bacteria cells.
Bacteria can cause many diseases in humans. Here are some of the more common bacterial diseases:
- The others are viruses, protozoans, fungi, and helminths (worms).
- But not chloroplasts, as plants use.
- The word 'saprotroph’ is preferred for organisms such as bacteria, 'as saprophyte’ implies that
they are plants (reflecting obsolete taxonomy systems). Oxford Paperback Encyclopedia,
© Oxford University Press 1998