Bacteria are the common name for a vast group of one-celled microscopic organisms that encompasses the smallest, simplest, and perhaps first form of cell life that evolved. They are unicellular and furnish both the raw material and the chemical machinery for their own reproduction, whereas viruses, for example, do not.

SIZE AND HABITAT

Thirty trillion bacteria of average size would weigh about 28 g (1 oz). Bacteria are measured in microns (0.001 micrometers, about 0.00004 in) and most types range in size from 0.1 to 4.0 microns in width and 0.2 to 50 microns in length. Bacteria are found everywhere. Approximately 2,000 species have been identified, many of them living in conditions that would destroy any other organism. They have been found in the almost airless reaches of the upper atmosphere, 10 km (6 mi) below the surface of the ocean, living in frozen soil, and attached to rocks in hot springs. Some bacteria produce a resting stage, the endospore, which is the most resistant living thing known and cannot be killed except by boiling in steam under pressure for many hours.

STRUCTURE

Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA forms a tangle known as a nucleoid, but there is no membrane around the nucleoid, and the DNA is not bound to proteins as it is in eukaryotes. Whereas eukaryote DNA is organized into linear pieces, the chromosomes, bacterial DNA forms loops. Bacteria contain plasmids, or small loops of DNA, that can be transmitted from one cell to another, either in the course of sex (yes, bacteria have sex) or by viruses. This ability to trade genes with all comers makes bacteria amazingly adaptible; beneficial genes, like those for antibiotic resistance, may be spread very rapidly through bacterial populations. It also makes bacteria favorites of molecular biologists and genetic engineers; new genes can be inserted into bacteria with ease.

Bacteria do not contain membrane-bound organelles such as mitochondria or chloroplasts, as eukaryotes do. However, photosynthetic bacteria, such as cyanobacteria, may be filled with tightly packed folds of their outer membrane. The effect of these membranes is to increase the potential surface area on which photosynthesis can take place.

The cell membrane is surrounded by a cell wall in all bacteria except one group, the Mollicutes, which includes pathogens such as the mycoplasmas. The composition of the cell wall varies among species and is an important character for identifying and classifying bacteria. In this diagram, the bacterium has a fairly thick cell wall made of peptidoglycan (carbohydrate polymers cross-linked by proteins); such bacteria retain a purple color when stained with a dye known as crystal violet, and are known as Gram-positive (after the Danish bacteriologist who developed this staining procedure). Other bacteria have double cell walls, with a thin inner wall of peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. Such bacteria do not stain purple with crystal violet and are known as Gram-negative.

PHYSIOLOGY

In terms of metabolism, the diversity of life is much more evident in bacteria than in other organisms. Bacteria display a staggering variety of mechanisms for obtaining energy that have no parallel in higher organisms.

Bacteria may be classified on the basis of their requirements for free atmospheric oxygen.

Those requiring oxygen are aerobes.

Those which cannot live in the presence of oxygen are obligate anaerobes.

Those which do well with oxygen but can survive without it are facultative anaerobes. These are phototrophs: they can photosynthesise and do have chlorophyll.

REPRODUCTION

Most bacteria reproduce asexually by binary fission, in which a single cell divides in two after developing a wall across its width. Many species divide as often as every 20 minutes under favorable conditions. If all the descendants survived, the initial cell would result in about 500,000 new cells after 6 hours. Increases to large numbers in a short period of time help to explain the rapid development of disease, food spoilage, decay, and the speed at which certain chemical processes used in industry take place.

Certain bacteria such as E. coli reproduce by conjugation, which resembles sexual reproduction, in that the two bacteria join (mate) and exchange genes. As in true sexual reproduction, the genetic material, or "nuclear" chromosomes, recombine with one another. In the process of recombination, a fragment of a chromosome transmitted from one bacterium is incorporated in the chromosome of the recipient. Conjugation and recombination increase the total number of different hereditary characteristics in a population of bacteria, increasing the bacteria's chances of survival.

Conjugation can occur between bacteria of different species. Thus a resistance for a particular antibiotic may be passed on from a harmless bacteria to one that can produce disease.

Conjugation