- Singular:
- bacterium
- Related Topics:
- rickettsia
- campylobacter
- nitrogen-fixing bacteria
- sulfur bacterium
- eubacterium
- On the Web:
- Arizona State University - Ask A Biologist - Bacteria (Apr. 14, 2025)
The genetic information of all cells resides in the sequence of nitrogenous bases in the extremely long molecules of DNA. Unlike the DNA in eukaryotic cells, which resides in the nucleus, DNA in bacterial cells is not sequestered in a membrane-bound organelle but appears as a long coil distributed through the cytoplasm. In many bacteria the DNA is present as a single circular chromosome, although some bacteria may contain two chromosomes, and in some cases the DNA is linear rather than circular. A variable number of smaller, usually circular (though sometimes linear) DNA molecules, called plasmids, can carry auxiliary information.
The sequence of bases in the DNA has been determined for hundreds of bacteria. The amount of DNA in bacterial chromosomes ranges from 580,000 base pairs in Mycoplasma genitalium to 4,700,000 base pairs in E. coli to roughly 9,450,000 base pairs in Myxococcus xanthus. Sorangium cellulosum, a myxobacterium, has one of the largest bacterial genomes, containing in excess of 13 million base pairs. The length of the E. coli chromosome, if removed from the cell and stretched to its fullest extent, is about 1.2 mm, which is striking in view of the fact that the length of the cell is about 0.001 mm.
As in all organisms, bacterial DNA contains the four nitrogenous bases adenine (A), cytosine (C), guanine (G), and thymine (T). The rules of base pairing for double-stranded DNA molecules require that the number of adenine and thymine bases be equal and that the number of cytosine and guanine bases also be equal. The relationship between the number of pairs of G and C bases and the number of pairs of A and T bases is an important indicator of evolutionary and adaptive genetic changes within an organism. The proportion, or molar ratio, of G + C can be measured as G + C divided by the sum of all the bases (A + T + G + C) multiplied by 100 percent. The extent to which G + C ratios vary between organisms may be considerable. In plants and animals, the proportion of G + C is about 50 percent. A far wider range in the proportion of G + C is seen in prokaryotes, extending from about 25 percent in most Mycoplasma to about 50 percent in E. coli to nearly 75 percent in Micrococcus, actinomycetes, and fruiting myxobacteria. The G + C content within a species in a single genus, however, is very similar.
Cytoplasmic structures
The cytoplasm of bacteria contains high concentrations of enzymes, metabolites, and salts. In addition, the proteins of the cell are made on ribosomes that are scattered throughout the cytoplasm. Bacterial ribosomes are different from ribosomes in eukaryotic cells in that they are smaller, have fewer constituents (consist of three types of ribosomal RNA [rRNA] and 55 proteins, as opposed to four types of rRNA and 78 proteins in eukaryotes), and are inhibited by different antibiotics than those that act on eukaryotic ribosomes.
There are numerous inclusion bodies, or granules, in the bacterial cytoplasm. These bodies are never enclosed by a membrane and serve as storage vessels. Glycogen, which is a polymer of glucose, is stored as a reserve of carbohydrate and energy. Volutin, or metachromatic granules, contains polymerized phosphate and represents a storage form for inorganic phosphate and energy. Many bacteria possess lipid droplets that contain polymeric esters of poly-β-hydroxybutyric acid or related compounds. This is in contrast to eukaryotes, which use lipid droplets to store triglycerides. In bacteria, storage granules are produced under favorable growth conditions and are consumed after the nutrients have been depleted from the medium. Many aquatic bacteria produce gas vacuoles, which are protein-bound structures that contain air and allow the bacteria to adjust their buoyancy. Bacteria can also have internal membranous structures that form as outgrowths of the cytoplasmic membrane.
Biotypes of bacteria
The fact that pathogenic bacteria are constantly battling their host’s immune system might account for the bewildering number of different strains, or types, of bacteria that belong to the same species but are distinguishable by serological tests. Microbiologists often identify bacteria by the presence of specific molecules on their cell surfaces, which are detected with specific antibodies. Antibodies are serum proteins that bind very tightly to foreign molecules (antigens) in an immune reaction aimed at removing or destroying the antigens. Antibodies have remarkable specificity, and the substitution of even one amino acid in a protein might prevent that protein from being recognized by an antibody.
For many bacterial species there are thousands of different strains (called serovars, for serological variants), which differ from one another mainly or solely in the antigenic identity of their lipopolysaccharide, flagella, or capsule. Different serovars of enteric bacteria—such as E. coli and Salmonella enterica, for example—are often found to be associated with the ability to inhabit different host animals or to cause different diseases. Formation of these numerous serovars reflects the ability of bacteria to respond effectively to the intense defensive actions of the immune system.
Bacterial reproduction
Reproductive processes
Binary fission
Most prokaryotes reproduce by a process of binary fission, in which the cell grows in volume until it divides in half to yield two identical daughter cells. Each daughter cell can continue to grow at the same rate as its parent. For this process to occur, the cell must grow over its entire surface until the time of cell division, when a new hemispherical pole forms at the division septum in the middle of the cell. In gram-positive bacteria the septum grows inward from the plasma membrane along the midpoint of the cell; in gram-negative bacteria the walls are more flexible, and the division septum forms as the side walls pinch inward, dividing the cell in two. In order for the cell to divide in half, the peptidoglycan structure must be different in the hemispherical cap than in the straight portion of the cell wall, and different wall-cross-linking enzymes must be active at the septum than elsewhere.
Budding
A group of environmental bacteria reproduces by budding. In this process a small bud forms at one end of the mother cell or on filaments called prosthecae. As growth proceeds, the size of the mother cell remains about constant, but the bud enlarges. When the bud is about the same size as the mother cell, it separates. This type of reproduction is analogous to that in budding fungi, such as brewer’s yeast (Saccharomyces cerevisiae). One difference between fission and budding is that, in the latter, the mother cell often has different properties from the offspring. In some Pasteuria strains, the daughter buds have a flagellum and are motile, whereas the mother cells lack flagella but have long pili and holdfast appendages at the end opposite the bud. The related Planctomyces, found in plankton, have long fibrillar stalks at the end opposite the bud. In Hyphomicrobium a hyphal filament (prostheca) grows out of one end of the cell, and the bud grows out of the tip of the prostheca, separated by a relatively long distance from the mother cell.
Sporulation
Many environmental bacteria are able to produce stable dormant, or resting, forms as a branch of their life cycle to enhance their survival under adverse conditions. These processes are not an obligate stage of the cell’s life cycle but rather an interruption. Such dormant forms are called endospores, cysts, or heterocysts (primarily seen in cyanobacteria), depending on the method of spore formation, which differs between groups of bacteria.
The ability to form endospores is found among bacteria in a number of genera, predominantly gram-positive groups, including the aerobic rod Bacillus, the microaerophilic rod Sporolactobacillus, the anaerobic rods Clostridium and Desulfotomaculum, the coccus Sporosarcina, and the filamentous Thermoactinomyces. The formation of a spore occurs in response to nutritional deprivation. Consequently, endospores do not possess metabolic activity until nutrients become available, at which time they are able to differentiate from spores into vegetative cells. Only one spore is formed inside each bacterial cell during sporulation. The formation of a spore begins with invagination of the cytoplasmic membrane around a copy of the bacterial chromosome, thus separating the contents of the smaller cell from the mother cell. The membrane of the mother cell engulfs the smaller cell within its cytoplasm, effectively providing two concentric unit membranes to protect the developing spore. A thin spore membrane and a thick cortex of a peptidoglycan are laid down between the two unit membranes. A rigid spore coat forms outside the cortex, enclosing the entire spore structure. The spore coat has keratin-like properties that are able to resist the lethal effects of heat, desiccation (dehydration), freezing, chemicals, and radiation. The ability of endospores to resist these noxious agents may ensue from the extremely low water content inside the spore. Methane-oxidizing bacteria in the genus Methylosinus also produce desiccation-resistant spores, called exospores.
Cysts are thick-walled structures produced by dormant members of Azotobacter, Bdellovibrio (bdellocysts), and Myxococcus (myxospores). They are resistant to desiccation and other harmful conditions but to a lesser degree than are endospores. In encystment by the nitrogen-fixing Azotobacter, cell division is followed by the formation of a thick, multilayered wall and coat that surround the resting cell. The filamentous actinomycetes produce reproductive spores of two categories: conidiospores, which are chains of multiple spores formed on aerial or substrate mycelia, or sporangiospores, which are formed in specialized sacs called sporangia.
