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Bacteria are classified into 5 groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios) or corkscrew (spirochaetes).
When conditions are favourable such as the right temperature and nutrients are available, some bacteria like Escherichia coli can divide every 20 minutes. Natural genetic transformation is believed to be the essential mechanism for the attainment of genetic plasticity in many species of bacteria. Transduction - the transfer of DNA by phage requires that the donor and recipient share cell surface receptors for phage binding and thus is usually limited to closely related bacteria; the length of DNA transferred is limited by the size of the phage head.
Each of these methods of genetic exchange can introduce sequences of DNA that share little homology with the remaining DNA of the recipient cell. A common way that Bacteria and Archae seem to different environmental conditions is via the acquisition of mosaic genes. Often, the integrated sequence contains a selectable genetic marker, such as antibiotic resistance.
One of the most well-characterized examples of mosaic genes are those that encode the penicillin-resistant binding proteins found in Streptococcus pneumoniae. These modified proteins are most likely obtained through intragenic recombination as a result of transformation events.
Therefore, these resistant strains that carry the mosaic allele express a phenotype that is favored by selection and the frequency of such genes that encode for antibiotic resistance tends to rise in a given population.
Another way that Bacteria and Archae can adapt to new environmental conditions is via the acquisition of large contiguous fragments of DNA.
Regions of the chromosome that seem to be acquired by the inheritance of long DNA sequences can be identified by comparison of genomes from closely related bacteria -- a region of DNA that is present in one strain of bacteria but lacking in other closely related strains indicates that either an insertion occured in the strain with the sequence or a deletion of that sequence occured in the other strain.
Inheritance of many genes simultaneously allows the inheritance of complex characteristics in a single step. The cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles.

In this process the bacterium, which is a single cell, divides into two identical daughter cells.
These are dormant structures, which are extremely resistant to hostile physical and chemical conditions such as heat, UV radiation and disinfectants. During bacterial evolution, the ability of Bacteria and Archaea to adapt to new environments most often results from the acquistion of new genes through horizontal transfer rather than by the alteration of gene functions through numerous point mutations.
If there are homologous sequences shared between the donor DNA and the recipient chromosome, the donor sequences can be stably incorporated into the recipient chromosome by genetic recombination. Therefore, horizontal exchange through transformation permits movement of alleles in bacterial lineages, resulting in mosaics that increase the opportunities for the spread of such genes as those that encode for antibiotic resistance. These high molecular weight proteins are the lethal targets of the B-Lactams of penicillin. This results in novel alleles that are mosaics containing genes from other streptococcal species that encode resistant proteins.
These regions are called "indels" or "islands" (sometimes shorter regions with these properties are called "islets"). Instead their control centre containing the genetic information is contained in a single loop of DNA. Viral transmission involves a virus (pink, lower left) obtaining a resistant gene, and passing it to a bacterial cell that incorporates it into its plasmid. If the homologous sequences flank sequences that are absent in the recipient, the recipient may acquire an insertion from another strain of unrelated bacteria. There are many examples of horizontal genetic exchange through both transformation and conjugation in bacteria species that have enabled them to better adapt to their environment. Therefore, some species have acquired modified proteins with low affinity for the B-lactams, resulting in a form of target-mediated anitbiotic resistance. Examples include "pathogenesis islands" (PAI) and islands that enable complex metabolic pathways (such as the genes required for vitamin B12 biosynthesis in Salmonella).

The bacterial cell then elongates and splits into two daughter cells each with identical DNA to the parent cell. Many endospore-producing bacteria are nasty pathogens, for example Bacillus anthracis is the cause of anthrax.
This type of resistance occurs when the species acquires a variant of the particular target of the antibiotic that has a normal or near-normal metabolic function but a low binding-affinity for the antibiotic. Evolution of coenzyme B12 synthesis among enteric bacteria: evidence for loss and reacquisition of a multigene complex.
The plasmid often contains genes that give the bacterium some advantage over other bacteria.
Mutations (not seen) may also occur, which may be antibiotic resistant and thus allow the bacteria to survive and reproduce. Some bacteria live in the soil or on dead plant matter where they play an important role in the cycling of nutrients. Large insertions that have been acquired from another bacterium (often inferred from differences in GC content or codon usage) and are absent from related strains of bacteria are called "islands". For example it may contain a gene that makes the bacterium resistant to a certain antibiotic. Some types cause food spoilage and crop damage but others are incredibly useful in the production of fermented foods such as yoghurt and soy sauce. Adaptation to the environment: Streptococcus pneumoniae, a paradigm for recombination-mediated genetic plasticity? Relatively few bacteria are parasites or pathogens that cause disease in animals and plants.

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06.02.2016, author: admin

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