Bacteria - Microbial Genetics

BACTERIA

The genes essential for growth and metabolism of bacteria are normally contained on a chromosome of double-stranded DNA, which is in the form of a covalently closed circle (and so designated ccc ds DNA). Additional genes that usually just confer upon the cell a survival advantage under certain circumstances may also be contained upon plasmids; these are usually similar in structure to chromosomes but much smaller and replicate independently (Chapters 3 and 13). The total complement of genes possessed by a cell, i.e. those in the chromosome, plasmid(s) and any received from other sources, e.g. bacteriophages (bacterial viruses), is referred to as the genome of the cell.

Typically bacterial chromosomes are 1 mm or more in length and contain about 1000–3000 genes. As many bacterial cells are approximately 1 µm long, it is clear that the chromosome has to be tightly coiled in order to fit in the available volume. Although all the genes are contained on a single chromosome (rather than being distributed over two or more), it is possible for a cell to contain several copies of that chromosome at any one time. Usually there are multiple copies during periods of rapid cell division, but some species seem to have many copies all the time. The mechanisms by which bacterial genes may be transferred from one organism to another are described in Chapter 3.

Plasmids usually resemble chromosomes except that they are approximately 0.1–1.0% of the size of a bacterial chromosome, and there are a few that are linear rather than circular. Plasmid genes are not essential for the normal functioning of the cell but may code for a property that affords a survival advantage in certain environmental conditions; bacteria possessing the plasmid in question would therefore be selected when such conditions exist. Properties which can be coded by plasmids include the ability to utilize unusual sugars or food sources, toxin production, production of pili that facilitate the attachment of a cell to a substrate (e.g. intestinal epithelium) and antibiotic resistance. A cell may contain multiple copies of any one plasmid and may contain two or more different plasmids. However, some plasmid combinations cannot coexist inside the same cell and are said to be incompatible; this phenomenon enables plasmids to be classified into incompatibility groups.

Plasmids replicate independently of the chromosome within the cell, so that both daughter cells contain a copy of the plasmid after binary fission. Plasmids may also be passed from one cell to another by various means . Some plasmids exhibit a marked degree of host specificity and may only be transmitted between different strains of the same species, although others, particularly those commonly found in Gram-negative intestinal bacteria, may cross between different species within a genus or between different genera. Conjugative (self transmissible) plasmids code for genes that facilitate their own transmission from one cell to another by the production of pili. These sex pili initially establish contact between the two cells and then retract, drawing the donor and recipient cells together until membrane fusion occurs.