Bacterial Conjugation
Bacterial conjugation is a process by which bacteria transfer genetic material, such as plasmids, from one cell to another. This transfer occurs through direct cell-to-cell contact facilitated by a specialized structure called a pilus. Bacterial conjugation plays a significant role in the spread of antibiotic resistance and genetic diversity among bacterial populations.
6 Key excerpts on "Bacterial Conjugation"
eBook - ePub- Jeremy W. Dale, Simon F. Park(Authors)
- 2013(Publication Date)
- Wiley(Publisher)
...These methods are dealt with in Chapter 8. 6.2 Conjugation Conjugation is the direct transmission of DNA from one bacterial cell to another. In most cases this involves the transfer of plasmid DNA, although with some organisms chromosomal transfer can also occur. As with other modes of gene transfer in bacteria, there is a one-way transfer of DNA from one parent (donor) to the other (recipient). In the simplest of cases, conjugation is achieved in the laboratory by mixing the two strains together and, after a period of incubation to allow conjugation to occur, plating the mixture onto a medium that does not allow either parent to grow, but on which a transconjugant that contains genes from both parents will grow. For example, in the experiment illustrated in Figure 6.1, one strain (the donor) carries a plasmid that confers resistance to ampicillin, while the second strain does not have a plasmid but has a chromosomal mutation that makes it resistant to nalidixic acid. After incubating the mixed culture, a sample is plated onto a medium containing both antibiotics. Neither parent can grow on this medium, so the colonies that are observed are due to the transfer of a copy of the plasmid from the donor to a recipient cell. Although this event may be quite infrequent, the powerful selection provided by the two antibiotics means that we can readily detect plasmid transfer even if only, say, 1 in 10 6 recipients have received a copy of it. Conjugation is most easily demonstrated among members of the Enterobacteriaceae and other Gram-negative bacteria (such as vibrios and pseudomonads)...
eBook - ePub- David P. Clark(Author)
- 2009(Publication Date)
- Academic Cell(Publisher)
...18.11).The donor cell manufactures a sex pilus that binds to a suitable recipient and draws the two cells together. Next, a conjugation bridge forms between the two cells and provides a channel for DNA to move from donor to recipient. In real life, mating bacteria actually tend to cluster together in groups of five to ten (Fig. 18.12). Figure 18.11 Bacterial Conjugation Certain plasmids, called Tra + or transfer positive, are able to move a copy of their DNA into a different cell through a mechanism called Bacterial Conjugation. First the cell containing a Tra + plasmid manufactures a rod-like extension on the surface of the outer membrane, called a sex pilus. The sex pilus binds to a nearby cell and pulls the two cells together. Once the cells are in contact, a connection is made between the two cells called the conjugation bridge. This connects the cytoplasm of the two cells, so the plasmid can transfer a copy of itself to the recipient cell. Figure 18.12 Conjugating Cells of Escherichia coli False-color transmission electron micrograph (TEM) of a male Escherichia coli bacterium (bottom right) conjugating with two females. This male has attached two F-pili to each of the females. The tiny bodies covering the F-pili are bacteriophage MS2, a virus that attacks only male bacteria and binds specifically to F-pili. Magnification: ×11,250. Credit: Dr. L. Caro, Photo Researchers, Inc. attλ Lambda attachment site—site where lambda inserts its DNA into the bacterial chromosome defective phage Mutant phage that lacks genes for making virus particles helper phage Phage that provides the necessary genes so allowing a defective phage to make virus particles transferability Ability of certain plasmids to move themselves from one bacterial cell to another The genes for formation of the sex pilus and conjugation bridge and for overseeing the DNA transfer process are known as tra genes and are all found on the plasmid itself...
- Tina M. Henkin, Joseph E. Peters(Authors)
- 2020(Publication Date)
- ASM Press(Publisher)
...As noted in the introduction, Lederberg and Tatum suspected that bacteria of the two strains exchanged DNA—that is, two parental strains mated to produce progeny unlike themselves but with characteristics of both parents. At that time, however, plasmids were unknown, and it was not until later that the basis for the mating was understood. The plasmid that Lederberg and Tatum were unknowingly working with—called the fertility plasmid, or F plasmid —has been the focus of much of the research about the process of Bacterial Conjugation. The central role conjugation systems played in the development of bacterial genetics contrasts with the more notorious role these plasmids play as vectors for the spread of antibiotic resistance between bacteria. While historically studied with plasmids, conjugation is now also known to be used very frequently in the transfer of elements that are not maintained separately from the chromosome but instead are integrated into the chromosome and are therefore called integrating conjugative elements (ICEs). Their integration into the host chromosome typically uses the same type of recombination that is used for the integration of many bacteriophages. The same families of conjugation systems are found in ICEs and conjugal plasmids, indicating that over evolutionary time the same systems are alternatively utilized for intra- and extrachromosomal lifestyles (see Cury et al., Suggested Reading). Overview During plasmid conjugation, the two strands of an element separate in a process resembling rolling-circle replication (see “Mechanism of DNA Transfer during Conjugation in Proteobacteria” below), and one strand moves from the bacterium that originally contained the plasmid—the donor —into a recipient bacterium. The two single strands serve as templates for DNA replication concurrent with the process of DNA transfer to yield double-stranded DNA molecules in both the donor cell and the recipient cell...
eBook - ePubAdvanced Molecular Biology
A Concise Reference
- Richard Twyman(Author)
- 2018(Publication Date)
- Garland Science(Publisher)
...The first three processes are passive with respect to the plasmid, whereas conjugation is active and many plasmids carry genes which promote self-transfer between cells by this method. Bacterial plasmids may thus be classified into two major groups, conjugative and nonconjugative. Conjugative plasmids are subdivided into families based on their particular conjugation mechanism (Box 20.1). In eukaryotes, plasmid transfer mechanism is a less useful criterion for classification. Horizontal plasmid transfer generally occurs only when cells fuse (e.g. syngamy, or the formation of a hyphal network in fungi) or occasionally by mechanical transfer (viroids spread in this manner). Occasionally, bacteria transfer plasmids to eukaryotes, as occurs in bacteria to yeast conjugation and in the specialized case of the Agrobacterium tumorfaciens Ti plasmid (q.v.). 20.2 Plasmid replication and maintenance Plasmid copy number. Plasmid replication and maintenance is a totally intrinsic property applicable to all plasmids, allowing them to be classified according to replication and partition strategy and how their copy number is regulated. Table 20.3: Classification and nomenclature of plasmids based on phenotype Type of plasmid Phenotype Examples Bacteriocinogenic Encode bacteriocins (proteins which kill or inhibit growth of other bacteria, e.g. agrocins and colicins). Also encode immunity functions so that the host cell is not destroyed. Killer plasmids in yeast are analogous AgK84, CIoDF13, CoIE1-K30, CoIV, I-K94 Designation by encoded bacteriocin (e.g. agrocin 84, cloacin DF13, colicin E1, colicins V and Ia) Cryptic None 2μ, Mauriceville Degradative Catabolic Lac, TOL, Cit Designation by substrate (e.g. lactose, toluene, citrate) F Fertility (conjugal transfer) The F-plasmid (see Gene Transfer in Bacteria) R Resistance (e.g...
eBook - ePubHandbook of Microbiology
Condensed Edition
- Allen I Laskin(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
...Transfer of Genetic Information DR. HUBERT LECHEVALIER In eucaryotic organisms with a sexual cycle, conjugation between gametes is the usual mode of reproduction. It is a well-ordered series of events, which in its simplest form takes place between identical gametes or isogametes. With increased complexity, the conjugating gametes become different from each other, and the largest one, which is also the least active, is attributed to the female sex. In bacteria, at least in Escherichia coli, the mating type is determined by the presence or the absence of replicons called episomes, which may or may not be incorporated into the main replicon of the bacterial cell; the main replicon is called the bacterial chromosome. Bacterial cells containing an episome called the F factor are considered “male”. A male cell can transfer its F episome to “female” cells lacking it, thus producing a change in their “sex”. Another example of transfer of episomes is that of R factors, which results in the transfer of drug resistance from one bacterial cell to another. Thus, bacteria that have never been exposed to a certain inhibitory drug may become resistant to it. Other methods of transfer of genetic information in bacteria include transformation, transduction, transfection, and viral conversion. Transformation is the uptake of DNA coming from one bacterium by another bacterium (Figure 1). Transduction is the transfer of information from one bacterium to another through the agency of a phage (Figure 1). Transfection is the uptake of viral nucleic acid by bacteria, resulting in the production of complete virions by the affected bacteria. Viral conversion is a change in the properties of a bacterium due to the presence in its cell of viral nucleic acid that is not necessarily included in the bacterial genome (Figure 1). In asexual fungi, exchange of genetic information may take place through parasexual activities...
eBook - ePub- Graham F. Hatfull, William R. Jacobs, Graham F. Hatfull, William R. Jacobs(Authors)
- 2014(Publication Date)
- ASM Press(Publisher)
...Conjugation generally involves the transfer of plasmids, or integrative and conjugative elements (ICEs). ICEs are found in the chromosome, but excise to form a plasmid-like circle before transferring into a recipient and reintegrating into the chromosome (18). Most of the early characterization of conjugative plasmids and the transfer process was completed in Escherichia coli, but the mechanism of transfer is essentially the same for both plasmids and ICEs, regardless of bacterial species (16, 19). The first step in conjugation is establishment of cell-cell contact between the donor and recipient cells, or mating-pair formation (MPF). In Gram-negative species, MPF is facilitated by the plasmid-encoded pilus that is assembled by a type IV secretion system (T4SS) (20, 21). Gram-positive species also encode many of the T4SS proteins, but they are mainly localized to the cytoplasm or membrane and are therefore proposed to assemble as the conduit of DNA transfer and not to assemble pilus structures (21, 22). Instead, Gram-positive mating pairs are likely formed by other mechanisms including surface-exposed adhesins, which cause aggregation of donor and recipient enterococci cells (23, 24). Once MPF is established, DNA-processing enzymes mediate transfer of a single strand of DNA from the donor to the recipient (Fig. 1 A). A key protein is a relaxase, which induces a strand-specific nick within a sequence called the origin of transfer, oriT (16). The relaxase, bound to the 5′ end of the nicked DNA, mediates transfer of the DNA through the pore and into the recipient. On completion of plasmid transfer, the relaxase recognizes the 3′ end of oriT and recircularizes the single strand by a reversal of the nicking process. Complementary DNA synthesis in the recipient results in a transconjugant containing a copy of the conjugative plasmid and, thus, is now a donor...
