Microbiology, Lec 15.
Genetic exchange among bacteria
I. Transformation - a cell taking up free DNA from its surroundings
II. Conjugation - requires cell-to-cell contact for the transfer of DNA
III. Transduction - the transfer of DNA due to a bacteriophage (bacterial virus)
occurs naturally in some bacterial species while others have to be forced into taking up DNA in the lab. Transformation was first demonstrated by Griffith in 1928 using mice and 2 strains of Streptococcus pneumoniae (lec. 7), a smooth (S) capsule-forming strain and a rough (R) mutant that can not make a capsule (fig 13.1).Avery et al. (1944) later showed that the transforming principle was DNA (Fig. 13.2).
Figs. 14.16 and 14.17 show transformation......
is another means of transferring DNA but requires a donor and a recipient cell. This is also the closest thing to sexual reproduction that occurs in bacteria (although actual "reproduction" does NOT have to be part of the process).
The F-plasmid is one of the better understood examples of conjugation in E. coli (figs. 14.14, 14.15, 14.12, 14.13).
{Plasmids (see lec. 3) - extra-chromosomal, circular, double-stranded DNA molecules. Plasmids divide and are apportioned to both daughter cells during asexual reproduction in bacteria (lec. 14). Many plasmids can be transmitted between closely related species and some are not limited to close relatives.}
The F plasmid in E. coli:
Transfer of plasmid DNA involves a cell that contains an F plasmid (F+) and a cell that does not contain an F-plasmid (F-)
Fig. 14.14a shows the transfer process between an F+ and an F- cell. Note that the F- cell is converted to an F+ cell as a result of a successful mating.An Hfr (high frequency of recombination) cell has the F-plasmid incorporated into the chromosome (fig. 14.7). This situation results in the whole chromosome being mobilizable. Thus, during a mating of an Hfr cell and an F- cell, chromosomal DNA is transferred to the F- cell (but the F- usually does not become F+ or Hfr; why not?)
The Hfr condition can be reversed (i.e. the plasmid can pop back out). However, if the reversal process is not identical to the integration step, then the newly formed plasmid (F') may contain some chromosomal genes. The new F plasmid (F') can then carry chromosomal genes to a recipient cell (Fig. 14.15).
Discuss the experiments that demonstrated that Bacteria could conjugate...
Fig. 14.12 and 14.13
These experiments were done using auxotrophic mutants ("auxotrophs") - see Fig. 13.15 for how such mutants are obtained.
is the transfer of DNA from one bacterium to another due to infection by a bacteriophage (a virus that infects bacteria). Bacteriophage infection involves a virus attaching to a specific cell and injecting nucleic acids. The viral nucleic acids "reprogram" the host cell and causes it to replicate the virus that is then assembled and released from the host cell. These new viruses can infect other cells and repeat the process.
Generalized transduction - in this situation, newly assembled viruses may contain DNA from the host cell and upon infection of a new cell can transfer the newly acquired DNA (Fig. 14.19). This typically makes the virus defective because it does not have the necessary information to force a cell to produce new viruses. Once this DNA is injected into a new bacterial cell recombination can occur and incorporate this new DNA.E. coli O157:H7 is the virulent strain that has been in the news lately. It contains a shiga-like toxin that was transferred from a Shigella sp. through transduction. This toxin is a protein that causes severe hemorrhaging from epithelial cells that line the intestine.
Specialized transduction - Some viruses form prophages meaning that the viral DNA incorporates itself into the host chromosome and does not immediately reprogram the cell to make new virus particles. When the prophage is later excised from the chromosome it can take some of the cells chromosomal DNA with it. This new viral DNA that contains the bacterial DNA will then be replicated by the cell and produce new viruses. These can then infect new cells and transfer this DNA (Fig 14.20).
Other types of Plasmids (see lec. 4) - extrachromosomal, circular, double-stranded DNA molecules. Plasmids divide and are apportioned to both daughter cells during asexual reproduction in bacteria (lec. 14). Many plasmids can be transmitted between closely related species and some are not limited to close relatives. Plasmids are not usually essential for a given organism, rather they allow the organism to adapt to specific environmental conditions. Therefore, plasmids are often unstable in a host bacterium due to the increased metabolic load.
A plasmid can encode for any one or more of the functions listed below (see also Table 14.1).
A. Antibiotic resistance - usually by coding for an enzyme that renders the antibiotic non-functional. Beta-lactamase (inactivates Beta-lactams) is one example (read pgs 690 - 692). Penicillin is a Beta-lactam.
B. Virulence - there are a number of ways that a plasmid can confer virulence in a bacterium. (read box 14.1)
1) The production of one or more toxins that can be directed toward the host or towards other bacteria (bacteriocins).C. Special metabolic properties -2) The ability to form a capsule.
The recent anthrax scare is an example of both 1 and 2. Virulent Bacillus anthracis has 2 plasmids that encode for toxin production and capsule formation. An avirulent strain used for veterinary vaccination lacks the capsule forming plasmids and is harmless.
3) The production of siderophores that enable the bacterium to scavenge iron in the body.
4) Adhesins - proteins or glycoproteins that are usually a component of capsules or fimbriae that allow the bacterium to adhere to specific cells.
some plasmids allow bacteria to take advantage of situations that might be otherwise harmful.Another example of an important plasmid:Breakdown of complex organic molecules is often plasmid encoded......
see Figure of the NAH7 plasmid of Pseudomonas putida, which contains 2 gene clusters (operons). Gene cluster nah1 codes for the first 6 enzymes in the naphthalene catabolic pathway, and gene cluster nah2 codes for the final 6 steps which convert salicylate to pyruvate (and you already know what happens to pyruvate in an aerobic chemoheterotroph like P. putida). Organisms that live in a salicylate-rich environment often have a modified plasmid containing only the nah2 operon (such a plasmid would usually be designated as a SAL plasmid; see Table 14.1).
An example of combining virulence and special metabolic properties as well as other strange things is the relationship between Agrobacterium tumefaciens and plants. A. tumefaciens has a plasmid (pg. 899) that:.
1) induces crown gall formation in plants (via excess hormone production),See also box 15.2, pg. 331 for how this plasmid is used in genetic engineering of plants
2) forces the plant to produce new compounds (opines), and
3) allows the bacterium to metabolize opines as their carbon and energy source
Genetic mobility within bacteria
Transposition involves small segments of DNA that can move around a chromosome or plasmid ("jumping genes"). For example, F-plasmids have insertion sequences (Fig. 14.7) that allow the plasmid to integrate into the chromosome (this allows the process shown in Fig. 14.8 to occur). The result is the formation of an Hfr cell. Insertion sequences (see Table 14.2) are very simple and typically contain only the information needed for insertion.
Transposons are slightly more complex and may contain a number of genes that confer antibiotic resistance and/or toxin production (see Table 14.3). Can be used in experiments to disrupt specific genes.