cells is broken before the achievement of the process.
Episomes carry specific sequences called insertion sequences.
If a chromosome possesses similar insertion sequences,
homologous recombination events can occur between the
chromosome and the episome, leading to the insertion of the
plasmid into the chromosome. Once the episome is integrated,
the entire chromosome becomes mobilizable and thus transferrable via the conjugative process. Cells with a conjugative
plasmid integrated into their chromosomes are called HFR for
high-frequency recombination because they are able to transfer part of their chromosomes to recipient cells by conjugation.
HFR bacteria have played an important role in the history of
microbiology by allowing establishing the first physical maps
of bacterial chromosomes. In fact, it is possible to determine
the relative position of genes in chromosomes by measuring
the frequency of their cotransfer. For example, if E. coli K-12
markers B
+ and M
+ are systematically cotransferred during
conjugation, this indicates that the corresponding loci are close
on the chromosome. Conversely, if two loci are distant on the
chromosome, their cotransfer during conjugation will be less
frequent as the probability that the two cells are separated
before the transfer of the second marker is high. Precise
chromosomal maps have been established by measuring the
time required for the transfer of each genetic marker. Loci
located near to the episome insertion site penetrate first into the
recipient cell and are thus transferred faster than loci located at
the other end. The insertion of an episome into a chromosome
is reversible as the plasmid can be excised by reverse recombination events involving the same insertion sequences.
Because plasmids and chromosomes generally possess multiple
copies of these insertion sequences, excision events can occur
in such a way that chromosome fragments carrying chromosomal genes can be excised with the episome. If the resulting
episome is involved in new conjugation events, the genes of
chromosomal origin can be transferred to the recipient cell.
At the time of its discovery, conjugation was considered a
mechanism of horizontal gene transfer occurring among
cells belonging to the same species. This idea was
questioned after the discovery of similar conjugative
plasmids in cells belonging to very different taxonomic
groups. Such taxonomic distribution suggested that conjugation could occur among distant microbial species. This
hypothesis has been confirmed by laboratory experiments
which have also showed that conjugation can occur among
organisms from different domains (e.g., from bacterial to
yeast cells, namely, between bacteria and eukaryotes)
(Heinemann and Sprague 1989).
12.2.1.3 Transduction
This mechanism consists in transferring DNA between bacterial cells via bacteriophages. Bacteriophages were discovered in 1915 (Twort 1915). At that time, Twort did not make
the link between these agents and the viruses infecting the
cells of plants and animals that had been described a few
years earlier (Duckworth 1976). This link was made few
years later by Felix d’Herelle, who isolated agents capable
of destroying the bacteria responsible of dysentery. He proposed to call bacteriophages the viruses having the capacity
of killing bacteria (d’Herelle 1917). These viruses would
have been now called “lytic phages” because they are able
to induce the immediate lysis of the target cells through a
lytic cycle. They are opposed to the “temperate phages,”
which are able to integrate at a specific site of the genome
of the target cells, where they can remain silent for many
generations. Temperate phages are able to induce a lysogenic cycle in response to an environmental stimulus or
stress. In that case, the genome of the phage is excised
from the bacterial chromosome and the lytic cycle is
initiated. This leads to the lysis of the cell and the release
of new phage particles.
The involvement of bacteriophages in HGT was
demonstrated later by Zinder during his 2-year stay in the
laboratory headed by Lederberg (Zinder 1992). His work
aimed at extending the work of Lederberg on bacterial
conjugation. To do so, he worked on Salmonella
typhimurium strains (Zinder and Lederberg 1952) and
observed that the crossing of strain LT-22 (auxotroph for
phenylalanine and tryptophan, due to a double mutation in
the pathway of aromatic amino acid synthesis) and strain
LT-2 (auxotroph for methionine and histidine) produced
recombinant phenotypes at higher frequencies than in the
case of E. coli (10
À5 compared to 10
À6 /10
À7 ). Rapidly,
significant differences became apparent between the studied
phenomenon and conjugation. First, the physical contact
between the two partners was not required. Second, selfcrosses between pairs of S. typhimurium mutants belonging
to the same strain but carrying complementary mutations
failed, whereas all the E. coli K-12 strains were interfertile.
In addition, irrespective of the considered markers, the
recombination rates were always constant (10
À5
). Furthermore, in contrast to transformation, the filter-sterilized
supernatants from the separate cultures were without effect,
whereas the supernatant of a coculture could convert LT-22
but not LT-2 mutants. Finally, the recombinant colonies
presented a genomic background similar to that of strain
LT-22, suggesting that strain LT-22 was the recipient.
Shortly after, Zinder and Lederberg put in evidence that
the strain LT-22 carries a lysogenic phage (PLT-22 or P22)
that can infect and lyse strain LT-2. During this process,
genome fragments of LT-2 can be incorporated to the
genomes of newly produced phages and transferred to LT22 cells, restoring the aromatic amino acid biosynthesis
pathway of this strain (Zinder 1992). This new mechanism
of HGT was named by its discoverers “transduction.”
Research works have revealed two types of transduction
mechanisms: the generalized transduction and the
12 Horizontal Gene Transfer in Microbial Ecosystems
449
Episomes carry specific sequences called insertion sequences.
If a chromosome possesses similar insertion sequences,
homologous recombination events can occur between the
chromosome and the episome, leading to the insertion of the
plasmid into the chromosome. Once the episome is integrated,
the entire chromosome becomes mobilizable and thus transferrable via the conjugative process. Cells with a conjugative
plasmid integrated into their chromosomes are called HFR for
high-frequency recombination because they are able to transfer part of their chromosomes to recipient cells by conjugation.
HFR bacteria have played an important role in the history of
microbiology by allowing establishing the first physical maps
of bacterial chromosomes. In fact, it is possible to determine
the relative position of genes in chromosomes by measuring
the frequency of their cotransfer. For example, if E. coli K-12
markers B
+ and M
+ are systematically cotransferred during
conjugation, this indicates that the corresponding loci are close
on the chromosome. Conversely, if two loci are distant on the
chromosome, their cotransfer during conjugation will be less
frequent as the probability that the two cells are separated
before the transfer of the second marker is high. Precise
chromosomal maps have been established by measuring the
time required for the transfer of each genetic marker. Loci
located near to the episome insertion site penetrate first into the
recipient cell and are thus transferred faster than loci located at
the other end. The insertion of an episome into a chromosome
is reversible as the plasmid can be excised by reverse recombination events involving the same insertion sequences.
Because plasmids and chromosomes generally possess multiple
copies of these insertion sequences, excision events can occur
in such a way that chromosome fragments carrying chromosomal genes can be excised with the episome. If the resulting
episome is involved in new conjugation events, the genes of
chromosomal origin can be transferred to the recipient cell.
At the time of its discovery, conjugation was considered a
mechanism of horizontal gene transfer occurring among
cells belonging to the same species. This idea was
questioned after the discovery of similar conjugative
plasmids in cells belonging to very different taxonomic
groups. Such taxonomic distribution suggested that conjugation could occur among distant microbial species. This
hypothesis has been confirmed by laboratory experiments
which have also showed that conjugation can occur among
organisms from different domains (e.g., from bacterial to
yeast cells, namely, between bacteria and eukaryotes)
(Heinemann and Sprague 1989).
12.2.1.3 Transduction
This mechanism consists in transferring DNA between bacterial cells via bacteriophages. Bacteriophages were discovered in 1915 (Twort 1915). At that time, Twort did not make
the link between these agents and the viruses infecting the
cells of plants and animals that had been described a few
years earlier (Duckworth 1976). This link was made few
years later by Felix d’Herelle, who isolated agents capable
of destroying the bacteria responsible of dysentery. He proposed to call bacteriophages the viruses having the capacity
of killing bacteria (d’Herelle 1917). These viruses would
have been now called “lytic phages” because they are able
to induce the immediate lysis of the target cells through a
lytic cycle. They are opposed to the “temperate phages,”
which are able to integrate at a specific site of the genome
of the target cells, where they can remain silent for many
generations. Temperate phages are able to induce a lysogenic cycle in response to an environmental stimulus or
stress. In that case, the genome of the phage is excised
from the bacterial chromosome and the lytic cycle is
initiated. This leads to the lysis of the cell and the release
of new phage particles.
The involvement of bacteriophages in HGT was
demonstrated later by Zinder during his 2-year stay in the
laboratory headed by Lederberg (Zinder 1992). His work
aimed at extending the work of Lederberg on bacterial
conjugation. To do so, he worked on Salmonella
typhimurium strains (Zinder and Lederberg 1952) and
observed that the crossing of strain LT-22 (auxotroph for
phenylalanine and tryptophan, due to a double mutation in
the pathway of aromatic amino acid synthesis) and strain
LT-2 (auxotroph for methionine and histidine) produced
recombinant phenotypes at higher frequencies than in the
case of E. coli (10
À5 compared to 10
À6 /10
À7 ). Rapidly,
significant differences became apparent between the studied
phenomenon and conjugation. First, the physical contact
between the two partners was not required. Second, selfcrosses between pairs of S. typhimurium mutants belonging
to the same strain but carrying complementary mutations
failed, whereas all the E. coli K-12 strains were interfertile.
In addition, irrespective of the considered markers, the
recombination rates were always constant (10
À5
). Furthermore, in contrast to transformation, the filter-sterilized
supernatants from the separate cultures were without effect,
whereas the supernatant of a coculture could convert LT-22
but not LT-2 mutants. Finally, the recombinant colonies
presented a genomic background similar to that of strain
LT-22, suggesting that strain LT-22 was the recipient.
Shortly after, Zinder and Lederberg put in evidence that
the strain LT-22 carries a lysogenic phage (PLT-22 or P22)
that can infect and lyse strain LT-2. During this process,
genome fragments of LT-2 can be incorporated to the
genomes of newly produced phages and transferred to LT22 cells, restoring the aromatic amino acid biosynthesis
pathway of this strain (Zinder 1992). This new mechanism
of HGT was named by its discoverers “transduction.”
Research works have revealed two types of transduction
mechanisms: the generalized transduction and the
12 Horizontal Gene Transfer in Microbial Ecosystems
449