Bacteria engage in horizontal, or lateral, gene transfer, meaning that genes are exchanged
between cells of the same generation. In contrast, vertical gene transfer occurs when parents
pass genes to their progeny. The three methods of horizontal gene transfer that bacteria
employ are conjugation, transformation, and transduction, with conjugation being the most
common. Transformation and transduction typically take place between bacteria of the same or
closely-related species. Conjugation is encoded by plasmids or transposons. Plasmids are circular
DNA sections that replicate independently of chromosomes. Transposons, also known as
transposable elements, or jumping genes, are mobile sections of DNA that can move within
or between genomes. Conjugation with plasmids is more common than conjugation with transposons.
Conjugation requires direct cell-cell contact. A conjugative plasmid or conjugative transposon
is self-transmissible – in other words, it has all the genes need to connect with
another bacterium and transmit itself to another bacterium via conjugation. We still don’t
know how the majority of gram-positive bacteria achieve the intimate association of cell surfaces
required, however, in gram-negative bacteria, this typically involves a conjugation pilus,
also known as an F pilus, or a sex pilus. The conjugation pilus binds the other bacterium,
then retracts to pull the two cells together. Once a bridge is formed with an opening between
the two bacteria, they are now called a “mating pair”. A nuclease breaks one strand of the
plasmid DNA at the oriT, or origin of transfer site. The nicked strand enters the other bacterium
while the other strand remains behind in the donor. They can now both produce a complementary
copy of the plasmid so the DNA is double stranded again. Now each bacterium has a copy of the
plasmid and both can make a conjugation pilus! Note that in a population of bacteria in which
some have a conjugative plasmid and others do not, eventually all cells will acquire
a conjugative plasmid. The enzyme transposase catalyzes cutting and resealing of DNA during
transposition. Note that once mating pairs are formed by conjugative plasmids or transposons,
this also allows nonconjugative transposons and plasmids to be transferred to another
bacterium. Transformation occurs when bacteria take up extracellular DNA and incorporate
it into their genomes. Typically, this occurs when one bacteria lyses, or splits open, releasing
its genetic contents, and then another bacteria comes by and acquires it. Bacteria able to
bind large amounts of DNA are termed “competent”. Competency is a state of increased cell wall
and cell membrane permeability that allows cells to uptake DNA. Many bacteria are naturally
competent, and so actively bind environmental DNA. After transport into their cytoplasm,
the bacterium can incorporate the new DNA into its genome through the process of “recombination”.
Recombination is the rearrangement of donor and recipient genomes into new, hybrid genomes.
This can result in new phenotypes – for example, the bacteria can acquire pathogenicity
or antibiotic resistance. Some competent bacteria actually kill noncompetent bacteria to release
DNA for transformation! Transduction occurs when DNA is transferred from one cell to another
by a bacteriophage – a type of virus that infects bacteria. Viruses cannot replicate
on their own – they are obligate parasites that rely on host machinery. Many bacteriophages
can switch between a state of lysogeny and a lytic cycle. When the bacteriophage is in
a state of lysogeny, the virus combines its genome with the bacterial chromosome. The
viral genome hangs out there for many generations as the bacterium replicates. When induction
occurs, the virus switches to the lytic cycle. The cell becomes a virus-producing factory
until it gets so full of virus that it lyses, or bursts open, releasing virus particles
into its surroundings. There are two kinds of transduction – generalized and specialized.
During generalized transduction, the phage capsid accidentally assembles around a fragment
of bacterial DNA or a plasmid. When the assembled viral particle infects a new bacterium, it
injects its previous host’s DNA into its new host. Specialized transduction occurs
during the lysogenic life cycle of the virus. Incorrect excision of DNA during induction
results in a fragment of bacterial DNA being picked up instead of a part of the viral genome,
which then stays in the bacterial nucleoid. Now, the bacterial DNA replicates as part
of the bacteriophage genome, is packaged into phage capsids, and is injected into new bacteria.
Horizontal gene transfer, along with mutations, allows bacteria to achieve genetic diversity.
These processes allow bacteria to adapt to new environments because they enable them
to respond to selective pressures. Mutation is a slow process, and most mutations are
harmful or neutral, not beneficial to the bacterium. Meanwhile, horizontal gene transfer
is a rapid way to acquire large chunks of DNA from another bacterium all at once. So
if bacteria keep acquiring new snippets of DNA from other bacteria, do their genomes
keep getting larger and larger? No! If the transferred genes don’t provide a selective
advantage, they are usually lost by deletion, so the genome size remains constant over time.