Antibiotic same way as antibiotic resistant bacteria when exposed

Antibiotic
resistance is posing an increasing threat to our society. Bacteria are
constantly mutating and finding new ways to survive antibiotics, meaning
diseases like pneumonia and tuberculosis are becoming harder and harder to
treat. Testing possible ways to kill antibiotic resistant bacteria is risky as
there is the distinct possibility of creating a superbug that cannot be killed
by any antibiotics. This means healthcare authorities are currently more
focused on preventing the spread and development of antibiotic resistance. However,
recently it was noticed that when the bacteria E. coli is altered slightly, it superficially acts in the same way
as antibiotic resistant bacteria when exposed to the chemical IPTG. This means
that we many be able to use these E. coli
as a safe model to test possible techniques that will help us to combat antibiotic
resistance without the high risk associated with actual antibiotic resistant
bacteria.

A
suggested technique to test on this new model of antibiotic resistance is to
‘trick’ the bacteria into mutating so much that once it is put back into a
normal environment, it can no longer survive. This is known as driving the
bacteria into an evolutionary dead end. Testing this technique on antibiotic
resistant bacteria could lead to a strain of bacteria resistant to most types
of antibiotic, potentially leading to catastrophic consequences if it ever
infected anyone. Using these bacteria as a new model could lead to a whole new
mindset when it comes to treating and destroying antibiotic resistant bacteria.
Instead of trying to limit the damage that antibiotic resistant bacteria have
already caused, we may be able to decrease the bacterial population and stop its
development entirely before it becomes a more serious threat.

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The possibility
of a safe model for antibiotic resistance was first put forward when the
similarity between antibiotic resistant bacteria and E. coli that were undergoing the pET expression system was noticed.
The pET expression system is a technique that allows a scientist to produce
large quantities of a particular target protein. In order to produce a large
volume of target protein, a small loop of DNA, called a vector plasmid, is
inserted into the bacteria. This DNA codes for the target protein and half of
the information the cell will need to make the target protein. This information
is called the expression system and contains a repression system that controls how
quickly the target protein is produced. The other half of the expression system
is already present in the bacterial DNA, known as the bacterial genome. The
repressor is a small protein called lacI which binds a section of DNA, blocking
the DNA transcriber from attaching to the DNA. The DNA transcriber is a protein which reads
and transcribes the DNA so it can be translated into proteins by machinery in
the cell. IPTG is a small, sugar-like molecule that binds to the lacI and stops
it from binding to the DNA, leaving the DNA transcriber protein able to bind
and produce the T7 (DNA) transcriber. The T7 transcriber can then freely
produce the target protein. The whole process can be seen more clearly in
figure 1.

The
lack of a repressor means the cell makes far more of the protein than is
healthy and puts the cell under extreme stress. The high workload on the
protein production machinery in the cell causes them to malfunction after a
short time and proteins are made incorrectly, this eventually leads to cell
death. Usually the cell responds to this by expressing proteins that stop the
cell from producing more proteins, this process is the general stress response.
Bacteria in the pET expression system partially respond to stress with a
heat-shock like response which activates the synthesis of proteins that disable
some of the protein synthesis machinery. The main line of defence however, the
general stress response, isn’t activated and the cells die.

Figure 2 shows
how increasing the IPTG concentration dramatically decreases the number of
bacteria that survive. The cells that produce large numbers of the target
protein are coloured green by a protein called green fluorescent protein (GFP)
which can be tagged to the target protein. This makes it easier to see which
cells are expressing the largest amount of the target protein. One group of
cells contain a vector plasmid that codes for pure GFP so the other colonies
can be compared to it.

The
colonies that survive this initial IPTG exposure are then permanently resistant
to IPTG and can grow normally in very high concentrations of IPTG. These
colonies have developed a mutation somewhere in the bacterial DNA or the
plasmid that allows them to survive and prosper in the hostile environment. Though
the exact mutation or mutations are as yet unknown, it is likely that the
mutation makes the T7 transcriber less likely to bind to its binding site on
the plasmid. This means the protein is produced at a slower pace and the cell
is put under less stress, allowing it to survive. This theory is backed up by a
second experiment performed in which protein expression in the mutant bacteria
was compared to the expression of protein in E. coli that had never been exposed to IPTG. At low IPTG
concentrations, the mutant bacteria produced considerably less protein than the
newly exposed bacteria.

While
the mechanism that allows them to survive is most likely different to
antibiotic resistant bacteria, superficially the result is the same. The
number of surviving bacteria dramatically decreases as the concentration of the
stress-inducing compound increases. Bacteria that survive have a genetic
mutation which makes them permanently resistant to the compound and can grow in
environments with high concentrations of the compound. This means that E. coli pET expression system has the
potential to open many research pathways that could lead to the safe treatment
of antibiotic resistant bacteria.