Hacking bacteria to fight cancer Tal Danino
In 1884, a patient’s luck seemed
to go from bad to worse.
This patient had a rapidly growing
cancer in his neck,
and then came down with an unrelated
bacterial skin infection.
But soon, something unexpected happened:
as he recovered from the infection,
the cancer also began to recede.
When a physician named William Coley
tracked the patient down 7 years later,
no visible signs of the cancer remained.
Coley believed something remarkable
was happening:
that the bacterial infection
had stimulated the patient’s immune system
to fight off the cancer.
Coley’s fortunate discovery
led him to pioneer
the intentional injection of bacteria
to successfully treat cancer.
Over a century later, synthetic biologists
have found an even better way
to use these once unlikely allies—
by programming them to safely
deliver drugs directly to tumors.
Cancer occurs when normal functions
of cells are altered,
causing them to rapidly multiply
and form growths called tumors.
Treatments like radiation, chemotherapy,
and immunotherapy
attempt to kill malignant cells,
but can affect the entire body
and disrupt healthy tissues
in the process.
However, some bacteria like E. coli
have the unique advantage of being able
to selectively grow inside tumors.
In fact, the core of a tumor forms
an ideal environment
where they can safely multiply,
hidden from immune cells.
Instead of causing infection,
bacteria can be reprogrammed
to carry cancer-fighting drugs,
acting as Trojan Horses
that target the tumor from within.
This idea of programming bacteria
to sense and respond in novel ways
is a major focus of a field called
Synthetic Biology.
But how can bacteria be programmed?
The key lies in manipulating their DNA.
By inserting particular genetic sequences
into bacteria,
they can be instructed
to synthesize different molecules,
including those
that disrupt cancer growth.
They can also be made
to behave in very specific ways
with the help of biological circuits.
These program different behaviors
depending on the presence, absence,
or combination of certain factors.
For example, tumors have low oxygen
and pH levels
and over-produce specific molecules.
Synthetic biologists can program bacteria
to sense those conditions,
and by doing so, respond to tumors
while avoiding healthy tissue.
One type of biological circuit,
known as a synchronized lysis circuit,
or SLC,
allows bacteria
to not only deliver medicine,
but to do so on a set schedule.
First, to avoid harming healthy tissue,
production of anti-cancer drugs
begins as bacteria grow,
which only happens
within the tumor itself.
Next, after they’ve produced the drugs,
a kill-switch causes
the bacteria to burst
when they reach a critical population
threshold.
This both releases the medicine
and decreases the bacteria’s population.
However, a certain percentage
of the bacteria remain alive
to replenish the colony.
Eventually their numbers grow large enough
to trigger the kill switch again,
and the cycle continues.
This circuit can be fine-tuned
to deliver drugs
on whatever periodic schedule
is best to fight the cancer.
This approach has proven promising
in scientific trials using mice.
Not only were scientists able
to successfully eliminate lymphoma tumors
injected with bacteria,
but the injection also stimulated
the immune system,
priming immune cells
to identify and attack untreated lymphomas
elsewhere in the mouse.
Unlike many other therapies,
bacteria don’t target a specific type
of cancer,
but rather the general characteristics
shared by all solid tumors.
Nor are programmable bacteria
limited to simply fighting cancer.
Instead, they can serve
as sophisticated sensors
that monitor sites of future disease.
Safe probiotic bacteria could perhaps
lie dormant within our guts,
where they’d detect, prevent,
and treat disorders
before they have the chance
to cause symptoms.
Advances in technology
have created excitement around a future
of personalized medicine
driven by mechanical nanobots.
But thanks to billions of years
of evolution
we may already have a starting point
in the unexpectedly biological
form of bacteria.
Add synthetic biology to the mix,
and who knows what might soon be possible.