Hi everyone! Welcome to week two of the
"Evolution and Biology of Sex". This week's
topic will be evolution and natural
selection and in lab this week we'll be
looking at the experimental evolution of
E coli. Escherichia coli is a bacteria
that is commonly found in the human gut.
It is used in research and for medical
purposes, such as for producing insulin.
However, wild strains can be pathogenic.
Triclosan is the antibacterial agent
that we'll be working with this week in
lab. It is used to clean surfaces and it
was recently banned from soaps because
of adverse health effects. Our question
this week in lab is going to be "Can we
evolve ecoli in the lab to make it
resistant to triclosan, the
antibacterial agent". I'm going to ask two
questions that I want you to consider.
The first one is evolution. How the e
coli population compared to the start
e coli population if evolution has
happened. And the second question; if we
do get evolution of e coli what would be
the most likely mechanism given our
experimental procedure? How do you know which mechanism is most likely? I'm going
to continue by going over the
experimental procedure for this week in
lab. So, our first step is to take an
un-inoculated plate. So, this plate will just
be filled with agar, which is a growth
medium for the bacteria. Then you're
going to plate a lawn of e.coli. After
this lawn has been plated you're going
to place a paper with triclosan in the
center. Remember, triclosan is our anti
bacterial agent. Take a moment to think
about the following questions. If
bacteria is not resistant triclosan, where will it grow where will it
not grow? It's important to note that the
concentration of triclosan varies with
distance from the paper; meaning
that the concentration of the
antibacterial agent is probably the most
dense right on the paper or near the dot
and becomes less dense as we get farther outwards. Take a moment and consider that
these three brown boxes are different
bacterial colonies. Which of these
colonies do you think are resistant and
which are probably not resistant to the
antibacterial agent? Here you can see
that the colony that is closest to the
dot is probably resistant to the
antibacterial agent and those that are
farther away are probably not resistant.
Following our procedure we initially
played it a lawn of E coli,
thus inoculating the plate. A piece of
paper dipped in triclosan was placed in
the center and then we waited 24 hours.
You can see that there should be a
clearance zone where no E. coli is growing
and around this zone there will be E. coli
growth surrounding the outsides of the
plate. After measuring this clearance
zone where there's no growth we're going
to take some colonies from right around
the edge of the no growth zone which are
most likely our most resistant colonies
or bacterial cells. We're going to
scrape some of those up and put them in
a liquid bacterial growth medium, so, this
un-inoculated liquid. Step five
highlights this saying that we will be
re-innoculating cells from the
periphery of the clearance zone. Finally,
we will be repeating this entire
procedure four times in the hope of
evolving our E. coli species over these
four generations. A final reminder
that it is important to label your
plates and tubes to ensure that we're
keeping everything straight. Good luck
and have fun in lab this week!
