Wednesday, October 2, 2013

What I've been doing in my lab

...for those who are curious.

I took this picture a while ago so I don't remember exactly what was growing, but it definitely included some strain of a soil bacterium.

The point of the complicated setup is to be able to keep these bacteria in a co
nstant state of healthy proliferation; that is, about every 5-10 hours (depending on the strain), every cell in the tank in the center will have divided into two new cells, effectively doubling the population.

This is called exponential growth, because when this takes place in a flask of a given amount of broth, nothing added or removed, the size of the population doubles every 5-10 hours, so the number of cells in the population, if you start with 2, is 2 to the nth power, where n is the number of 5-to-10-hour periods
 (depending on the strain) that have passed. So after 5-10 hours, there will be 4 cells, then after 10-15 hours, 8 cells, then 16, then 32, then 64, and it goes up exponentially.

However, this is not the case in this picture, because rather than having a given amount of broth, there's a bottle of fresh broth outside the frame up above, and new broth is being added to tank where the bacteria are growing, so their food is being constantly replenished at a certain rate. At the same time, the volume in the tank is being kept constant by removing excess material, including the cells in it, thus reducing the size of the population. So the size of the population of bacteria effectively remains constant over time.

The point of this is to study the behavior of the bacteria. There are some good reasons to maintain a population of bacteria this way; for one thing, you can repeatedly make changes to the broth you're feeding them, or other environmental factors, to see how the behavior changes, without having to repeatedly clean out and sterilize the tank, add fresh medium, and inoculate it again.

In my case, what I'm currently doing, is trying to figure out how much hydrogen gas my bacteria produce when I feed them a certain amount of sugar. As long as they have everything else that they need, it should be the case that the more sugar I give them, the more hydrogen they produce, and I'm trying to quantify the relationship between those two factors.

In order to do that, I make a tank of broth with a known amount of sugar, feed it to the bacteria until the population size stabilizes, and measure the amount of hydrogen they produce with that white box to the left. I'm also measuring the size of the population using other methods. Then, I increase the concentration of sugar in the broth, keeping everything else the same, let the population stabilize, and measure the new values of hydrogen and population size. Once I've done this a few times, I can make a graph of hydrogen produced per gram of sugar added or something.

The other cool thing about the setup in this picture is that it allows me to control many environmental factors and take measurements in real time: the unit on the right is a unit that controls and takes measurements, and it can measure the amount of oxygen in the tank, the pH, the temperature, and how much broth is left in the tank, and it uploads these values to a computer on the other side of the bench. It feeds in air (for oxygen) at a certain rate, controls the stirring (to help the oxygen dissolve), and pumps water through a jacket around the tank to control the temperature. There are also sensors on the other side that measure the concentrations of certain gases that come out of the tank, and the computer records those too. So it's a lot of control and information.

That's about it for this kind of experiment. In case I've kept your attention this long and you're interested in more, I started a new blog here to help myself review papers for my own research, so feel free to check it out.