hi I'm Rico and I am symbiosis pasta working with professors Casey Huang and Julie Theriot and the general area of my research is the mechanics of single cells so we are interested in understanding how it is that cells exert and control the forces that they need in order to do some of the most basic things it is that cells do such as grow or divide or move well there's a few different ways by which single cells can exert forces but the biggest force that cells have at their disposal is the one provided by osmotic pressure so what is osmotic pressure well in many ways a bacterial cell can be thought of as a microscopic water balloon in the sense that within the cell there is a very high water pressure which pushes out on the sides of the cell and it's called an osmotic pressure because it results from a high osmolarity within the cell that is a high concentration of solutes such as DNA and proteins and smaller molecules such as amino acids and ions and so this results in a pressure much the same way like in a normal balloon pressure results from the fact that you have a higher concentration of air molecules inside the balloon and relatively low concentration outside the balloon and so this osmotic pressure is sort of a blessing and a curse for these little cells in the sense that it's a it's a blessing in the sense that they can actually use this force in order to perform physical processes which I'll get to in a moment on the other hand it's sort of a curse in the sense that it can be a very high pressure it can be much higher for example than the air pressure in your car tires and so how is it that these little cells don't explode in response to to this osmotic pressure and the answer is that they are encased in a cell wall and so the cell wall is a polymeric structure that actually has about the same stiffness as rubber and its role of course is to sustain the osmotic pressure thereby preventing the cells from exploding but on the other hand this cell wall actually presents a problem to these cells because how is it that you grow and you need to grow in order to reproduce and divide how do you grow if you're encased in a relatively stiff rubber like sheath and so the hypothesis that we wanted to test was that the cells actually exploit osmotic pressure in order to continuously stretch the cell wall during growth much like the process of inflating a rubber balloon and so in order to test this hypothesis that cell wall expansion during cell growth is pressure dependent what we do is we make movies of cells under the microscope and we label their cell walls so we can see what's happening to that structure and then we alter their osmotic pressure by changing the osmolarity of the medium in which they grow and we just see what happens what happens to cell growth when we change their osmotic pressure and so in the movie that you're seeing now these are a number of e.coli cells and like I said their cell wall has been stained with a fluorescent label and as you can see they're growing but they are also swelling and shrinking and as they're growing and that is because the osmotic pressure within the cells is being toggled up and down as I change the osmolarity of their growth medium from one that's high to one that's low and back and forth so for e.coli which is the type of cells that you just saw which is a common bacteria that lives in your intestine we found that blurring the osmotic pressure did not have a huge effect on cell growth in fact you could even deplete the osmotic pressure and the cells would basically grow just fine on the other hand when we looked at a soil bacterium which had a much thicker cell wall we found that by reducing the osmotic pressure within the cells we reduced we could reduce the growth rate proportionally and furthermore that when we tried to increase the osmotic pressure within mesilla subtlest this soil bacterium the cells responded very drastically and the growth rate actually ocellated around a steady-state value until these these oscillations were damped out as if the cells were sort of frantically trying to react will rate their growth rate in response to this perturbation and this is not something that we saw with e.coli and so in conclusion what we've discovered is that there are at least two mechanisms for cell wall expansion in bacterium one in which pressure is not a critical factor for cell wall expansion and one in which cell wall expansion and therefore cell growth is highly dependent on osmotic pressure and furthermore that these two mechanisms may have co-evolved with the ultra structure of the cell wall of these bacteria that is the thickness of the cell wall currently we're trying to model these processes from a dynamical system standpoint by integrating the solid mechanics of the cell wall with the mechanisms that the bacteria have for regulating their osmotic pressure in an attempt to fit the experimental data that we see under the microscope I hope you've gotten a feel for the kind of research that we do and I hope that you'll email me if you have questions or are interested further Thanks you