In this video, we're going to begin our lesson on microbial growth curves in a closed system. And so when cells are grown in a lab with a fixed volume of liquid enclosed in a container or a flask, we refer to this as a closed system. And so a closed system is really just a scenario where additional nutrients cannot be added to the container or flask, and waste products cannot be removed from the container or the flask. And so you can pretty much imagine a closed system somewhat like a closed door where things cannot go through the door, things cannot be added to the room and the door, and things cannot be removed from the room, through the door.

With this closed system, the nutrients, because additional nutrients cannot be added, the nutrients in this closed system are going to be limited, and therefore a closed system cannot support infinite growth. And so it turns out that bacterial growth in a closed system creates a very characteristic microbial growth curve. And a microbial growth curve is really just this graph or this plot that you see right here in this image. And so notice, just to orient you, on the y-axis of this plot, what we have is the number of cells in a logarithmic scale. And so the lower the y-axis is, the fewer cells there are. And the higher the curve is on the y-axis the more cells there are. And then on the x-axis what we have is the amount of time that the cells are allowed to be in the closed system.

What you'll note is that bacterial growth in a closed system actually occurs in 4 distinct phases that we have numbered down below 1 through 4. And it's important to note that these numbers 1 through 4 for these phases correspond with the numbers 1 through 4 that we have in the curve down below. And so, keep that in mind as we move forward. The very first distinct phase of bacterial growth in a closed system is the lag phase. And so in the lag phase, things are going to be lagging behind a little bit, things are going to be delayed a little bit. And so the cells from a pure colony are going to be synthesizing enzymes that are required for cell growth. And so they don't have those enzymes yet, so they're not able to grow. They have to first synthesize the enzymes that are needed to grow before they can actually grow. And so things are going to be lagging behind a little bit, and there is going to be somewhat of a delay before you see tremendous cell growth.

If we take a look at our first phase down below in this image, the lag phase, you'll notice that things are going to be lagging behind a little bit. Things are going to be delayed. And so notice that the cell here is saying, "Well, this is starting off slow." And notice that there is a relatively low number of cells here. The curve is relatively low on the y-axis. There's not a lot of growth occurring in this lag phase. The numbers are pretty low, and they're remaining low in this period of time. This is because the cells are adjusting to their new environment, and they're beginning to synthesize the enzymes that they need in order for cell growth to occur. And so after the lag phase, the second phase is the log phase, also sometimes referred to as the exponential phase.

In this phase, as its name implies, there is going to be exponential growth. Cell division is going to occur at a continuous rate and there will be active growth during this phase. So sometimes it's referred to as the active growth phase. And so if we take a look at our image down below at the second stage here, we have the log phase or the exponential phase. So during the lag phase, the cells are synthesizing the enzymes that they need for growth. And so at the end of the lag phase and at the beginning of the log phase, the cells are able to initiate cell division and they start to increase in the number of cells. So notice that the number of cells is beginning to increase, the curve is going up in relation to the y-axis. And so notice the cells are saying let's grow.

Now after the log or exponential phase, the third phase is going to be the stationary phase. And this is when nutrient levels are going to become limited. And limited nutrients are going to somewhat make the curve appear stationary, meaning that it's not really appearing to move or change with respect to the y-axis. And so it appears that cells stop growing, and really it's not that the cells stop growing, it's that they are growing at an equal rate that they are dying. The population is growing at an equal rate as it is dying. And so it appears that there is no growth. And so when we take a look at the third phase here, we have the stationary phase. And notice that the curve appears to plateau here. It gets flat. It does not increase or decrease overall in the number of cells. This is because the population is going to be growing and dying at an equal rate. And this is because it's starting to get crowded in the closed system, the nutrients are beginning to get limited, and so, it's not able to allow for continuous exponential growth in the closed system.

And so after the stationary phase, the fourth and final phase here is going to be the decline phase, also sometimes referred to as the death phase. Because in this phase, the amount of death is going to begin to increase significantly and the number of viable cells are going to start to die off. But what you'll notice if we take a look at our image down below of the death phase, you'll notice that the curve is definitely decreasing in the number of cells. And so, ultimately, the number of cells is going to decrease a lot here. The cells are going to begin to die. And this is once again because the nutrients are limited and there's not really enough nutrients in this closed system to allow for infinite growth. And so, ultimately the cells will die, all die in a closed system.

But what you'll notice is that this curve does not drop off super immediately. It doesn't go straight down like this. And what you'll notice is that the curve kinda drops off relatively slowly. Cells are dying, but it's going down pretty slow in comparison to a curve that you might see dropping down like this, immediately. And the reason for this is because as cells start to die, they actually will release their contents into the environment, and those contents can serve as additional nutrients that can be used by living cells that are still alive. And so that can allow for the prolonged extension, the prolonged extension of the ones that are alive and kind of delay their death. And so that's why the curve goes down relatively slowly. Now, one thing to note is that the growth rate on these microbial growth curves is plotted using a logarithmic scale or a log scale, for the number of cells. And so that's exactly what you see over here on this side, the log of the number of cells.

And so it is going to be critical for you guys to know each of these phases and to be able to identify these phases and know somewhat of what happens during each of these phases. This is something that is commonly tested on by professors. And so this here concludes our lesson on microbial growth curves in a closed system. And we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.