For the last couple of lectures we've been talking about the structure of bacteria, the structure of eukaryotic cells, the structure of viruses, and today we're going to shift focus and ask questions about what do bacteria need to grow, what kind of environments do they do well in. We're going to ask about how they divide. We're going to learn a little bit about the structures they make called spores that allow some bacteria to survive when conditions get tough.
And we're going to learn about bacteria growing in groups and actually communicating with each other. So we're shifting our focus here to think about what they do and what they need and a little bit less just about how they're built. Now just like you, bacteria have physical requirements and chemical requirements. So they're affected by the temperature around them.
Because they often live in aqueous environments, they're affected by pH. They don't want to be in a really acidic environment or a really basic environment. and they're affected by the amount of salt that's in the water. This results in changes in osmotic pressure that they would need to adapt to.
Just like you, they need nutrition, so they need a carbon source, as well as some of the major biological elements that we associate with living things, and then trace elements, you know, just like you need a little bit of iron in your diet, bacteria also have trace elements that they require for survival. I'm going to start by talking about temperature. And one of the things that I want you to be thinking about is this idea that bacteria live everywhere on Earth, from the hottest hot springs to the coldest places in the Arctic.
We see bacteria living in almost every environment. And this really is a function of evolution and this idea that those environments have exerted selective pressure on bacteria to be able to survive under those conditions. And as a result, we have bacteria that we call psychrophiles.
And these are... cold-loving bacteria. So this is a temperature scale here in degrees Celsius. So 10 degrees Celsius is quite cold. Most refrigerators and houses are about four degrees Celsius.
So this is just maybe slightly warmer than your fridge. And these bacteria do best at very cold temperatures. In fact, you get up to about 20 degrees Celsius and they stop growing. Mesophiles do best at human body temperature. So right about 37 degrees Celsius here.
You can see this bacteria has a growth peak at that temperature, and if it gets too much hotter, their growth begins to slow down. Then we have thermophiles. They like it much warmer, so we're peaking here around 65 degrees Celsius, and then the hyperthermophiles that live in almost boiling water, 100 degrees Celsius, is boiling water.
Actually, they're surviving in boiling water. Their growth rate is a little bit diminished. So bacteria successfully have navigated every environment on earth and again this is just because the environment is selected for bacteria with those traits that can survive under those conditions.
So bacteria grow within a range. They have a minimal growth temperature, a maximum growth temperature, and then an optimal temperature where they're going to divide the fastest. And the reality is that it's the environment that has selected for the kinds of enzymes that function.
under those temperatures, under those conditions. They have evolved to select for the kinds of cell membrane lipids that keep membrane fluidity just right. So it's evolution and this idea that the environment selects for traits that allow organisms to survive and reproduce that result in bacteria really living almost everywhere on earth successfully. We said that mesovials are the most common type of human pathogen and you like to be at 37 degrees, so it makes sense that bacteria that are resulting in human infections, that are causing human infections, are going to be happy at 37 degrees Celsius.
If you take a pathogen like E. coli, for example, which is also, I should add, a commensal organism that we all have in our gut, so it's not always a pathogen. When you increase the temperature, the higher the temperature is, the faster those bacteria will die. Really important point is that refrigeration and freezing slows bacteria down, but it doesn't kill them.
So, for example, if you buy chicken at the grocery store and it has some salmonella on it, as many chicken does, much chicken does, the salmonella won't divide in the fridge, but it also won't be killed. So you still have to cook that chicken to make it safe to eat. And there are psychrophiles, as I mentioned earlier, that actually enjoy being in your refrigerator.
Here's just a thermometer, and here we have our human body temperature, just to kind of give you an idea. Your refrigerator is probably about 4 degrees Celsius and about minus 18, minus 20 degrees Celsius in the freezer. When we think about Louis Pasteur, he developed this method called pasteurization.
There are multiple methods of pasteurization. So one is called the holding method, where you hold, in the case of dairy, you hold the milk at 63 for 30 minutes. So that will kill pathogenic bacteria.
If you want to undergo pasteurization more quickly, you can do something called flash pasteurization. You increase the temperature nine degrees, and now you can kill all the bacteria in 15 seconds. So... Heat and time, they have a relationship. The hotter it is, the shorter amount of time you need to treat the bacteria in order to kill them.
Autoclaves kill most bacteria in 15 minutes. Now we're not talking just about pathogenic bacteria, but we're talking about all bacteria, and in fact, even the endospores, which we'll talk about more in a little bit, that are really difficult to kill. There's an ultra-high temperature pasteurization method that kills bacteria in three seconds, and then you can use a hot air oven.
which would kill spores in two hours at 160 degrees. And we'll talk about why some of these methods like an autoclave that use moist heat, why they're so much faster than even a hotter dry heat when we talk about germicides and physical methods of microbial control in lab. All right, so our next lecture video, we're going to talk about the liquid environment and how the things that are in the liquid environment that bacteria live in affect them.