Pasteur's Experiments on Spontaneous Generation - Online Tutor, Practice Problems & Exam Prep

Louis Pasteur's swan neck flask experiments in the 1800s demonstrated that microorganisms do not spontaneously generate. By boiling a non-sterile broth in a specially designed flask, he allowed air to enter while trapping dust and microbes in the neck. The broth remained sterile until the flask was tilted, mixing the trapped microbes, which then led to microbial growth. This supported the theory of biogenesis, confirming that life arises from preexisting life, and disproved the concept of spontaneous generation.

Pasteur's Experiments on Spontaneous Generation

In this video, we're going to begin our lesson on Louis Pasteur's experiments on spontaneous generation. Louis Pasteur was a French chemist from the 1800s who conducted many experiments, including some that demonstrated that microbes are all around us, including in the air that we breathe. Based on Spallanzani's results, Louis Pasteur wanted to show that air was actually the source of microbial contamination. He designed a simple set of experiments using a specialized type of flask that today we call a swan neck flask because of the resemblance that this flask has to a swan's neck.

If you take a look at the image below, notice on the left we have an image of Louis Pasteur, who lived from 1822 to 1895 and conducted the swan neck flask experiments in 1859. What you see here is actually the swan neck flask. Notice that the swan neck flask has a long curved tube that leads into the top of the flask. This long curved tube that you see here resembles the curve that you see in a swan's neck, which is why it's called a swan neck flask.

These swan neck flasks are really just specialized flasks that were designed by Louis Pasteur to have a long and curved tube leading into the top of the flask, just as what you see in the image below. Now, the swan neck flasks are really clever in their design because they allowed air to enter into the flask. However, the dust and the microbes that would be in the air would get trapped in the neck's bend and they would not be able to enter all the way into the flask. If we take a look at the image below, notice that dust and microbes that are found in the air can enter but will get trapped in the neck's bend. They will not be able to make their way back up this uphill portion of the flask. Whereas the air itself is able to go all the way up, air is able to get in except the microbes and dust in the air do get trapped over here in the neck's bend and they can't continue.

This was a really, really clever design in that fashion because the vital source or this unknown source that some people claimed was needed for spontaneous generation would be able to reach the broth, but the dust and microbes that were in the air would not be able to reach the broth. Using this swan neck flask, Louis Pasteur was able to conduct an experiment that ultimately disproved spontaneous generation once and for all.

We'll be able to talk more about this swan neck flask experiment in our next video. But for now, this here concludes our brief introduction to Louis Pasteur's experiments on spontaneous generation, and we'll get to learn more as we move forward. So I'll see you all in our next video.

Louis Pasteur designed swan-necked flasks to:

• A. Keep maggots away from decaying meat.
• B. Pasteurize beer and wine.
• C. Trap microorganisms from the air in the neck of the flask.
• D. Allow dust to reach sterile infusions.

Why did Pasteur design swan-necked flasks in his experiments on spontaneous generation?

• A. The neck excluded oxygen from entering the flask.
• B. The neck stopped microorganisms in the air from contaminating the contents of the flask.
• C. The neck excluded flies and maggots from entering the flask.
• D. The neck served as a handle when heating the flask.
• E. The neck prevented Pasteur from contaminating his sample by sneezing on it.

Pasteur’s Swan-Neck Experiment

In this video, we're going to talk more details about Louis Pasteur's swan neck experiment. Pasteur's experiment actually consisted of the following 5 steps that notice we have numbered down below in the text. Notice that the numbers in the text actually correspond with the numbers that we have throughout our image down below, so keep that in mind.

In the very first step of Louis Pasteur's swan neck experiment, the flask is actually going to be filled with a non-sterile liquid broth. By non-sterile, we mean that there are microbes in that broth. Notice that in step number 1 over here on the far left, we have a flask and it is filled with a broth that is non-sterile, meaning that there are microbes in that broth.

Then, in step number 2, the swan neck of the swan neck flask is going to be formed using heat. If you take a look at step number 2 in the image down below, notice that the swan neck of the swan neck flask is going to be created. It's created using heat as you can see here. There's a flame showing the heat allowing for the bending of the glass and the creation of the swan neck flask.

Then in step number 3, the broth, the liquid broth in the swan neck flask, is going to be boiled for a long period of time using an extensive amount of heat. This long boiling is going to help to create a sterile environment to kill all of the microbes in the broth. Taking a look at step number 3 down below, notice that the liquid broth is going to be boiled for a long period of time, allowing for the liquid broth to become sterile, meaning that all microbes have been killed.

Then in step number 4, as the flask is allowed to cool from the extensive amounts of heat and boiling, the dust and microbes from the air are actually going to get trapped in the bend of the swan neck flask. When we take a look at the image down below for step number 4, notice that the dust and microbes that are in the air, they're able to enter originally into the flask, but they get trapped in the bend of the swan neck, and they're not able to continue to contaminate. And because they get trapped right here, this means that the broth remains sterile and that it is not contaminated. As long as this flask stays in its upright position like this, the broth in the flask would remain sterile indefinitely, basically forever.

As long as it remained upright, there would be no microbes in the liquid broth. It would remain free of microorganisms, which shows that spontaneous generation does not occur. The broth remains sterile as we mentioned already indefinitely, which basically means forever. It remains, sterile indefinitely or until step number 5 of the experiment.

In step number 5, the flask is actually tilted. Tilting the flask, or turning it sideways, will allow for the mixing of the trapped microbes with the sterile broth. Once the microbes have been mixed with the sterile broth, that allows for microbial growth to be observed. If you take a look down here at step number 5, notice that the swan neck flask is tilted to allow the liquid broth to mix with the microbes from the air, and that mixing of the broth with the microbes allows for microbial growth to be observed. Over here you can see that microbes are present in the liquid after tilting the flask and mixing it.

What this shows is that spontaneous generation of microbes is not going to happen, and that microbial growth only comes from pre-existing microbes. These are the conclusions that Louis Pasteur was able to come up with. Number 1, microorganisms cannot spontaneously generate. They do not spontaneously generate. Otherwise, we would have seen spontaneous generation of microbes when the flask is upright. But the second conclusion that he was able to make is that life requires life to form, that he would only see significant microbial growth when the microbes that were trapped were mixed into the broth. This supported the theory of biogenesis that new life only comes from pre-existing life.

This here concludes our introduction to Louis Pasteur's Swan Neck Experiment and how it supported biogenesis and disproved spontaneous generation. 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.

Which of the following is not a valid conclusion and/or outcome of Pasteur's experiment refuting the theory of spontaneous generation?

• A. That microorganisms are present everywhere, even in the air.
• B. The development of aseptic techniques when working with microbial cultures.
• C. Microorganisms can cause disease.
• D. Living cells can only develop from pre-existing cells.

Here’s what students ask on this topic:

What was the purpose of Louis Pasteur's swan neck flask experiment?

The purpose of Louis Pasteur's swan neck flask experiment was to disprove the theory of spontaneous generation, which posited that microorganisms could arise from non-living matter. By designing a flask that allowed air to enter but trapped dust and microbes in its curved neck, Pasteur demonstrated that microbial growth only occurred when preexisting microbes were introduced into a sterile broth. This experiment supported the theory of biogenesis, which states that life arises from preexisting life, and conclusively showed that microorganisms do not spontaneously generate.

How did the swan neck flask design help in disproving spontaneous generation?

The swan neck flask design was crucial in disproving spontaneous generation because it allowed air to enter the flask while trapping dust and microbes in its curved neck. This design ensured that the broth inside the flask remained sterile as long as the flask was upright. When the flask was tilted, the trapped microbes mixed with the sterile broth, leading to microbial growth. This demonstrated that microbial life did not spontaneously generate but came from preexisting microbes, thus supporting the theory of biogenesis.

What were the main steps in Louis Pasteur's swan neck flask experiment?

The main steps in Louis Pasteur's swan neck flask experiment were: 1) Filling the flask with non-sterile broth containing microbes. 2) Forming the swan neck using heat. 3) Boiling the broth to sterilize it, killing all microbes. 4) Allowing the flask to cool, during which dust and microbes from the air got trapped in the neck. 5) Tilting the flask to mix the trapped microbes with the sterile broth, leading to microbial growth. These steps demonstrated that microbial growth only occurred from preexisting microbes, disproving spontaneous generation.

What conclusions did Louis Pasteur draw from his swan neck flask experiment?

Louis Pasteur drew two main conclusions from his swan neck flask experiment: 1) Microorganisms do not spontaneously generate. This was evidenced by the fact that no microbial growth occurred in the sterile broth as long as the flask remained upright. 2) Life requires preexisting life to form. Microbial growth was only observed when the flask was tilted, allowing the trapped microbes to mix with the sterile broth. These conclusions supported the theory of biogenesis and definitively disproved the concept of spontaneous generation.

How did Pasteur's experiment support the theory of biogenesis?

Pasteur's experiment supported the theory of biogenesis by demonstrating that life arises from preexisting life. In his swan neck flask experiment, the broth remained sterile as long as the flask was upright, indicating no spontaneous generation of microbes. When the flask was tilted, allowing trapped microbes to mix with the sterile broth, microbial growth occurred. This showed that new microbial life originated from existing microbes, not from non-living matter, thus confirming the theory of biogenesis and disproving spontaneous generation.