Chapter 1 - 10

10 Biology

Listen to a lecture in a biology class. Fill in the diagram with the information that you hear.

W: It looks like we’re already halfway through class, so I want to move on now and speak about a pretty difficult concept. Are you all up for it? Of course you are.

Well, then, let me get started. What I want to talk about now is a scientist from the late 19th and early 20th century named Robert Koch. More, uh, more exactly, I want to talk about his theory of disease, which is sometimes called the Germ Theory of Disease.

Now, the reason I’m going to describe the theory itself is−well, not to make you think the theory itself isn’t important − but the way that applied the theory to tuberculosis changed our understanding of the disease forever. So, well, let me just tell you about the theory now.

To understand the theory, um, I’ll−I mean, you’ll need to understand Koch’s Postulates. Now this is just the larger part of his overall Germ Theory, which basically states that microorganisms can cause disease.

We know this now, but it was only speculation until 1876, when Koch finally proved it. Anyway, let’s talk about the four postulates, uh, meaning requirements, which Koch said were needed to prove that a specific microorganism caused a disease.

Are you all ready to write? OK.

The first postulate said that the germ being studied must always be present in a sick subject, and never present in a healthy animal.

This basically just assures that no oversights were allowed. Pretty simple, right?

The second requirement is that the culture−uh, culture here meaning a sample of the microorganism−the culture must be grown separately from the animal and from other microorganisms.

This stresses the importance of isolation, just meaning that, to get accurate results, you don’t want any other germs in there that could affect the results.

OK, moving on to number three. Postulate three says that the pure culture−the one in which the suspected organism is the only one present−the pure culture must cause the disease in a healthy animal.

So, in doing the experiments, they would infect an animal that they know is healthy.

Then, after infection, they would see whether the disease becomes present in the animal−whether the animal has become infected.

OK, ready for the last one?

the fourth postulate said that, after the animal has been infected, a sample should be taken from the infected animal−a blood sample, usually.

Then, the germ must be found to be the same one as the germ from the culture. This just assures that it wasn’t actually a different disease−or even a combination of diseases−that ended up infecting the animal.

That was a lot of information… any questions? OK, so how did this apply to tuberculosis, and why was it so revolutionary?

Well, we don’t think as much about tuberculosis anymore, but in 1881, one in seven deaths was caused by it. Tuberculosis is an infectious disease that often affects the lungs.

What Koch wanted to do was use the postulates he created to identify the organism that caused it. Here’s how he did it.

Basically, the problem with identifying the tubercle bacterius−um, that’s the organism that causes tuberculosis… um, it was hard to identify because previous techniques to view the organism using staining didn’t work.

Before, staining processes couldn’t successfully color organisms with large amounts of lipid on their surfaces.

Koch, however, used a new brown stain to successfully identify the tubercle bacillus. However, he had to follow all of his own postulates in order to prove that it really was that organism that caused the disease… rather than something else.

Uh, so he went through all of his postulates, and they all checked out. He used guinea pigs as subjects… and after a lot of research, finally proved that tubercle bacillus caused the disease.

So, why was this such a big deal? Well, identifying the cause of tuberculosis led to measures to prevent and treat the deadly disease. Without the discovery, we might all hear much more today about tuberculosis, but luckily, it’s not nearly as common as it used to be.

1) what is the main topic of the lecture? 2) what aspect of Koch’s postulates does the professor mainly discuss?