巴朗托福听力文本-Test 6(下)

巴朗托福听力文本-Test 6(下) BIOLOGY CLASS

Narrator: Listen to part of a lecture in a biology class. The professor is talking about bacteria.

$ Professor:

$ Bacteria is the common name for a very large group of one-celled microscopic organisms that, we believe, may be the smallest, simplest, and perhaps even the very first forms of cellular life that evolved on Earth. Because they’re so small, bacteria must be measured in microns, with one micron measuring about 0.00004 inches long. Most bacteria range from about 0.1 microns to about 4 microns wide and about 0.2 microns to almost 50 microns long. So how can we observe them? I’ll give you one guess.

$ Under the microscope, of course. As I said, bacteria are very primitive and simple. In fact, they’re uni cellular, which means that they’re made up of a single cell. We think they probably evolved about three and a half billion years ago. Some of the oldest fossils are bacterial organisms. They’ve been found almost everywhere on Earth, including all the continents, seas, and fresh water habitats, and in the tissues of both plants and animals.

$ Well, since they’re so prevalent, you might ask, how do they reproduce? Okay, they grow in colonies and can reproduce, quite rapidly, in fact, by a process called fis sion. In fission, the cell, and remember, there’s only one in bacteria, one cell. So the cell increases in size and then splits in two parts. Fission is also referred to in your text as asexual budding. Now you’ll also read about conjugation, and that’s wh en two separate bacteria exchange pieces of DNA, so there are two ways that reproduction can occur, but we think that fission is more common.

$ Okay. Bacteria were virtually unknown until about 1600 when microscopes were introduced, and at that time, bac teria were observed and classified into three main types according to their shapes, and that classification hasn’t really changed that much over the years. So that’s what I want to talk about today—the main types of bacteria. The slides that I’m going to s how you are enlargements of bacteria that I observed under the microscope in the lab earlier today. Now, this first slide is an example of bacilli.

$ The bacilli are a group of bacteria that occur in the soil and air. As you can see, they’re shaped like rods, and if you were to see them in motion, they’d be rolling or tumbling under the microscope. Of course, you can’t see that because this is a still visual, but later, when you go into the lab, you’ll see that rolling motion in examples of bacilli. These are kind of a greenish blue, but some are yellow. So don’t try to identify them by their color. Look at the shape. These bacilli are largely responsible for food spoilage.

$ Okay, the next slide is a very different shape of bacteria. It’s referred to as the cocci group, and it tends to grow in clusters or chains, like this example. This specimen is one of the common streptococci bacteria that cause strep throat.

$ Finally, let’s look at the spiral-shaped bacteria called the spirilla.

$ This is the sp irilla. They look a little like corkscrews, and they’re responsible for a number of diseases in humans. But I don’t want you to get the wrong idea. It’s true that some species of bacteria do cause diseases, but for the most part, bacteria are benign. There’s a lot of bacteria in this room in fact. We all have it on us. They live harmlessly on the skin, in the mouth, and in the intestines. In fact, bacteria are very helpful to researchers because bacterial cells resemble the cells of other life forms in many ways, and may be studied to give us insights. For example, we have a major research project in genetics here at the university. Since bacteria reproduce very rapidly, we’re using them to determine how certain characteristics are inherited.

$ Okay, now, let me review these three types with you . . . cocci are spheres, bacilli are rods, and spirilli are spirals. One of my students came up with a way to remember them. Just try to visualize the first letter in the name of each of the different types: Cocci starts with C like the shape of half a sphere. Bacilli starts with a straight line on the B, and a rod is straight. Spirilla starts with S, and that’s a spiral shape.