When we hear the word bacteria, a lot of people think germs or disease, but bacteria can be good or bad and if it weren't for the good bacteria we wouldn't be able to survive, but before getting into that let me reiterate that bacteria are virtually everywhere. There are bacteria living in soil, living on windowsills, they're living on us, and they're living in us. As you know, bacteria are organisms; tiny, single-celled organisms. They move, they breathe, they do a lot of the things that we do. They have metabolic activities. They can break down sugars, build proteins, and secrete things and all these things happen just on a much smaller and faster scale. A number of research studies have shown and now, this is hard to believe, but the number of bacterial cells in and on our body actually exceeds the number of our human cells. If you consider all the human and bacterial cells that exist in and on our body, only one in every ten cells in our body is human. The rest are mostly bacteria. Now, because the bacteria cells are much smaller, that doesn't mean that we are 90% bacteria, but still, I find that amazing. Nine often cells are bacteria. So, why do we have all these bacteria? At least 500 different species in the digestive systemalone and why do I say they're helpful? Lets look at some examples. 。。。。。余下托福听力真题原文省略！
Question 1 of 6
What is the main purpose of the lecture?
O A. To contrast the functions of probiotics and antibiotics
O B. To describe some ways that bacteria contribute to human health
O C. To describe some differences between beneficial and harmful bacteria
O D. The explain the relationship between bacteria and vitamins
Question 2 of 6
The professor mentions research findings on the number of bacterial cells in the human body. What is her opinion of these findings?
O A. She is confused by the inconsistency of the various findings.
O B. She accepts the validity of the findings.
O C. She expects that future research will show that the estimates are too high.
O D. She regrets that researchers did not include counts of bacteria in the digestive system.