What point does the professor make about the bacteria present in yogurt?

Listen to part of a lecture in a biology class.
Professor: 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 do a lot of things that we do. For example, they move and they breathe. 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 this amazing. Nine out of ten cells are bacteria. So, why do we have all these bacteria? At least 500 different species in the digestive system alone and why do I say they’re helpful? Let’s look at some examples.
    First, our bodies can’t make certain vitamins, like certain B-vitamins and vitamin-K, which plays an important role in blood clotting. So, we have these good bacteria in our digestive system. They eat the food we eat and secrete those vitamins for us. Also, some bacteria help us digest food that our own digestive enzymes cannot otherwise process. For example, at least one bacterial species breaks down the complex carbohydrates that are found in vegetables and other plant foods. Our own enzymes can’t do this for us. The bacteria in our digestive system also compete for the same nutrients that bad bacteria need, you know, the ones that if allowed to grow unchecked can be harmful to us. There’re so many more good bacteria than bad bacteria normally that the good ones overwhelm the bad ones by taking up most of the available nutrients.
    The bacteria on our skin do a similar thing. They prevent the bad bacteria from getting a foothold, so they really serve as another layer of protection against infection. So when you wash your hands with antibacterial soap, you know, the soap that helps kill bacteria when you use it, well, what does that do? It’s going to kill the bacteria, and not just the bad bacteria. The soap doesn’t discriminate between the good and bad. A lot of the good bacteria that’s protecting you will be destroyed as well.
    Another thing that inadvertently kills good bacteria are antibiotics. Most of them are what we call broad-spectrum antibiotics, which means they kill all kinds of bacteria, not just the bad ones. So to replenish some of the good bacteria, we can take probiotics like the ones contained in yogurt. There’s a bacterium used to help make yogurt and it’s the same kind of bacterium that can aid digestion, so if you get sick and you’re given antibiotics, the doctor might tell you to eat yogurt. That’s one form of probiotic therapy. The doctor would actually say to you to eat yogurt with live cultures, with live bacteria. As we know, yogurt is made with live bacteria. The problem is that some yogurt brands destroy the bacteria in the processing.
    Probiotics can help treat or prevent certain digestive problems. We know there is plenty of evidence to support this. But if you go into a health food store you may see probiotic supplements claiming to lower blood pressure and do things beyond improving gastrointestinal health. While these claims may very well be documented in the future, right now they’re merely postulations based on laboratory data, test tube experiments. So it is not a reflection of what happens when people actually take probiotics.