In this season of flu and so many other germs, we're told again and again that hand sanitizers and soap are the best defense. Last year, americans spent more than $170 million on hand sanitizers alone. So, we wondered, what works best? And how do the sanitizers compare in a showdown with soap? Well, tonight, our chief medical editor dr. Richard besser put them to the test and gets some real answers. Reporter: Clean? As many as 400,000 germs per hand. That's how many bacteria travel with us every day. So, here at the university of maryland food safety lab, six brave grad students and I signed up for a nasty experiment -- voluntarily dousing our hands in a toxic brew of millions of e-coli bacteria. Yep, e-coli. Good? Something a little odd about rubbing e. Coli. To see what would take it off. First up, hand sanitizers. We tested two kinds you'll find at the store. One with 60% alcohol and one so-called natural formula containing no alcohol. The key with hand sanitizers is to use enough of it. Your hands should take a full 15 seconds or longer to dry. Now let that air dry, okay? Then, we pressed our hands onto special incubation plates to grow any remaining bacteria. The results? Dramatic. Here, those graying spots are the e. Coli that grew from unwashed hands. After, using hand sanitizer with 60% alcohol. But the formula without alcohol? Just look at how many gee left behind. So, right now get your hands all wet. Alcohol-based hand sanitizers work so much better because alcohol breaks up the bacteria's proteins and kills them. But if you really want to get clean, what's your best bet of all? Guess what? It's your old friend -- soap. Soap not only does as well on bacteria as the sanitizer, it gets more viruses, too. Surprisingly, studies show regular and anti-bacterialp out equally effective. It's how you use it, that's crucial. Most of us only spend five seconds at the sink. Just look how many germs remain, yuck. Go! You have to wash your hands for a full 20 seconds to really get the bugs off. And yes, that's singing happy birthday twice. So, let's review again. Hand sanitizers, if you can't get to a sink. Look for one with at least 60% alcohol, but washing with soap -- any kind -- is better as long as you wash for long enough. Dr. Richard besser, abc news, college park, maryland.