Although there are some pathogenic one, some bacteria help our everyday life as we can see from this blog. We learned that bacteria is really important for the carbon, nitrogen and sulfur cycle in our life. For instance, without bacterias' help, we cannot use nitrogen since the nitrogen as a waste is too stable for us to use. And as everyone knows, bacteria are also used for various kinds of food which usually promote more health benefits for us.
Natto (fermented soy beans) is the Japanese traditional food which we Japanese people especially eat for a breakfast. There are differences about the origin of natto among some sources, but people started to eat long time ago. Originally, people stored soybeans in straws for preservation since Japanese people grew rice as a staple diet, and they got natto from them. How did the soybeans become natto? That is the job bacillus natto do! Bacillus natto live in straws, and they ferment soybeans, which leads to more healthy soybean food. By fermenting soybeans, it is also said that proteins are more easy to assimilate to our body compare to the non-fermented soybeans. Moreover, the well-known job bacillus natto does is the production of the enzyme called nattokinase. Natto contains a lot of nattokinase which lyses clotting in blood stream, and we can prevent incidence of thrombosis such as myocardial and cerebral infarction.
Natto is rich in source of vitamin K since it is made from soybeans. Vitamin K is pro-coagulants which is converted from vitamin K in a liver as a clotting factors. Therefore you have to pay attention since vitamin K is associated with some blood clotting activity, and especially people taking certain medicine (such as warfrin) have to attention the excess intake of natto. So don't take natto too much although it is highly nutritional. I believe moderate intake works best no matter how nutritious the food is.
It is hard to get natto here, but you can get it at some Japanese grocery stores. As a microbiology student, why don't you try one:)?
http://en.wikipedia.org/wiki/Natt%C5%8D (also image)