Bacteria are microscopic unicellular organisms. They are spherical, rod-like, spiral or threadlike in shape, often clumped into colonies. A very few species of bacteria cause disease, most perform essential roles in nature. You could not digest certain types of food, for example, without the help of bacteria flourishing in your digestive system. People sometimes get nervous when they hear the term “bacteria”, because they have only heard the word in connection with diseases. The bacteria that you will add to your pond water are entirely beneficial.