A bacterium is a biological unicellular microorganism with no organelles and systematized nucleus. Bacteria can flourish in different kinds of environments and are among the first life forms known to exist. They are around 0.2 to 10 micrometers long and come in various shapes such as spirals, ovals, and rods. Good bacteria such as the ones in your gut are helpful in digesting food, some are important in the process of decomposition, and they are also being used to clean up oil spills. On the other hand, bad bacteria are notorious for spreading different kinds of diseases like tetanus, tuberculosis, and cholera.