Bacteria are living one-celled creatures that most often reproduce by an original cell splitting in half, a process known as binary fission. This process begins once a cell has grown to be big enough — about twice its original size.
There are four, or sometimes five, steps to binary fission. First, the original cell (called a parent cell) has to grow large enough to begin the fission process. In the second step, the cell duplicates its chromosome so that it has two exact copies of the cell's genetic material. These copies attach themselves to the cell's plasma membrane. Then, the cell grows yet more, which pushes the two copies of genetic material even further apart. Fourth, a new wall is built across the center of the cell, dividing it into two parts, which each contain a copy of the chromosome. This cell wall is referred to as a septum.
What happens next depends on the type of microbe. Some microbes stop here and remain linked together by the septum. Other types of microbes take one more step and completely separate along the septum. Either way, both cells become completely independent adult cells whether they remain attached to each other or not.