Most bacteria are much smaller and simpler than human cells, and generally have only one chromosome.
In addition to this single chromosome, bacteria often carry plasmids. Plasmids like chromosomes are made of genetic material but, unlike chromosomes, are non-essential. However, a plasmid, which is small ring of DNA, can change the characteristics of bacteria. It can make a bacterium resistant to antibiotics, for instance. This is important if the bacterium happens to cause a disease and a doctor wants to cure the sick patient with antibiotics.