Bacteriology is a part of microbiology which encompasses the study of bacteria, viruses, and all other sorts of microorganisms. Bacteria are single-celled microorganisms which can live as independent organisms or, dependently, as parasites. Among the better-known bacteria are streptococcus, staphylococcus, and the agents of tuberculosis and leprosy. The Bacteria are a group of single-cell microorganisms with prokaryotic cellular configuration. The genetic material (DNA) of prokaryotic cells exists unbound in the cytoplasm of the cells. There is no nuclear membrane, which is the definitive characteristic of eukaryotic cells such as those that make up, fungi, Protista, plants and animals. Until recently, bacteria were the only known type of prokaryotic cell, and the discipline of biology related to their study is called bacteriology.