屬於真細菌類，原核有機體兩大分類之一，也是活體有機體三大分類的其中一類。根據核醣體核醣核酸的研究，細菌被分為兩類，而細菌又與古菌相異，和他們與真核生物 (植物與動物)之間的差異相同。有人認為在地球的演化初期，細菌與古菌 (過去被稱為真細菌與古細菌)有相同的祖先，爾後才各自進化。細菌與古菌的主要特徵有所不同，例如核醣體蛋白質的數量以及核醣體次單元的大小與形狀皆不同。一般而言，細菌指的是微小的單細胞生物。其形狀為球狀、棒狀、或是螺旋狀，亦可能群集成絲狀的菌落。細菌繁殖的方式包括細胞分裂繁殖、無性孢子繁殖、以及仍待進一步確認的有性繁殖。許多種細菌會產生化學反應，例如發酵作用、固氮作用、以及腐敗作用，亦可能導致疾病產生。Members of the group of true bacteria, which is one of two major groups of prokaryotic organisms, and one of the three major divisions of living organisms. The division of the bacteria into two groups arises from studies of their ribosomal RNA, which indicates that they are as different from each other as they are from eukaryotes (plants and animals). Bacteria and Archaea (formerly called Eubacteria and Archaebacteria) are thought to have evolved separately from a common ancestor early in Earth's history. Bacteria and Archaea differ in important characteristics, such as the number of ribosomal proteins and the size and shape of the ribosomal S unit. Use generally for unicellular microscopic protists, spherical, rodlike, or spiral in form, or clustered into filamentous colonies, reproducing by cell division, by asexual spores, or by imperfectly understood sexually processes. Various types have chemical effects, such as fermentation, nitrogen fixation, and putrefaction, or cause diseases.