Bacteria are prokaryotic, single-celled, microscopic organisms (Two exceptions have been discovered that can reach sizes just visible to the naked eye. They are Epulopiscium fishelsoni, a bacillus-shaped bacterium that is typically 80 micrometers (µm) in diameter and 200-600 µm long, and Thiomargarita namibiensis, a spherical bacterium between 100 and 750 µm in diameter.) Bacteria are generally much smaller than eukaryotic cells and very complex despite their small size. Structurally, a typical bacterium usually consists of (1) a cytoplasmic membrane surrounded by a peptidoglycan cell wall and maybe an outer membrane, (2) a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and (3) often various external structures such as a glycocalyx, flagella, and pili. Because a cytoplasmic membrane surrounds all cells in nature, we will start with this structure. Next we will study the bacterial cell wall. Then we will look at the anatomical parts located within the cytoplasm. Finally we will examine those structures that lie external to the cell wall.
Thumbnail: Electron micrograph of Treponema pallidum on cultures of cotton-tail rabbit epithelium cells (Sf1Ep). Treponema pallidum is the causative agent of syphilis. Image used with permission (Public Domain; CDC / Dr. David Cox).