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.
- 2.1: Sizes, Shapes, and Arrangements of Bacteria
- There are three basic shapes of bacteria: coccus, bacillus, and spiral. Based on planes of division, the coccus shape can appear in several distinct arrangements: diplococcus, streptococcus, tetrad, sarcina, and staphylococcus. The bacillus shape can appear as a single bacillus, a streptobacillus, or a coccobacillus. The spiral shape can appear in several forms: vibrio, spirillum, and spirochete.
- 2.2: The Cytoplasmic Membrane
- The bacterial cytoplasmic membrane is a fluid phospholipid bilayer that encloses the bacterial cytoplasm. The cytoplasmic membrane is semipermeable and determines what molecules enter and leave the bacterial cell. Passive diffusion is the net movement of gases or small uncharged polar molecules such as water across a membrane from an area of higher concentration to an area of lower concentration.
- 2.3: The Peptidoglycan Cell Wall
- The vast majority of the domain Bacteria have a rigid cell wall composed of peptidoglycan. The peptidoglycan cell wall surrounds the cytoplasmic membrane and prevents osmotic lysis. Peptidoglycan is composed of interlocking chains of building blocks called peptidoglycan monomers.
- 2.4: Cellular Components within the Cytoplasm
- Various anatomical parts that make up the anatomy of a Prokaryotic Cell bacterium. As mentioned in the introduction to this section, a typical bacterium usually consists of: a cytoplasmic membrane surrounded by a peptidoglycan cell wall and maybe an outer membrane; a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and often various external structures such as a glycocalyx, flagella, and pili.
- 2.5: Structures Outside the Cell Wall
- In this section on Prokaryotic Cell Anatomy we are looking at the various anatomical parts that make up a bacterium. We will now look at the following structures located outside the cell wall of many bacteria: (1) glycocalyx (capsule) and S-layer, (2) flagella, and (3) pili.
- 2.E: The Prokaryotic Cell: Bacteria (Exercises)
- These are homework exercises to accompany Kaiser's "Microbiology" TextMap. Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell (unicellular), cell clusters or no cell at all (acellular). This includes eukaryotes, such as fungi and protists, and prokaryotes. Viruses and prions, though not strictly classed as living organisms, are also studied.
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).