Prokaryotic cells lack autonomous plastids, such as mitochondria and chloroplasts; the electron transport enzymes are localized instead in the cytoplasmic membrane. The photosynthetic pigments (carotenoids, bacteriochlorophyll) of photosynthetic bacteria are contained in intracytoplasmic membrane systems of various morphologies. Mem brane vesicles (chromatophores) or lamellae are commonly observed membrane types. Some photosynthetic bacteria have specialized nonunit membrane-enclosed structures called chlorosomes. In some cyanobacteria (formerly known as blue-green algae), the photosynthetic membranes often form multilayered structures known as thylakoids (Figure1). The major accessory pigments used for light harvesting are the phycobilins found on the outer surface of the thylakoid membranes.

Fig1. Thin section of Synechocystis during division. Many structures are visible. (Reproduced from Stanier RY: The position of cyanobacteria in the world of phototrophs. Carlsberg Res Commun 42:77-98, 1977. With kind permission of Springer + Business Media.)

Bacteria often store reserve materials in the form of insoluble granules, which appear as refractile bodies in the cytoplasm when viewed by phase-contrast microscopy. These so-called inclusion bodies almost always function in the storage of energy or as a reservoir of structural building blocks. Most cellular inclusions are bounded by a thin non unit membrane consisting of lipid, which serves to separate the inclusion from the cytoplasm proper. One of the most common inclusion bodies consists of poly-β-hydroxybutyric acid (PHB), a lipid-like compound consisting of chains of  β-hydroxybutyric acid units connected through ester link ages. PHB is produced when the source of nitrogen, sulfur, or phosphorous is limited and there is excess carbon in the medium (Figure 2-A). Another storage product formed by prokaryotes when carbon is in excess is glycogen, which is a polymer of glucose. PHB and glycogen are used as car bon sources when protein and nucleic acid synthesis are resumed. A variety of prokaryotes are capable of oxidizing reduced sulfur compounds, such as hydrogen sulfide and thiosulfate, producing intracellular granules of elemental sulfur (Figure 2-B). As the reduced sulfur source becomes limiting, the sulfur in the granules is oxidized, usually to sul fate, and the granules slowly disappear. Many bacteria accumulate large reserves of inorganic phosphate in the form of granules of polyphosphate. These granules can be degraded and used as sources of phosphate for nucleic acid and phospholipid synthesis to support growth. These granules are sometimes termed volutin granules or metachromatic gran ules because they stain red with a blue dye. They are characteristic features of Corynebacterium .

Fig2. Inclusion bodies in bacteria. A: Electron micrograph of B. megaterium (30,500×) showing poly-β hydroxybutyric acid inclusion body, PHB; cell wall, CW; nucleoid, N; plasma membrane, PM; “mesosome,” M; and ribosomes, R. (Reproduced with permission. © Ralph A. Slepecky/ Visuals Unlimited.) B: Cromatium vinosum, a purple sulfur bacterium, with intracellular sulfur granules, bright field microscopy (2000×). (Reproduced with permission from Holt J (editor): The Shorter Bergey’s Manual of Determinative Bacteriology, 8th ed. Williams & Wilkins, 1977. Copyright Bergey’s Manual Trust.)

Certain groups of autotrophic bacteria that fix carbon dioxide to make their biochemical building blocks contain polyhedral bodies surrounded by a protein shell (carboxysomes) containing the key enzyme of CO₂ fixation, ribulosebisphosphate carboxylase (see Figure1). Magnetosomes are intra cellular crystal particles of the iron mineral magnetite (Fe₃O₄ ) that allow certain aquatic bacteria to exhibit magnetotaxis (ie, migration or orientation of the cell with respect to the earth’s magnetic field). Magnetosomes are surrounded by a nonunit membrane containing phospholipids, proteins, and glycoproteins. Gas vesicles are found almost exclusively in microorganisms from aquatic habitats, where they provide buoyancy. The gas vesicle membrane is a 2-nm-thick layer of protein, impermeable to water and solutes but permeable to gases; thus, gas vesicles exist as gas-filled structures surrounded by the constituents of the cytoplasm (Figure 3).

Fig3. Transverse section of a dividing cell of the cyanobacterium Microcystis species showing hexagonal stacking of the cylindric gas vesicles (31,500×). (Micrograph by HS Pankratz. Reproduced with permission from Walsby AE: Gas vesicles. Microbiol Rev 1994;58:94.)

The most numerous intracellular structure in most bacteria is the ribosome, the site of protein synthesis in all living organisms. All prokaryotes have 70S ribosomes, while eukaryotes contain larger 80S ribosomes in their cytoplasm. The 70S ribosome is made up of 50S and 30S subunits. The 50S subunit contains the 23S and 5S ribosomal RNA (rRNA), while the 30S subunit contains the 16S rRNA. These rRNA molecules are complexed with a large number of ribosomal proteins. The bacterial cytoplasm also contains homologs of all the major cytoskeletal proteins of eukaryotic cells as well as additional proteins that play cytoskeletal roles (Figure 4).

Fig4. The prokaryotic cytoskeleton. Visualization of the MreB-like cytoskeletal protein (Mbl) of B. subtilis. The Mbl protein has been fused with green fluorescent protein, and live cells have been examined by fluorescence microscopy. A: Arrows point to the helical cytoskeleton cables that extend the length of the cells. B: Three of the cells from A are shown at a higher magnification. (Courtesy of Rut Carballido-Lopez and Jeff Errington.)

Actin homologs (eg, MreB and Mbl) perform a variety of functions, helping to determine cell shape, segregate chromosomes, and localize proteins within the cell. Nonactin homologs (eg, FtsZ) and unique bacterial cytoskeletal proteins (eg, SecY and MinD) are involved in determining cell shape and in regulation of cell division and chromosome segregation.