Figure 4. Schematic drawing of a typical bacterium.

Surface Structures-Appendages

Flagella are filamentous protein structures attached to the cell surface that provide swimming movement for most motile procaryotic cells. The flagellar filament is rotated by a motor apparatus in the plasma membrane allowing the cell to swim in fluid environments. Bacterial flagella are powered by proton motive force (chemiosmotic potential) established on the bacterial membrane, rather than ATP hydrolysis which powers eucaryotic flagella and cilia. Procaryotes are known to exhibit a variety of types of tactic behavior, i.e., the ability to move (swim) in response to environmental stimuli. For example, during chemotaxis a bacterium can sense the quality and quantity of certain chemicals in their environment and swim towards them (if they are useful nutrients) or away from them (if they are harmful substances).

Figure 5. Vibrio cholerae has a single polar flagellum for swimming movement. Electron Micrograph of Vibrio cholerae by Leodotia Pope, Department of Microbiology, University of Texas at Austin.

Fimbriae and Pili are interchangeable terms used to designate short, hair-like structures on the surfaces of procaryotic cells. Fimbriae are shorter and stiffer than flagella, and slightly smaller in diameter. Like flagella, they are composed of protein. A specialized type of pilus, the F or sex pilus, in some way stabilizes the transfer of DNA between mating bacteria, but the function of the smaller, more numerous common pili is quite different. Common pili (often called fimbriae) are usually involved in adherence (attachment) of procaryotes to surfaces in nature. In medical situations, they are major determinants of bacterial virulence because they allow pathogens to attach to (colonize) tissues and to resist attack by phagocytic white blood cells.

Figure 6. Fimbriae of Neisseria gonorrhoeae allow the bacterium to adhere to tissues. Electron micrograph by David M. Phillips, Visuals Unlimited, with permission.

The Cell Envelope

Most procaryotes have a rigid cell wall. The cell wall is an essential structure that protects the delicate cell protoplast from osmotic lysis. The cell wall of Bacteria consists of a polymer of disaccharides cross-linked by short chains of amino acids (peptides). This molecule is a type of peptidoglycan, which is called murein. In the Gram-positive bacteria (those that retain the purple crystal violet dye when subjected to the Gram-staining procedure) the cell wall is a thick layer of murein. In the Gram-negative bacteria (cells which do not retain the crystal violet dye) the cell wall is relatively thin and is composed of a thin layer of murein surrounded by a membranous structure called the outer membrane. Murein is a substance unique in nature to bacterial cell walls. Also, the outer membrane of Gram-negative bacteria invariably contains a unique component, lipopolysaccharide (LPS or endotoxin), which is toxic to animals. The cell walls of Archaea may be composed of protein, polysaccharides, or peptidgolycan-like molecules, but never do they contain murein. This feature distinguishes the Bacteria from the Archaea.

Although procaryotes lack any intracellular organelles for respiration or photosynthesis, many species possess the physiologic ability to conduct these processes, usually as a function of their plasma membrane. For example, the electron transport system that couples aerobic respiration and ATP synthesis is found in the plasma membrane. The photosynthetic chromophores that harvest light energy for conversion into chemical energy are located in the membrane. Hence, the plasma membrane is the site of oxidative phosphorylation and photophosphorylation in procaryotes, analogous to the functions of mitochondria and chloroplasts in eucaryotic cells. The procaryotic plasma membrane is also a permeability barrier, and it contains a variety of different transport systems that selectively mediate the passage of substances into and out of the cell.

The membranes of Bacteria are structurally similar to the cell membranes of eucaryotes, except that bacterial membranes consist of saturated or monounsaturated fatty acids (rarely polyunsaturated fatty acids) and do not normally contain sterols. The membranes of Archaea form phospholipid bilayers functionally equivalent to bacterial membranes, but archaeal lipids are saturated, branched, repeating isoprenoid subunits that attach to glycerol via an ether linkage, as opposed to the ester linkage found in glycerides of eucaryotic and bacterial membrane lipids. The structure of archaeal membranes is thought to be an adaptation to their existence in extreme environments.

Most bacteria contain some sort of a polysaccharide layer outside of the cell wall or outer membrane. In a general sense, this layer is called a capsule or glycocalyx. bacteria and archaea may also have an additional proteinaceous coat called an S-layer. Capsules, slime layers, glycocalyx and s-layer are known to mediate attachment of bacterial cells to particular surfaces. Capsules also protect bacteria from engulfment by predatory protozoa or white blood cells (phagocytes) and from attack by antimicrobial agents of plant or animal origin. Capsules in certain soil bacteria protect them from perennial effects of drying or desiccation.

Importance of Surface Components

All of the various surface components of a procaryotic cell are important in its ecology since they mediate the contact of the cell with its environment. The only "sense" that a procaryote has results from its immediate contact with its environment. It must use its surface components to assess the environment and respond in a way that supports its own existence and survival in that environment. The surface properties of a procaryote are determined by the exact molecular composition of its plasma membrane cell wall, including LPS, and the function of surface structures such as flagella, fimbriae and capsules. Some important ways that procaryotes use their surface components are (1) as permeability barriers that allow selective passage of nutrients and exclusion of harmful substances; (2) as "adhesins" used to attach or adhere to specific surfaces or tissues; (3) for protection against engulfment by phagocytic white blood cells or predatory protozoa: (4) as enzymes to mediate specific reactions on the cell surface important in the survival of the procaryote; (5) as "sensing proteins" that can respond to temperature, osmolarity, salinity, light, oxygen, nutrients, etc. resulting in a signal to the genome of the cell that will cause a biological response to a changing environment.

Figure 7. The complex surface of Streptococcus pyogenes. Electron micrograph of Streptococcus pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller University.

Cytoplasmic Constituents

The cytoplasmic constituents of bacteria invariably include the procaryotic chromosome and ribosomes. The chromosome is typically one large circular molecule of DNA, more or less free in the cytoplasm, although intermittently associated with membranes.  Procaryotes sometimes possess smaller extrachromosomal pieces of DNA called plasmids. The total DNA content of a cell is referred to as the cell genome. During cell growth and division, the procaryotic chromosome is replicated in the usual semi-conservative fashion before for distribution to progeny cells. However, the eucaryotic processes of meiosis and mitosis are absent in procaryotes. Replication and segregation of procaryotic DNA is coordinated by the plasma membrane.

The distinct granular appearance of procaryotic cytoplasm is due to the presence and distribution of ribosomes The ribosomes of procaryotes are smaller than cytoplasmic ribosomes of eucaryotes. Procaryotic ribosomes are 70S in size, being composed of 30S and 50S subunits. The 80S ribosomes of eucaryotes are made up of 40S and 60S subunits. Ribosomes are involved in the process of translation (protein synthesis), but some details of their activities differ in eucaryotes, Bacteria and Archaea. Protein synthesis using bacterial 70S ribosomes occurs in eucaryotic mitochondria and chloroplasts, and this is taken as a major line of evidence that these organelles are descended from bacteria.

Often contained in the cytoplasm of procaryotic cells is one or another of some type of inclusion granule. Inclusions are distinct granules that may occupy a substantial part of the cytoplasm. Inclusion granules are usually reserve materials of some sort. For example, carbon and energy reserves may be stored as glycogen (a polymer of glucose) or as polybetahydroxybutyric acid (a type of fat) granules. Polyphosphate inclusions are reserves of PO₄ and possibly energy; elemental sulfur (sulfur globules) are stored by some phototrophic and some lithotrophic procaryotes as reserves of energy or electrons. Some inclusion bodies are actually membranous vesicles or intrusions into the cytoplasm which contain photosynthetic pigments or specialized enzyme complexes.

Figure 8. Bacterial colonies growing in a petri dish containing nutrients.
Hans Knoll Institute, Jena, Germany.