Bacterial Cell Components

Bacterial cell components can be divided into those that make up the cell envelope and its appendages and those associated with the cell's interior. Importantly, the cellular structures work together to function as a complex and integrated unit.

Cell Envelope

As shown in Figure 2-13, the outermost structure, the cell envelope, comprises:

  • An outer membrane (in gram-negative bacteria only)
  • A cell wall composed of the peptidoglycan macromolecule (also known as the murein layer)
  • Periplasm (in gram-negative bacteria only)
  • The cytoplasmic membrane, interior to which is the cytoplasm

Outer Membrane. Outer membranes are only found in gram-negative bacteria, and they function as the cell's initial barrier to the environment. These membranes serve as primary permeability barriers to hydrophilic and hydrophobic compounds, and they retain essential enzymes and other proteins located in the periplasmic space. The membrane is a bilayered structure composed of lipopolysaccharide, which gives the surface of gram-negative bacteria a net negative charge; it also plays a significant role in the ability of certain bacteria to cause disease.

Scattered throughout the lipopolysaccharide macromolecules are protein structures called porins. These water-filled structures control the passage of nutrients and other solutes, including antibiotics, through the outer membrane. The number and types of porins vary with bacterial species, and these differences can substantially influence the extent to which various substances pass through the outer membranes of different bacteria. In addition to porins, other proteins (murein lipoproteins) facilitate attachment of the outer membrane to the next deeper layer in the cell envelope, the cell wall.

Cell Wall (Murein Layer). The cell wall, also referred to as the peptidoglycan, or murein layer, is an essential structure found in nearly all clinically important bacteria. This structure gives the bacterial cell shape and strength to withstand changes in environmental osmotic pressures that would otherwise result in cell lysis. The murein layer also protects against mechanical disruption of the cell and offers some barrier to the passage of larger substances. Because this structure is essential for the survival of bacteria, its synthesis and structure have been the primary target for the development and design of several antimicrobial agents.

The structure of the cell wall is unique in nature and is composed of disaccharide-pentapeptide subunits. The disaccharides N-acetylglucosamine and N-acetylmuramic acid are the alternating sugar components (moieties), with the amino acid chain only linked to N-acetylmuramic acid molecules (Figure 2-14). Polymers of these subunits crosslink to one another via peptide bridges to form peptidoglycan sheets. Layers of these sheets are, in turn, crosslinked with one another to give a multilayered, crosslinked structure of considerable strength. Referred to as the murein sacculus, or sack, this peptidoglycan structure surrounds the entire cell.

A notable difference between the cell walls of gram-positive and gram-negative bacteria is that the peptidoglycan layer in gram-positive bacteria is substantially thicker (see Figure 2-13). Additionally, the cell wall of gram-positive bacteria contains teichoic acids (i.e., glycerol or ribitol phosphate polymers that are combined with various sugars, amino acids, and amino sugars). Some teichoic acids are linked to N-acetylmuramic acid, and others (e.g., lipoteichoic acids) are linked to the next underlying layer, the cellular membrane. Still other gram-positive

Peptide nam nam—nam nam



Amino acid chain


Figure 2-14 Peptidoglycan sheet (A) and subunit (B) structure. Multiple peptidoglycan layers compose the murein structure, and different layers are extensively crosslinked by peptide bridges (NAG, N-acetylglucosamine; NAM, N-acetylmuramic acid). Note that amino acid chains only derive from NAM. (Modified from Saylers AA, Whitt DD: Bacterial pathogenesis: a molecular approach, Washington, DC, 1994, American Society for Microbiology Press.)

Amino acid chain




Figure 2-14 Peptidoglycan sheet (A) and subunit (B) structure. Multiple peptidoglycan layers compose the murein structure, and different layers are extensively crosslinked by peptide bridges (NAG, N-acetylglucosamine; NAM, N-acetylmuramic acid). Note that amino acid chains only derive from NAM. (Modified from Saylers AA, Whitt DD: Bacterial pathogenesis: a molecular approach, Washington, DC, 1994, American Society for Microbiology Press.)

bacteria (e.g., mycobacteria) fortify their murein layer with waxy substances, such as mycotic acids, that make their cells even more refractory to toxic substances, including acids.

Periplasmic Space. The periplasmic space is only found in gram-negative bacteria and is bounded by the internal surface of the outer membrane and the external surface of the cellular membrane. This area, which contains the murein layer, is not just an empty space but consists of gel-Hke substances that help to secure nutrients from the environment. This space also contains several enzymes that degrade macromolecules and detoxify environmental solutes, including antibiotics, that enter through the outer membrane.

Cytoplasmic (Inner) Membrane, the cytoplasmic membrane is present in both gram-positive and gramnegative bacteria and is the deepest layer of the cell envelope. In both gam-positive and gram-negative bacteria, the structure of the cell membrane is similar, this lipid bilayer is heavily laced with various proteins, including a number of enzymes vital to cellular metabolism. Besides being an additional osmotic barrier, the cytoplasmic membrane is functionally similar to several of the eukaryotic cell's organelles (e.g., mitochondria, Golgi complexes, lysosoroes). The cytoplasmic membrane functions include:

  • Transport of solutes into and out of the cell
  • Housing enzymes involved in outer membrane synthesis, cell wall synthesis, and the assembly and secretion of extracytoplasmic and extracellular substances
  • Generation of chemical energy (i.e.* ATP)
  • Cell motility
  • Mediation of chromosomal segregation during replication
  • Housing molecular sensors that monitor chemical and physical changes in the environment

Cellular Appendages. In addition to the components of the cell envelope proper, there are also cellular appendages (i.e., capsules, fimbriae, and flagella) that are associated with or proximal to this portion of the cell. The presence of these appendages, which can play a role in causing infections and in laboratory identification, varies among bacterial species and even among strains within the same species.

The capsule is immediately exterior to the murein layer of gram-positive bacteria and the outer membrane of gram-negative bacteria. Often referred to as the slime layer, the capsule is composed of high-molecular-weight polysaccharides whose production may depend on the environment and growth conditions surrounding the bacterial cell. The capsule does not function as an effective permeability barrier or add strength to the cell envelope but does protect bacteria from attack by cells of the human defense system. The capsule also facilitates and maintains bacterial colonization of biologic (e.g., teeth) and inanimate (e.g., prosthetic heart valves) surfaces through the formation of biofilms.

Fimbriae, or pili, are hairlike, proteinaceous structures that extend from the cell membrane into the external environment; some may be up to 2 pun in length, two general types are known: common pili and sex pili. Common pili are adhesins that help bacteria attach to animal host cell surfaces, often as the first step in establishing infection, The sex pilus, well characterized in the gram-negative bacillus E. coli, serves as the conduit for the passage of DNA from donor to recipient during conjugation.

Flagella are complex structures, mostly imposed of the protein flagellin, intricately embedded in the cell envelope. These structures are responsible for bacterial motility. Although not all bacteria are motile, motility plays an important role in survival and the ability of certain bacteria to cause disease. Depending on the bacterial species, flagella may be located at one end of the cell (monotrichous flagella) or at both ends of the cell (lophotrichous flagella), or the entire cell surface may be covered with flagella (peritrichous flagella).

Cell Interior

Those structures and substances that are bounded internally by the cytoplasmic membrane compose the cell interior and include the cytosol, polysomes, inclusions, the nucleoid, plasmids, and endospores.

The cytosol, where nearly all other functions not conducted by the cell membrane occur, contains thousands of enzymes and is the site of protein synthesis. The cytosol has a granular appearance caused by the presence of many polysomes (mRNA complexed with several ribosomes during translation and protein synthesis) and inclusions (i.e., storage reserve granules). The numher and nature of the inclusions vary depending on the bacterial species and the nutritional state of the organism's environment. Two common types of granules include glycogen, a storage form of glucose, and polyphosphate granules, a storage form for inorganic phosphates that are microscopically visible in certain bacteria stained with specific dyes.

Unlike eukaryotic chromosomes, the bacterial chromosome is not enclosed within a membrane-bound nucleus. Instead the bacterial chromosome exists as a nucleoid in which the highly coiled DNA is intermixed with RNA, polyamines, and various proteins that lend structural support. At times, depending on the stage of cell division, there may be more than one chromosome per bacterial cell. Plasmids are the other genetic elements that exist independently in the cytosol, and their numbers may vary from none to several per bacterial cell.

The final bacterial structure to be considered is the endospore. Under adverse physical and chemical conditions, or when nutrients are scarce, some bacterial genera are able to form spores (iev sporulate). Sporulation involves substantial metabolic and structural changes in the bacterial cell. Essentially, the cell transforms from an actively metabolic and growing state to a dormant state, with a decrease in cytosol and a concomitant increase in the thickness and strength of the cell envelope. The spore state is maintained until favorable conditions for growth are again encountered. This survival tactic is demonstrated by a number of clinically relevant bacteria and frequently challenges our ability to thoroughly sterilize materials and foods for human use.

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