Based on key characteristics, all cells are classified into two basic types: prokaryotic and eukaryotic. Although these two cell types share many common features, they have many important differences in terms of structure, metabolism, and genetics.

EUKARYOTIC AND PROKARYOTIC CELLS

Among clinically relevant organisms, bacteria are single cell prokaryotic microorganisms. Fungi and parasites are single-cell or multicellular eukaryotic organisms, as are plants and all higher animals. Viruses are dependent on host cells for survival and therefore are not considered cellular organisms but rather infectious agents.

A notable characteristic of eukaryotic cells, such as those of parasites and fungi, is the presence of membrane enclosed organelles that have specific cellular functions. Examples of these organelles and their respective functions include:

• Endoplasmic reticulum—process and transport proteins

• Golgi body—modification of substances and trans port throughout the cell, including internal delivery of molecules and exocytosis or secretion of other molecules

• Mitochondria—generate energy (ATP)

• Lysosomes—provide environment for controlled enzymatic degradation of intracellular substances

• Nucleus—provide membrane enclosure for chromosomes

Additionally, eukaryotic cells have an infrastructure, or cytoskeleton, that provides support for cellular structure, organization, and movement.

Prokaryotic cells, such as bacteria, do not contain organelles. All functions take place in the cytoplasm or cytoplasmic membrane of the cell. Prokaryotic and eukaryotic cell types differ considerably at the macromolecular level, including protein synthesis machinery, chromosomal organization, and gene expression. One notable structure present only in prokaryotic bacterial cells is a cell wall composed of peptidoglycan. This structure has an immeasurable impact on the practice of diagnostic bacteriology and the management of bacterial diseases.

BACTERIAL MORPHOLOGY

Most clinically relevant bacterial species range in size from 0.25 to 1 µm in width and 1 to 3 µm in length, thus requiring microscopy for visualization. Just as bacterial species and genera vary in their metabolic processes, their cells also vary in size, morphology, and cell-to-cell arrangements and in the chemical composition and structure of the cell wall. The bacterial cell wall differences provide the basis for the Gram stain, a fundamental staining technique used in bacterial identification schemes. This staining procedure separates almost all medically relevant bacteria into two general types: gram positive bacteria, which stain a deep blue or purple, and gram-negative bacteria, which stain a pink to red (see Figure 1). This simple but important color distinction is due to differences in the constituents of bacterial cell walls that influence the cell’s ability to retain differential dyes following treatment with a decolorizing agent.

Fig1. Gram stain procedures and principles. A, Gram-positive bacteria observed under oil immersion appear purple. B, Gram-negative  bacteria observed under oil immersion appear pink. (Modified from Atlas RM: Principles of microbiology, St Louis, 2006, Mosby.)

Common bacterial cellular morphologies include cocci (circular), coccobacilli (ovoid), and bacillus (rod shaped), as well as fusiform (pointed end), curved, or spiral shapes. Cellular arrangements are also noteworthy. Cells may characteristically occur singly, in pairs, or grouped as tetrads, clusters, or in chains (see Figure 2 for examples of bacterial staining and morphologies). The determination of the Gram stain reaction and the cell size, morphology, and arrangement are essential aspects of bacterial identification.

Fig2. Examples of common bacterial cellular morphologies,  Gram staining reactions, and cellular arrangements. 

BACTERIAL CELL COMPONENTS

 Bacterial cell components can be divided into those that make up the outer cell structure and its appendages (cell envelope) and those associated with the cell’s interior. It is important to note that the cellular structures work together to function as a complex and integrated unit.

Cell Envelope

As shown in Figure 1, the outermost structure, the cell envelope, comprises:

• An outer membrane (in gram-negative bacteria only)

• A cell wall composed of the peptidoglycan macro molecule (also known as the murein layer)

 • Periplasm (in gram-negative bacteria only)

• The cytoplasmic or cell membrane, which encloses the cytoplasm

Fig3. General structures of the gram-positive and gram-negative bacterial cell envelopes. The outer membrane and periplasmic space  are present only in the gram-negative envelope. The murein layer is substantially more prominent in gram-positive envelopes. (Modified from  Niedhardt FC, Ingraham JL, Schaechter M, editors: Physiology of the bacterial cell: a molecular approach, Sunderland, Mass, 1990, Sinauer  Associates.)

Outer Membrane. Outer membranes, which are found only in gram-negative bacteria, function as the cell’s initial barrier to the environment. These membranes serve as primary permeability barriers to hydrophilic and hydrophobic compounds and contain 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. The outer membrane also plays a significant role in the ability of certain bacteria to cause disease.

Scattered throughout the lipopolysaccharide macro molecules 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. 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 the attachment of the outer membrane to the next internal 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 relevant bacteria. This structure gives the bacterial cell shape and strength to with stand changes in environmental osmotic pressures that would otherwise result in cell lysis. The murein layer 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 are often the primary targets for the development and design of several antimicrobial agents.

The structure of the cell wall is unique and is com posed of disaccharide-pentapeptide subunits. The disaccharides N-acetylglucosamine and N-acetylmuramic acid are the alternating sugar components (moieties), with the amino acid chain linked to N-acetylmuramic acid molecules (Figure 4). Polymers of these subunits cross-link to one another by means of peptide bridges to form peptidoglycan sheets. In turn, layers of these sheets are cross-linked with one another, forming a multilayered, cross-linked structure of considerable strength. Referred to as the murein sacculus, or sack, this peptidoglycan structure surrounds the entire cell.

Fig4. Peptidoglycan sheet (A) and subunit (B) structure.  Multiple peptidoglycan layers compose the murein structure, and  different layers are extensively cross-linked by peptide bridges. Note  that amino acid chains are only derived from NAM. NAG, N acetylglucosamine; NAM, N-acetylmuramic acid. (Modified from  Saylers AA, Whitt DD: Bacterial pathogenesis: a molecular approach,  Washington, DC, 2010, American Society for Microbiology Press.)

A notable difference between the cell walls of gram positive and gram-negative bacteria is the substantially thicker peptidoglycan layer in gram-positive bacteria (see Figure 3). Additionally, the cell wall of gram-positive bacteria contains teichoic acids (i.e., glycerol or ribitol phosphate polymers 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. Other gram-positive bacteria (e.g., mycobacteria) have waxy substances within the murein layer, such as mycolic acids. Mycolic acids make the cells more refractory to toxic substances, including acids. Bacteria with mycolic acid in the cell walls require unique staining procedures and growth media in the diagnostic laboratory.

Periplasmic Space. The periplasmic space typically is found only in gram-negative bacteria (whether it is present in gram-positive organisms is the subject of debate). The periplasmic space 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, consists of gellike substances that assist in the capture of nutrients from the environment. This space also contains several enzymes involved in the degradation of macromolecules and detoxification of environmental solutes, including antibiotics that enter through the outer membrane.

Cytoplasmic (Inner) Membrane. The cytoplasmic (inner) membrane is present in both gram-positive and gram-negative bacteria and is the deepest layer of the cell envelope. The cytoplasmic membrane is heavily laced with various proteins, including a number of enzymes vital to cellular metabolism. The cell membrane serves as an additional osmotic barrier and is functionally similar to the membranes of several of eukaryotic cellular organelles (e.g., mitochondria, Golgi complexes, lysosomes). The cytoplasmic membrane functions include:

 • Transport of solutes into and out of the cell

 • Housing of 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 of molecular sensors that monitor chemical and physical changes in the environment

Cellular Appendages. In addition to the components of the cell envelope proper, cellular appendages (i.e., capsules, fimbriae, and flagella) are associated with or proximal to this portion of the cell. The presence of these appendages, which can play a role in the mediation of infection 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, the production of which 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 it does protect bacteria from attack by components of the human immune 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. A biofilm consists of a monomicrobic or polymicrobic group of bacteria housed in a complex polysaccharide matrix.

Fimbriae, or pili, are hairlike, proteinaceous structures that extend from the cell membrane into the external environment; some may be up to 2 µm long. Fimbriae may serve as adhesins that help bacteria attach to animal host cell surfaces, often as the first step in establishing infection. In addition, a pilus may be referred to as a sex pilus; this structure, which is well characterized in the gram-negative bacillus E. coli, serves as the conduit for the passage of DNA from donor to recipient during conjugation. The sex pilus is present only in cells that produce a protein referred to as the F factor. F-positive cells initiate mating or conjugation only with F-negative cells, thereby limiting the conjugative process to cells capable of transporting genetic material through the hollow sex pilus.

Flagella are complex structures, mostly composed 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 con ducted 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 number 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, more than one chromosome may be present per bacterial cell. Plasmids are the other genetic elements that exist independently in the cytosol, and their numbers may vary from none to several hundred 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 (i.e., 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 remains in a dormant state until favor able 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 food for human use.

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مواضيع ذات صلة

Bacterial Metabolism

Bacterial Genetics: Nucleic Acid Structure and Organization

Detection of Bacteremia: Culture Techniques

Detection of Bacteremia: Types of Blood Culture Bottle

Detection of Bacteremia: Specimen Collection

The Study of the Intestinal Microbiota

Chlamydia psittaci and Psittacosis

Chlamydia Pneumonia and Respiratory Infections

Chlamydia trachomatis Genital Infections and Inclusion Conjunctivitis

Trachoma

Coxiella burnetii

Ehrlichia and Anaplasma