which drawing in the figure is a tetrad?

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Baron Due south, editor. Medical Microbiology. quaternary edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996.

Chapter two Construction

Milton R.J. Salton and Kwang-Shin Kim.

General Concepts

Gross Morphology

Bacteria have feature shapes (cocci, rods, spirals, etc.) and often occur in characteristic aggregates (pairs, chains, tetrads, clusters, etc.). These traits are usually typical for a genus and are diagnostically useful.

Jail cell Structure

Prokaryotes take a nucleoid (nuclear body) rather than an enveloped nucleus and lack membrane-jump cytoplasmic organelles. The plasma membrane in prokaryotes performs many of the functions carried out by membranous organelles in eukaryotes. Multiplication is past binary fission.

Surface Structures

Flagella: The flagella of motile bacteria differ in structure from eukaryotic flagella. A basal body anchored in the plasma membrane and cell wall gives rise to a cylindrical protein filament. The flagellum moves by whirling about its long centrality. The number and arrangement of flagella on the cell are diagnostically useful.

Pili (Fimbriae): Pili are slender, hairlike, proteinaceous appendages on the surface of many (peculiarly Gram-negative) bacteria. They are of import in adhesion to host surfaces.

Capsules: Some leaner course a thick outer capsule of high-molecular-weight, viscous polysaccharide gel; others accept more baggy slime layers. Capsules confer resistance to phagocytosis.

Of import Chemical Components of Surface Structures

Cell Wall Peptidoglycans: Both Gram-positive and Gram-negative leaner possess cell wall peptidoglycans, which confer the feature cell shape and provide the jail cell with mechanical protection. Peptidoglycans are unique to prokaryotic organisms and consist of a glycan backbone of muramic acid and glucosamine (both N-acetylated), and peptide chains highly cross-linked with bridges in Gram-positive bacteria (east.1000., Staphylococcus aureus) or partially cross-linked in Gram-negative bacteria (e.g., Escherichia coli). The cross-linking transpeptidase enzymes are some of the targets for b-lactam antibiotics.

Teichoic Acids: Teichoic acids are polyol phosphate polymers bearing a potent negative charge. They are covalently linked to the peptidoglycan in some Gram-positive leaner. They are strongly antigenic, but are generally absent in Gram-negative bacteria.

Lipoteichoic Acids: Lipoteichoic acids as membrane teichoic acids are polymers of amphiphitic glycophosphates with the lipophilic glycolipid and anchored in the cytoplasmic membrane. They are antigenic, cytotoxic and adhesins (e.m., Streptococcus pyogenes).

Lipopolysaccharides: 1 of the major components of the outer membrane of Gram-negative bacteria is lipopolysaccharide (endotoxin), a complex molecule consisting of a lipid A anchor, a polysaccharide core, and chains of carbohydrates. Sugars in the polysaccharide chains confer serologic specificity.

Wall-Less Forms: Ii groups of bacteria devoid of prison cell wall peptidoglycans are the Mycoplasma species, which possess a surface membrane structure, and the Fifty-forms that ascend from either Gram-positive or Gram-negative bacterial cells that have lost their ability to produce the peptidoglycan structures.

Cytoplasmic Structures

Plasma Membrane: The bacterial plasma membrane is composed primarily of protein and phospholipid (about three:ane). It performs many functions, including transport, biosynthesis, and free energy transduction.

Organelles: The bacterial cytoplasm is densely packed with 70S ribosomes. Other granules represent metabolic reserves (e.thousand., poly-β-hydroxybutyrate, polysaccharide, polymetaphosphate, and metachromatic granules).

Endospores: Bacillus and Clostridium species can produce endospores: heat-resistant, dehydrated resting cells that are formed intracellularly and comprise a genome and all essential metabolic machinery. The endospore is encased in a complex protective spore coat.

Introduction

All leaner, both pathogenic and saprophytic, are unicellular organisms that reproduce past binary fission. Most bacteria are capable of independent metabolic existence and growth, but species of Chlamydia and Rickettsia are obligately intracellular organisms. Bacterial cells are extremely small and are almost conveniently measured in microns (ten^-six m). They range in size from large cells such as Bacillus anthracis (1.0 to 1.3 µm Ten iii to 10 µm) to very small cells such as Pasteurella tularensis (0.2 X 0.two to 0.7 µm) Mycoplasmas (atypical pneumonia group) are fifty-fifty smaller, measuring 0.1 to 0.2 µm in diameter. Bacteria therefore have a surface-to-volume ratio that is very loftier: nearly 100,000.

Leaner have characteristic shapes. The common microscopic morphologies are cocci (circular or ellipsoidal cells, such as Staphylococcus aureus or Streptococcus, respectively); rods, such as Bacillus and Clostridium species; long, filamentous branched cells, such as Actinomyces species; and comma-shaped and spiral cells, such equally Vibrio cholerae and Treponema pallidum, respectively. The arrangement of cells is likewise typical of diverse species or groups of leaner (Fig. 2-1). Some rods or cocci characteristically abound in chains; some, such as Staphylococcus aureus, form grapelike clusters of spherical cells; some circular cocci form cubic packets. Bacterial cells of other species grow separately. The microscopic appearance is therefore valuable in classification and diagnosis. The higher resolving ability of the electron microscope not only magnifies the typical shape of a bacterial cell but besides conspicuously resolves its prokaryotic organisation (Fig. 2-2).

Figure 2-i

Typical shapes and arrangements of bacterial cells.

Figure ii-2

Electron micrograph of a thin department of Neisseria gonorrhoeae showing the organizational features of prokaryotic cells. Note the electron-transparent nuclear region (n) packed with Deoxyribonucleic acid fibrils, the dense distribution of ribosomal particles in the cytoplasm, (more...)

The Nucleoid

Prokaryotic and eukaryotic cells were initially distinguished on the basis of structure: the prokaryotic nucleoidthe equivalent of the eukaryotic nucleusis structurally simpler than the true eukaryotic nucleus, which has a complex mitotic appliance and surrounding nuclear membrane. As the electron micrograph in Fig. 2-2 shows, the bacterial nucleoid, which contains the DNA fibrils, lacks a limiting membrane. Nether the light microscope, the nucleoid of the bacterial jail cell can be visualized with the aid of Feulgen staining, which stains DNA. Gentle lysis tin can exist used to isolate the nucleoid of most bacterial cells. The Deoxyribonucleic acid is then seen to exist a single, continuous, "giant" round molecule with a molecular weight of approximately iii X 10^nine (see Ch. v). The unfolded nuclear Dna would be well-nigh 1 mm long (compared with an average length of 1 to 2 µm for bacterial cells). The bacterial nucleoid, then, is a structure containing a unmarried chromosome. The number of copies of this chromosome in a prison cell depends on the stage of the cell cycle (chromosome replication, cell enlargement, chromosome segregation, etc). Although the mechanism of segregation of the two sister chromosomes following replication is not fully understood, all of the models proposed crave that the chromosome be permanently fastened to the cell membrane throughout the various stages of the cell cycle.

Bacterial chromatin does not contain basic histone proteins, but low-molecular-weight polyamines and magnesium ions may fulfill a office similar to that of eukaryotic histones. Despite the differences between prokaryotic and eukaryotic Deoxyribonucleic acid, prokaryotic DNA from cells infected with bacteriophage 𝛄, when visualized by electron microscopy, has a beaded, condensed appearance not dissimilar that of eukaryotic chromatin.

Surface Appendages

Two types of surface appendage tin can be recognized on certain bacterial species: the flagella, which are organs of locomotion, and pili (Latin hairs), which are as well known every bit fimbriae (Latin fringes). Flagella occur on both Gram-positive and Gram-negative bacteria, and their presence tin can be useful in identification. For example, they are found on many species of bacilli but rarely on cocci. In contrast, pili occur virtually exclusively on Gram-negative leaner and are establish on only a few Gram-positive organisms (eastward.g., Corynebacterium renale).

Some bacteria have both flagella and pili. The electron micrograph in Fig. 2-3 shows the characteristic wavy appearance of flagella and 2 types of pili on the surface of Escherichia coli.

Effigy 2-3

(A) Electron micrograph of negatively stained E. coli showing wavy flagella and numerous curt, thinner, and more rigid hairlike structures, the pili. (B) The long sex pilus tin can be distinguished from the shorter mutual pili by mixing East. coli cells with (more...)

Flagella

Structurally, bacterial flagella are long (iii to 12 µm), filamentous surface appendages most 12 to 30 nm in diameter. The protein subunits of a flagellum are assembled to grade a cylindrical construction with a hollow core. A flagellum consists of three parts: (ane) the long filament, which lies external to the cell surface; (2) the hook construction at the finish of the filament; and (iii) the basal body, to which the hook is anchored and which imparts motility to the flagellum. The basal torso traverses the outer wall and membrane structures. It consists of a rod and i or two pairs of discs. The thrust that propels the bacterial cell is provided past counterclockwise rotation of the basal body, which causes the helically twisted filament to whirl. The movement of the basal trunk is driven by a proton motive force rather than by ATP straight. The ability of leaner to swim by means of the propeller-like action of the flagella provides them with the mechanical means to perform chemotaxis (move in response to attractant and repellent substances in the surroundings). Response to chemic stimuli involves a sophisticated sensory system of receptors that are located in the cell surface and/or periplasm and that transmit information to methyl-accepting chemotaxis proteins that command the flagellar motor. Genetic studies take revealed the existence of mutants with altered biochemical pathways for flagellar movement and chemotaxis.

Chemically, flagella are constructed of a grade of proteins called flagellins. The hook and basal-trunk structures consist of numerous proteins. Mutations affecting any of these gene products may result in loss or impairment of motility. Flagellins are immunogenic and constitute a group of protein antigens called the H antigens, which are characteristic of a given species, strain, or variant of an organism. The species specificity of the flagellins reflects differences in the main structures of the proteins. Antigenic changes of the flagella known as the phase variation of H1 and H2 occurs in Salmonella typhimurium (see Ch. 21 and Ref. Seifert and Then).

The number and distribution of flagella on the bacterial surface are characteristic for a given species and hence are useful in identifying and classifying bacteria. Figure 2-iv illustrates typical arrangements of flagella on or around the bacterial surface. For example, Five. cholerae has a unmarried flagellum at one pole of the jail cell (i.due east., information technology is monotrichous), whereas Proteus vulgaris and E. coli accept many flagella distributed over the entire cell surface (i.e., they are peritrichous). The flagella of a peritrichous bacterium must amass as a posterior bundle to propel the prison cell in a frontward direction.

Effigy 2-4

Typical arrangements of bacterial flagella.

Flagella can be sheared from the cell surface without affecting the viability of the jail cell. The prison cell then becomes temporarily nonmotile. In time it synthesizes new flagella and regains motility. The protein synthesis inhibitor chloramphenicol, however, blocks regeneration of flagella.

Pili

The terms pili and fimbriae are normally used interchangeably to describe the thin, hairlike appendages on the surface of many Gram-negative bacteria and proteins of pili are referred to as pilins. Pili are more rigid in advent than flagella (Fig. 2-3). In some organisms, such as Shigella species and Due east. coli, pili are distributed profusely over the cell surface, with equally many equally 200 per cell. As is hands recognized in strains of E. coli, pili can come in two types: brusque, abundant common pili, and a small number (1 to six) of very long pili known as sexual activity pili. Sexual practice pili can be distinguished by their power to bind male-specific bacteriophages (the sexual practice pilus acts as a specific receptor for these bacteriophages) (Fig. two-3B). The sex pili attach male to female person bacteria during conjugation.

Pili in many enteric bacteria confer agglutinative properties on the bacterial cells, enabling them to adhere to various epithelial surfaces, to red blood cells (causing hemagglutination), and to surfaces of yeast and fungal cells. These adhesive properties of piliated cells play an important role in bacterial colonization of epithelial surfaces and are therefore referred to as colonization factors. The common pili found on Eastward. coli exhibit a carbohydrate specificity analogous to that of phytohemagglutinins and lectins, in that adhesion and hemagglutinating capacities of the organism are inhibited specifically by mannose. Organisms possessing this blazon of hemagglutination are called mannose-sensitive organisms. Other piliated organisms, such as gonococci, are adhesive and hemagglutinating, simply are insensitive to the inhibitory furnishings of mannose. Extensive antigenic variations in pilins of gonococci are well known (see Ref. Seifert and Then).

Surface Layers

The surface layers of the bacterial cell have been identified past various techniques: light microscopy and staining; electron microscopy of sparse-sectioned, freeze-fractured, and negatively stained cells; and isolation and biochemical label of individual morphologic components of the cell. The main surface layers are capsules and loose slime, the jail cell wall of Gram-positive bacteria and the circuitous cell envelope of Gram-negative bacteria, plasma (cytoplasmic) membranes, and mesosomal membrane vesicles, which arise from invaginations of the plasma membrane. In bacteria, the cell wall forms a rigid structure of uniform thickness around the jail cell and is responsible for the characteristic shape of the prison cell (rod, coccus, or spiral). Inside the cell wall (or rigid peptidoglycan layer) is the plasma (cytoplasmic) membrane; this is usually closely apposed to the wall layer. The topographic relationships of the prison cell wall and envelope layers to the plasma membrane are indicated in the thin section of a Gram-positive organism (Micrococcus lysodeikticus) in Figure 2-5A and in the freeze-fractured cell of a Gram-negative organism (Bacteroides melaninogenicus) in Figure 2-5B. The latter shows the typical fracture planes seen in most Gram-negative bacteria, which are weak cleavage planes through the outer membrane of the envelope and all-encompassing fracture planes through the bilayer region of the underlying plasma membrane.

Effigy 2-5

(A) Electron micrograph of a thin section of the Gram-positive One thousand. lysodeikticus showing the thick peptidoglycan cell wall (cw), underlying cytoplasmic (plasma) membrane (cm), mesosome (m), and nucleus (northward). (B) Freeze-fractured Bacteriodes cell showing (more...)

Capsules and Loose Slime

Some bacteria class capsules, which constitute the outermost layer of the bacterial cell and environs information technology with a relatively thick layer of viscous gel. Capsules may be up to 10 µm thick. Some organisms lack a well-defined capsule merely have loose, amorphous slime layers external to the cell wall or cell envelope. The α hemolytic Streptococcus mutans, the primary organism found in dental plaque is able to synthesis a large extracellular mucoid glucans from sucrose. Not all bacterial species produce capsules; nonetheless, the capsules of encapsulated pathogens are oft important determinants of virulence. Encapsulated species are establish amid both Gram-positive and Gram-negative leaner. In both groups, nearly capsules are composed of highmolecular-weight viscous polysaccharides that are retained as a thick gel outside the cell wall or envelope. The capsule of Bacillus anthracis (the causal agent of anthrax) is unusual in that it is composed of a γ-glutamyl polypeptide. Table 2-1 presents the various capsular substances formed by a selection of Gram-positive and Gram-negative bacteria. A plasma membrane stage is involved in the biosynthesis and associates of the capsular substances, which are extruded or secreted through the outer wall or envelope structures. Mutational loss of enzymes involved in the biosynthesis of the capsular polysaccharides can result in the shine-to-crude variation seen in the pneumococci.

Table ii-1

Nature of Capsular Substances Formed by Various Leaner.

The capsule is not essential for viability. Viability is not affected when capsular polysaccharides are removed enzymatically from the cell surface. The verbal functions of capsules are non fully understood, but they practice confer resistance to phagocytosis and hence provide the bacterial cell with protection against host defenses to invasion.

Cell Wall and Gram-Negative Cell Envelope

The Gram stain broadly differentiates bacteria into Gram-positive and Gram-negative groups; a few organisms are consistently Gram-variable. Gram-positive and Gram-negative organisms differ drastically in the organization of the structures outside the plasma membrane but below the capsule (Fig. 2-half-dozen): in Gram-negative organisms these structures establish the cell envelope, whereas in Gram-positive organisms they are chosen a cell wall.

Figure 2-6

Comparing of the thick cell wall of Gram-positive bacteria with the comparatively sparse cell wall of Gram-negative leaner. Note the complexity of the Gram-negative cell envelope (outer membrane, its hydrophobic lipoprotein anchor; periplasmic space). (more...)

Nigh Gram-positive leaner accept a relatively thick (nearly 20 to 80 nm), continuous cell wall (oftentimes called the sacculus), which is composed largely of peptidoglycan (also known as mucopeptide or murein). In thick prison cell walls, other prison cell wall polymers (such as the teichoic acids, polysaccharides, and peptidoglycolipids) are covalently attached to the peptidoglycan. In contrast, the peptidoglycan layer in Gram-negative bacteria is sparse (most v to 10 nm thick); in E. coli, the peptidoglycan is probably just a monolayer thick. Outside the peptidoglycan layer in the Gram-negative envelope is an outer membrane structure (about 7.5 to ten nm thick). In about Gram-negative bacteria, this membrane structure is anchored noncovalently to lipoprotein molecules (Braun's lipoprotein), which, in plough, are covalently linked to the peptidoglycan. The lipopolysaccharides of the Gram-negative prison cell envelope form part of the outer leaflet of the outer membrane structure.

The organization and overall dimensions of the outer membrane of the Gram-negative jail cell envelope are similar to those of the plasma membrane (about 7.five nm thick). Moreover, in Gram-negative bacteria such equally E. coli, the outer and inner membranes adhere to each other at several hundred sites (Bayer patches); these sites can interruption up the continuity of the peptidoglycan layer. Tabular array two-2 summarizes the major classes of chemical constituents in the walls and envelopes of Gram-positive and Gram-negative leaner.

Table 2-2

Major Classes of Chemic Components in Bacterial Walls and Envelopes.

The basic differences in surface structures of Gram-positive and Gram-negative leaner explain the results of Gram staining. Both Gram-positive and Gram-negative leaner accept upward the same amounts of crystal violet (CV) and iodine (I). The CV-I complex, nevertheless, is trapped inside the Gram-positive cell by the dehydration and reduced porosity of the thick cell wall as a result of the differential washing step with 95 percent ethanol or other solvent mixture. In dissimilarity, the thin peptidoglycan layer and probable discontinuities at the membrane adhesion sites do not impede solvent extraction of the CV-I circuitous from the Gram-negative cell. The above mechanism of the Gram stain based on the structural differences between the 2 groups has been confirmed past sophisticated methods of electron microscopy (come across Ref. Bereridge and Daries). The sequence of steps in the Gram stain differentiation is illustrated diagrammatically in Figure 2-7. Moreover, mechanical disruption of the cell wall of Gram-positive organisms or its enzymatic removal with lysozyme results in consummate extraction of the CV-I complex and conversion to a Gram-negative reaction. Therefore, autolytic wall-degrading enzymes that cause cell wall breakage may account for Gram-negative or variable reactions in cultures of Gram-positive organisms (such as Staphylococcus aureus, Clostridium perfringens, Corynebacterium diphtheriae, and some Bacillus spp.).

Figure 2-7

General sequence of steps in the Gram stain process and the resultant staining of Gram-positive and Gram-negative leaner.

Peptidoglycan

Unique features of almost all prokaryotic cells (except for Halobacterium halobium and mycoplasmas) are cell wall peptidoglycan and the specific enzymes involved in its biosynthesis. These enzymes are target sites for inhibition of peptidoglycan synthesis past specific antibiotics. The primary chemical structures of peptidoglycans of both Gram-positive and Gram-negative bacteria take been established; they consist of a glycan backbone of repeating groups of β1, 4-linked disaccharides of β1,4-N-acetylmuramyl-N-acetylglucosamine. Tetrapeptides of L-alanine-D-isoglutamic acid-50-lysine (or diaminopimelic acid)-n-alanine are linked through the carboxyl grouping by amide linkage of muramic acrid residues of the glycan chains; the D-alanine residues are directly cross-linked to the 𝛆-amino group of lysine or diaminopimelic acrid on a neighboring tetrapeptide, or they are linked past a peptide bridge. In S. aureus peptidoglycan, a glycine pentapeptide bridge links the two adjacent peptide structures. The extent of direct or peptide-bridge cross-linking varies from one peptidoglycan to another. The staphylococcal peptidoglycan is highly cantankerous-linked, whereas that of Eastward. coli is much less so, and has a more open peptidoglycan mesh. The diamino acrid providing the 𝛆-amino grouping for cross-linking is lysine or diaminopimelic acid, the latter being uniformly present in Gram-negative peptidoglycans. The structure of the peptidoglycan is illustrated in Figure two-8. A peptidoglycan with a chemical structure substantially different from that of all eubacteria has been discovered in sure archaebacteria. Instead of muramic acrid, this peptidoglycan contains talosaminuronic acid and lacks the D-amino acids found in the eubacterial peptidoglycans. Interestingly, organisms containing this wall polymer (referred to as pseudomurein) are insensitive to penicillin, an inhibitor of the transpeptidases involved in peptidoglycan biosynthesis in eubacteria.

Figure 2-8

Diagrammatic representation of peptidoglycan structures with next glycan strands cross-linked straight from the carboxyterminal D-alanine to the 𝛆-amino grouping of an side by side tetrapeptide or through a peptide cross bridge, N-acetylmuramic acid, (more...)

The ß-1,4 glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine is specifically broken by the bacteriolytic enzyme lysozyme. Widely distributed in nature, this enzyme is present in man tissues and secretions and can crusade consummate digestion of the peptidoglycan walls of sensitive organisms. When lysozyme is immune to digest the cell wall of Gram-positive bacteria suspended in an osmotic stabilizer (such every bit sucrose), protoplasts are formed. These protoplasts are able to survive and continue to abound on suitable media in the wall-less state. Gram-negative leaner treated similarly produce spheroplasts, which retain much of the outer membrane structure. The dependence of bacterial shape on the peptidoglycan is shown by the transformation of rod-shaped leaner to spherical protoplasts (spheroplasts) after enzymatic breakdown of the peptidoglycan. The mechanical protection afforded by the wall peptidoglycan layer is evident in the osmotic fragility of both protoplasts and spheroplasts. In that location are two groups of bacteria that lack the protective jail cell wall peptidoglycan construction, the Mycoplasma species, ane of which causes atypical pneumonia and some genitourinary tract infections and the L-forms, which originate from Gram-positive or Gram-negative bacteria and are so designated because of their discovery and description at the Lister Institute, London. The mycoplasmas and 50-forms are all Gram-negative and insensitive to penicillin and are bounded by a surface membrane structure. Fifty-forms arising "spontaneously" in cultures or isolated from infections are structurally related to protoplasts and spheroplasts; all three forms (protoplasts, spheroplasts, and L-forms) revert infrequently and only nether special conditions.

Teichoic Acids

Wall teichoic acids are found only in sure Gram-positive leaner (such as staphylococci, streptococci, lactobacilli, and Bacillus spp.); so far, they have not been found in gram- negative organisms. Teichoic acids are polyol phosphate polymers, with either ribitol or glycerol linked by phosphodiester bonds; their structures are illustrated in Figure 2-9. Substituent groups on the polyol chains can include D-alanine (ester linked), N-acetylglucosamine, N-acetylgalactosamine, and glucose; the substituent is characteristic for the teichoic acid from a particular bacterial species and can act every bit a specific antigenic determinant. Teichoic acids are covalently linked to the peptidoglycan. These highly negatively charged polymers of the bacterial wall can serve as a cation-sequestering machinery.

Figure 2-9

Structures of cell wall teichoic acids. (A) Ribitol teichoic acid with repeating units of 1,5-phosphodiester linkages of D-ribitol and D-alanyl ester on position 2 and glycosyl substituents (R) on position iv. The glycosyl groups may abe N-acetylglucosaminyl (more...)

Accessory Wall Polymers

In addition to the principal jail cell wall polymers, the walls of certain Gram-positive bacteria possess polysaccharide molecules linked to the peptidoglycan. For case, the C polysaccharide of streptococci confers group specificity. Acidic polysaccharides fastened to the peptidoglycan are called teichuronic acids. Mycobacteria take peptidoglycolipids, glycolipids, and waxes associated with the cell wall.

Lipopolysaccharides

A feature feature of Gram-negative bacteria is possession of various types of complex macromolecular lipopolysaccharide (LPS). And so far, merely i Gram-positive organism, Listeria monocytogenes, has been establish to contain an authentic LPS. The LPS of this bacterium and those of all Gram-negative species are likewise called endotoxins, thereby distinguishing these cell-bound, heat-stable toxins from oestrus-labile, poly peptide exotoxins secreted into civilisation media. Endotoxins possess an array of powerful biologic activities and play an important role in the pathogenesis of many Gram-negative bacterial infections. In addition to causing endotoxic shock, LPS is pyrogenic, tin can activate macrophages and complement, is mitogenic for B lymphocytes, induces interferon production, causes tissue necrosis and tumor regression, and has adjuvant backdrop. The endotoxic properties of LPS reside largely in the lipid A components. Ordinarily, the LPS molecules have three regions: the lipid A structure required for insertion in the outer leaflet of the outer membrane bilayer; a covalently attached core composed of 2-keto-3deoxyoctonic acrid (KDO), heptose, ethanolamine, N-acetylglucosamine, glucose, and galactose; and polysaccharide bondage linked to the core. The polysaccharide chains found the O-antigens of the Gram-negative bacteria, and the individual monosaccharide constituents confer serologic specificity on these components. Figure 2-10 depicts the structure of LPS. Although it has been known that lipid A is composed of β1,6-linked D-glucosamine disaccharide substituted with phosphomonester groups at positions 4' and one, uncertainties have existed most the attachment positions of the six fatty acid acyl and KDO groups on the disaccharide. The demonstration of the structure of lipid A of LPS of a heptoseless mutant of Salmonella typhimurium has established that amide-linked hydroxymyristoyl and lauroxymyristoyl groups are attached to the nitrogen of the ii- and two'-carbons, respectively, and that hydroxymyristoyl and myristoxymyristoyl groups are attached to the oxygen of the 3- and 3'-carbons of the disaccharide, respectively. Therefore, only position 6' is left for zipper of KDO units.

Figure ii-x

The iii major, covalently linked regions that grade the typical LPS.

LPS and phospholipids aid confer disproportion to the outer membrane of the Gram-negative bacteria, with the hydrophilic polysaccharide bondage outermost. Each LPS is held in the outer membrane past relatively weak cohesive forces (ionic and hydrophobic interactions) and tin be dissociated from the cell surface with surface-active agents.

As in peptidoglycan biosynthesis, LPS molecules are assembled at the plasma or inner membrane. These newly formed molecules are initially inserted into the outer-inner membrane adhesion sites.

Outer Membrane of Gram-Negative Bacteria

In thin sections, the outer membranes of Gram-negative bacteria appear broadly similar to the plasma or inner membranes; withal, they differ from the inner membranes and walls of Gram-positive bacteria in numerous respects. The lipid A of LPS is inserted with phospholipids to create the outer leaflet of the bilayer structure; the lipid portion of the lipoprotein and phospholipid class the inner leaflet of the outer membrane bilayer of most Gram-negative bacteria (Fig. 2-six).

In improver to these components, the outer membrane possesses several major outer membrane proteins; the most abundant is called porin. The assembled subunits of porin course a channel that limits the passage of hydrophilic molecules across the outer membrane barrier to those having molecular weights that are usually less than 600 to 700. Bear witness too suggests that hydrophobic pathways exist across the outer membrane and are partly responsible for the differential penetration and effectiveness of certain b-lactam antibiotics (ampicillin, cephalosporins) that are active against various Gram-negative leaner. Although the outer membranes act every bit a permeability barrier or molecular sieve, they do not appear to possess energy-transducing systems to drive agile transport. Several outer membrane proteins, all the same, are involved in the specific uptake of metabolites (maltose, vitamin B12, nucleosides) and iron from the medium. Thus, outer membranes of the Gram-negative leaner provide a selective barrier to external molecules and thereby prevent the loss of metabolite-binding proteins and hydrolytic enzymes (nucleases, alkali metal phosphatase) found in the periplasmic space. The periplasmic infinite is the region betwixt the outer surface of the inner (plasma) membrane and the inner surface of the outer membrane (Figure 2-vi). Thus, Gram-negative bacteria have a cellular compartment that has no equivalent in Gram-positive organisms. In add-on to the hydrolytic enzymes, the periplasmic space holds bounden proteins (proteins that specifically bind sugars, amino acids, and inorganic ions) involved in membrane transport and chemotactic receptor activities. Moreover, plasmid-encoded b-lactamases and aminoglycoside-modifying enzymes (phosphorylation or adenylation) in the periplasmic infinite produce antibiotic resistance by degrading or modifying an antibiotic in transit to its target sites on the membrane (penicillin-binding proteins) or on the ribosomes (aminoglycosides). These periplasmic proteins can exist released by subjecting the cells to osmotic shock and after treatment with the chelating agent ethylenediaminetetraacetic acid.

Intracellular Components

Plasma (Cytoplasmic) Membranes

Bacterial plasma membranes, the functional equivalents of eukaryotic plasma membranes, are referred to variously every bit cytoplasmic, protoplast, or (in Gram-negative organisms) inner membranes. Like in overall dimensions and advent in thin sections to biomembranes from eukaryotic cells, they are composed primarily of proteins and lipids (principally phospholipids). Protein-to-lipid ratios of bacterial plasma membranes are approximately three: one, shut to those for mitochondrial membranes. Different eukaryotic prison cell membranes, the bacterial membrane (except for Mycoplasma species and certain methylotrophic bacteria) has no sterols, and bacteria lack the enzymes required for sterol biosynthesis.

Although their composition is similar to that of inner membranes of Gram-negative species, cytoplasmic membranes from Gram-positive bacteria possess a class of macromolecules not present in the Gram-negative membranes. Many Gram-positive bacterial membranes contain membrane-bound lipoteichoic acrid, and species lacking this component (such equally Micrococcus and Sarcina spp.) contain an analogous membrane-bound succinylated lipomannan. Lipoteichoic acids are structurally similar to the cell wall glycerol teichoic acids in that they take basal polyglycerol phosphodiester 1-three linked bondage (Fig. 2-9). These chains terminate with the phosphomonoester end of the polymer, which is linked covalently to either a glycolipid or a phosphatidyl glycolipid moiety. Thus, a hydrophobic tail is provided for anchoring in the membrane lipid layers (Fig. ii-6A). As in the cell wall glycerol teichoic acid, the lipoteichoic acids can have glycosidic and D-alanyl ester substituents on the C-2 position of the glycerol.

Both membrane-spring lipoteichoic acid and membrane-jump succinylated lipomannan can be detected equally antigens on the cell surface, and the glycerol-phosphate and succinylated mannan chains announced to extend through the prison cell wall structure (Fig. 2-6). This course of polymer has non yet been found in the cytoplasmic membranes of Gram-negative organisms. In both instances, the lipoteichoic acids and the lipomannans are negatively charged components and can sequester positively charged substances. They have been implicated in adhesion to host cells, but their functions remain to be elucidated.

Multiple functions are performed by the plasma membranes of both Gram-positive and Gram-negative bacteria. Plasma membranes are the site of active transport, respiratory chain components, energy-transducing systems, the H⁺-ATPase of the proton pump (see Chapter 4), and membrane stages in the biosynthesis of phospholipids, peptidoglycan, LPS, and capsular polysaccharides. In essence, the bacterial cytoplasmic membrane is a multifunction structure that combines the mitochondrial transport and biosynthetic functions that are ordinarily compartmentalized in discrete membranous organelles in eukaryotic cells. The plasma membrane is also the anchoring site for DNA and provides the prison cell with a mechanism (as nonetheless unknown) for separation of sister chromosomes.

Mesosomes

Thin sections of Gram-positive bacteria reveal the presence of vesicular or tubular-vesicular membrane structures chosen mesosomes, which are plainly formed past an invagination of the plasma membrane. These structures are much more prominent in Gram-positive than in Gram-negative organisms. At one time, the mesosomal vesicles were idea to be equivalent to bacterial mitochondria; however, many other membrane functions take too been attributed to the mesosomes. At nowadays, there is no satisfactory testify to suggest that they have a unique biochemical or physiologic function. Indeed, electron-microscopic studies have suggested that the mesosomes, as usually seen in sparse sections, may arise from membrane perturbation and fixation artifacts. No general agreement exists about this theory, however, and some prove indicates that mesosomes may exist related to events in the cell division cycle.

Other Intracellular Components

In add-on to the nucleoid and cytoplasm (cytosol), the intracellular compartment of the bacterial jail cell is densely packed with ribosomes of the 70S type (Fig. ii-ii). These ribonucleoprotein particles, which accept a bore of 18 nm, are not arranged on a membranous rough endoplasmic reticulum as they are in eukaryotic cells. Other granular inclusions randomly distributed in the cytoplasm of various species include metabolic reserve particles such as poly-β-hydroxybutyrate (PHB), polysaccharide and glycogen-similar granules, and polymetaphosphate or metachromatic granules.

Endospores are highly estrus-resistant, dehydrated resting cells formed intracellularly in members of the genera Bacillus and Clostridium. Sporulation, the process of forming endospores, is an unusual holding of certain leaner. The series of biochemical and morphologic changes that occur during sporulation represent true differentiation inside the wheel of the bacterial prison cell. The process, which ordinarily begins in the stationary stage of the vegetative cell bike, is initiated by depletion of nutrients (usually readily utilizable sources of carbon or nitrogen, or both). The cell then undergoes a highly complex, well-defined sequence of morphologic and biochemical events that ultimately lead to the formation of mature endospores. As many every bit seven distinct stages have been recognized past morphologic and biochemical studies of sporulating Bacillus species: stage 0, vegetative cells with 2 chromosomes at the end of exponential growth; stage I, formation of axial chromatin filament and excretion of exoenzymes, including proteases; stage II, forespore septum formation and segregation of nuclear cloth into 2 compartments; stage Three, spore protoplast germination and elevation of tricarboxylic acrid and glyoxylate wheel enzyme levels; stage IV, cortex formation and refractile appearance of spore; stage V, spore coat protein germination; stage VI, spore maturation, modification of cortical peptidoglycan, uptake of dipicolinic acid (a unique endospore product) and calcium, and development of resistance to oestrus and organic solvents; and stage Seven, final maturation and liberation of endospores from female parent cells (in some species).

When newly formed, endospores appear as round, highly refractile cells within the vegetative cell wall, or sporangium. Some strains produce autolysins that digest the walls and liberate free endospores. The spore protoplast, or core, contains a complete nucleus, ribosomes, and free energy generating components that are enclosed within a modified cytoplasmic membrane. The peptidoglycan spore wall surrounds the spore membrane; on germination, this wall becomes the vegetative cell wall. Surrounding the spore wall is a thick cortex that contains an unusual blazon of peptidoglycan, which is rapidly released on germination. A spore glaze of keratinlike protein encases the spore contained within a membrane (the exosporium). During maturation, the spore protoplast dehydrates and the spore becomes refractile and resistant to oestrus, radiation, pressure, desiccation, and chemicals; these backdrop correlate with the cortical peptidoglycan and the presence of big amounts of calcium dipicolinate.

Contempo testify indicated that the spores of Bacillus spharicus were revived which had been preserved in bister for more than than 25 million years. Their claims need to exist reevaluated. Figure 2-xi illustrates the principal structural features of a typical endospore (Bacillus megaterium) on initiation of the germination process. The thin section of the spore shows the ruptured, thick spore coat and the cortex surrounding the spore protoplast with the germinal prison cell wall that becomes the vegetative wall on outgrowth.

Figure 2-xi

Electron micrograph of a thin department of a Bacillus megaterium spore showing the thick spore glaze (SC), germinal groove (M) in the spore coat, outer cortex layer (OCL) and cortex (Cx) germinal jail cell wall layer (GCW), underlying spore protoplast membrane (more...)

References

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Costerton JW, Ingram JM, Cheng KJ. Structure and function of the cell envelope of gram-negative bacteria. Bacteriol Rev. 1974;38:87. [PMC free article: PMC413842] [PubMed: 4601163]
Ghuysen J-One thousand, Hakenbeck R: Bacterial jail cell wall. Elsevier, 1994 .
Gould GW, Hurst A (eds): The Bacterial Spore. Bookish Press, San Diego, 1969 .
Jawetz E, Melnick JL, Adelberg EA: Medical Microbiology. Appleton & Lange, East Norwalk, CT, 1989 .
Rogers HJ: Bacterial Cell Structure. American Gild for Microbiology, Washington, D.C., 1983 .
Wright A, Tipper DJ: The outer membrane of gram-negative bacteria. p. 427. In Sokatch JR, Ornston LN (eds): The bacteria. Vol. 7. Academic Press, San Diego, 1979 .

Bookshelf ID: NBK8477 PMID: 21413343

Source: https://www.ncbi.nlm.nih.gov/books/NBK8477/

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