Cell Wall Of Gram Positive And Gram Negative Bacteria Pdf Reader

Cell wall of gram positive and gram negative bacteria pdf reader

2.3A: The Gram-Positive Cell Wall

NCBI Bookshelf. Baron S, editor. Medical Microbiology. Milton R.

Cell wall of gram positive and gram negative bacteria pdf reader

Salton and Kwang-Shin Kim. Bacteria have characteristic shapes cocci, rods, spirals, etc. These traits are usually typical for a genus and are diagnostically useful. Prokaryotes have a nucleoid nuclear body rather than an enveloped nucleus and lack membrane-bound cytoplasmic organelles.

The plasma membrane in prokaryotes performs many of the functions carried out by membranous organelles in eukaryotes. Multiplication is by binary fission. Flagella: The flagella of motile bacteria differ in structure from eukaryotic flagella.

Gram Staining

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 axis. 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 particularly Gram-negative bacteria. They are important in adhesion to host surfaces.

Capsules: Some bacteria form a thick outer capsule of high-molecular-weight, viscous polysaccharide gel; others have more amorphous slime layers. Capsules confer resistance to phagocytosis.

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Cell Wall Peptidoglycans: Both Gram-positive and Gram-negative bacteria possess cell wall peptidoglycans, which confer the characteristic cell shape and provide the 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 e. The cross-linking transpeptidase enzymes are some of the targets for b-lactam antibiotics.

Teichoic Acids: Teichoic acids are polyol phosphate polymers bearing a strong negative charge. They are covalently linked to the peptidoglycan in some Gram-positive bacteria. 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. Lipopolysaccharides: One 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: Two groups of bacteria devoid of cell wall peptidoglycans are the Mycoplasma species, which possess a surface membrane structure, and the L-forms that arise from either Gram-positive or Gram-negative bacterial cells that have lost their ability to produce the peptidoglycan structures.

Cell wall of gram positive and gram negative bacteria pdf reader

Plasma Membrane: The bacterial plasma membrane is composed primarily of protein and phospholipid about It performs many functions, including transport, biosynthesis, and energy transduction. Organelles: The bacterial cytoplasm is densely packed with 70S ribosomes.

Types of Staining

Other granules represent metabolic reserves e. Endospores: Bacillus and Clostridium species can produce endospores: heat-resistant, dehydrated resting cells that are formed intracellularly and contain a genome and all essential metabolic machinery.

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The endospore is encased in a complex protective spore coat. All bacteria, both pathogenic and saprophytic, are unicellular organisms that reproduce by 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 most conveniently measured in microns 10 -6 m. They range in size from large cells such as Bacillus anthracis 1.


Bacteria therefore have a surface-to-volume ratio that is very high: about , Bacteria have characteristic shapes. The common microscopic morphologies are cocci round 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 as Vibrio cholerae and Treponema pallidum , respectively.

The arrangement of cells is also typical of various species or groups of bacteria Fig. Some rods or cocci characteristically grow in chains; some, such as Staphylococcus aureus , form grapelike clusters of spherical cells; some round cocci form cubic packets.

Bacterial cells of other species grow separately. The microscopic appearance is therefore valuable in classification and diagnosis.

The principle of Gram staining

The higher resolving power of the electron microscope not only magnifies the typical shape of a bacterial cell but also clearly resolves its prokaryotic organization Fig.

Electron micrograph of a thin section of Neisseria gonorrhoeae showing the organizational features of prokaryotic cells. Note the electron-transparent nuclear region n packed with DNA fibrils, the dense distribution of ribosomal particles in the cytoplasm, more 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 apparatus and surrounding nuclear membrane.

As the electron micrograph in Fig. Under the light microscope, the nucleoid of the bacterial cell can be visualized with the aid of Feulgen staining, which stains DNA. Gentle lysis can be used to isolate the nucleoid of most bacterial cells. The DNA is then seen to be a single, continuous, "giant" circular molecule with a molecular weight of approximately 3 X 10 9 see Ch.

The bacterial nucleoid, then, is a structure containing a single chromosome. The number of copies of this chromosome in a 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 require that the chromosome be permanently attached 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 function similar to that of eukaryotic histones.

Two types of surface appendage can be recognized on certain bacterial species: the flagella, which are organs of locomotion, and pili Latin hairs , which are also known as fimbriae Latin fringes. Flagella occur on both Gram-positive and Gram-negative bacteria, and their presence can be useful in identification. For example, they are found on many species of bacilli but rarely on cocci.

In contrast, pili occur almost exclusively on Gram-negative bacteria and are found on only a few Gram-positive organisms e. Some bacteria have both flagella and pili.

Cell wall of gram positive and gram negative bacteria pdf reader

The electron micrograph in Fig. A Electron micrograph of negatively stained E. B The long sex pilus can be distinguished from the shorter common pili by mixing E. The protein subunits of a flagellum are assembled to form a cylindrical structure with a hollow core.

Cell wall of gram positive and gram negative bacteria pdf reader

A flagellum consists of three parts: 1 the long filament, which lies external to the cell surface; 2 the hook structure at the end of the filament; and 3 the basal body, to which the hook is anchored and which imparts motion to the flagellum.

The basal body traverses the outer wall and membrane structures. It consists of a rod and one or two pairs of discs. The thrust that propels the bacterial cell is provided by counterclockwise rotation of the basal body, which causes the helically twisted filament to whirl.

Functions of the Gram-Negative Cell Wall Components

The movement of the basal body is driven by a proton motive force rather than by ATP directly. The ability of bacteria to swim by means of the propeller-like action of the flagella provides them with the mechanical means to perform chemotaxis movement in response to attractant and repellent substances in the environment. Genetic studies have revealed the existence of mutants with altered biochemical pathways for flagellar motility and chemotaxis.

Chemically, flagella are constructed of a class of proteins called flagellins.

Structure and Composition of the Gram-Negative Cell Wall

The hook and basal-body structures consist of numerous proteins. Mutations affecting any of these gene products may result in loss or impairment of motility.

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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 primary structures of the proteins. Antigenic changes of the flagella known as the phase variation of H1 and H2 occurs in Salmonella typhimurium see Ch. Seifert and So. 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 illustrates typical arrangements of flagella on or around the bacterial surface. For example, V. The flagella of a peritrichous bacterium must aggregate as a posterior bundle to propel the cell in a forward direction. Flagella can be sheared from the cell surface without affecting the viability of the cell.

Cell wall of gram positive and gram negative bacteria pdf reader

The cell then becomes temporarily nonmotile. In time it synthesizes new flagella and regains motility. The protein synthesis inhibitor chloramphenicol, however, blocks regeneration of flagella.

Talk:Gram-positive bacteria

The terms pili and fimbriae are usually 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 appearance than flagella Fig.

Cell wall of gram positive and gram negative bacteria pdf reader

In some organisms, such as Shigella species and E. As is easily recognized in strains of E. Sex pili can be distinguished by their ability to bind male-specific bacteriophages the sex pilus acts as a specific receptor for these bacteriophages Fig.

The sex pili attach male to female bacteria during conjugation. Pili in many enteric bacteria confer adhesive 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 E. Organisms possessing this type of hemagglutination are called mannose-sensitive organisms.