Gram-positive bacteria, those species with peptidoglycan outer layers, are easier to kill - their thick peptidoglycan layer absorbs antibiotics and. One is the Gram-negative bacteria that have an outer membrane rich in In addition, the cell wall of Gram positive bacteria contains teichoic acids. . There is an excellent recent discussion of the diverse relationships that can occur as a. The cell wall of Gram-positive and the outer membrane in Gram-negative .. antimicrobial resistance is also related to increased virulence of bacteria .
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. A flagellum consists of three parts: 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.
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. 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. 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.
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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. The flagella of a peritrichous bacterium must aggregate as a posterior bundle to propel the cell in a forward direction.
Figure Typical arrangements of bacterial flagella. Flagella can be sheared from the cell surface without affecting the viability of the cell. 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. Pili 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. 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 hemagglutinationand 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. Other piliated organisms, such as gonococci, are adhesive and hemagglutinating, but are insensitive to the inhibitory effects of mannose. Extensive antigenic variations in pilins of gonococci are well known see Ref. Surface Layers The surface layers of the bacterial cell have been identified by various techniques: The principal surface layers are capsules and loose slime, the cell wall of Gram-positive bacteria and the complex 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 cell and is responsible for the characteristic shape of the 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 cell wall and envelope layers to the plasma membrane are indicated in the thin section of a Gram-positive organism Micrococcus lysodeikticus in Figure A and in the freeze-fractured cell of a Gram-negative organism Bacteroides melaninogenicus in Figure B. 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 extensive fracture planes through the bilayer region of the underlying plasma membrane.
A Electron micrograph of a thin section of the Gram-positive M. B Freeze-fractured Bacteriodes cell showing more Capsules and Loose Slime Some bacteria form capsules, which constitute the outermost layer of the bacterial cell and surround it with a relatively thick layer of viscous gel. Some organisms lack a well-defined capsule but have loose, amorphous slime layers external to the cell wall or cell envelope. Not all bacterial species produce capsules; however, the capsules of encapsulated pathogens are often important determinants of virulence.
Encapsulated species are found among both Gram-positive and Gram-negative bacteria. In both groups, most capsules are composed of highmolecular-weight viscous polysaccharides that are retained as a thick gel outside the cell wall or envelope. Table 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 assembly 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 smooth-to-rough variation seen in the pneumococci.
The capsule is not essential for viability. Viability is not affected when capsular polysaccharides are removed enzymatically from the cell surface. The exact functions of capsules are not fully understood, but they do confer resistance to phagocytosis and hence provide the bacterial cell with protection against host defenses to invasion.
Gram-positive and Gram-negative organisms differ drastically in the organization of the structures outside the plasma membrane but below the capsule Fig.
Figure Comparison of the thick cell wall of Gram-positive bacteria with the comparatively thin cell wall of Gram-negative bacteria. Note the complexity of the Gram-negative cell envelope outer membrane, its hydrophobic lipoprotein anchor; periplasmic space.
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Most Gram-positive bacteria have a relatively thick about 20 to 80 nmcontinuous cell wall often called the sacculuswhich is composed largely of peptidoglycan also known as mucopeptide or murein.
In thick cell walls, other 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 thin about 5 to 10 nm thick ; in E.
Outside the peptidoglycan layer in the Gram-negative envelope is an outer membrane structure about 7. In most Gram-negative bacteria, this membrane structure is anchored noncovalently to lipoprotein molecules Braun's lipoproteinwhich, in turn, are covalently linked to the peptidoglycan. The lipopolysaccharides of the Gram-negative 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 cell envelope are similar to those of the plasma membrane about 7. Moreover, in Gram-negative bacteria such as E. Table summarizes the major classes of chemical constituents in the walls and envelopes of Gram-positive and Gram-negative bacteria.
The basic differences in surface structures of Gram-positive and Gram-negative bacteria explain the results of Gram staining. Both Gram-positive and Gram-negative bacteria take up the same amounts of crystal violet CV and iodine I. The CV-I complex, however, 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 contrast, the thin peptidoglycan layer and probable discontinuities at the membrane adhesion sites do not impede solvent extraction of the CV-I complex from the Gram-negative cell.
The above mechanism of the Gram stain based on the structural differences between the two groups has been confirmed by sophisticated methods of electron microscopy see Ref. The sequence of steps in the Gram stain differentiation is illustrated diagrammatically in Figure Moreover, mechanical disruption of the cell wall of Gram-positive organisms or its enzymatic removal with lysozyme results in complete 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. General sequence of steps in the Gram stain procedure and the resultant staining of Gram-positive and Gram-negative bacteria.
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 by specific antibiotics. The extent of direct or peptide-bridge cross-linking varies from one peptidoglycan to another.
The staphylococcal peptidoglycan is highly cross-linked, whereas that of E. The structure of the peptidoglycan is illustrated in Figure A peptidoglycan with a chemical structure substantially different from that of all eubacteria has been discovered in certain archaebacteria.
Instead of muramic acid, 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.
Widely distributed in nature, this enzyme is present in human tissues and secretions and can cause complete digestion of the peptidoglycan walls of sensitive organisms.
When lysozyme is allowed to digest the cell wall of Gram-positive bacteria suspended in an osmotic stabilizer such as sucroseprotoplasts are formed.
These protoplasts are able to survive and continue to grow on suitable media in the wall-less state. Gram-negative bacteria 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 bacteria 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. There are two groups of bacteria that lack the protective cell wall peptidoglycan structure, the Mycoplasma species, one 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 L-forms are all Gram-negative and insensitive to penicillin and are bounded by a surface membrane structure.Antibiotics for Gram Negative Infections (Antibiotics - Lecture 5)
L-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 under special conditions. Teichoic Acids Wall teichoic acids are found only in certain Gram-positive bacteria such as staphylococci, streptococci, lactobacilli, and Bacillus spp. Teichoic acids are polyol phosphate polymers, with either ribitol or glycerol linked by phosphodiester bonds; their structures are illustrated in Figure Substituent groups on the polyol chains can include D-alanine ester linkedN-acetylglucosamine, N-acetylgalactosamine, and glucose; the substituent is characteristic for the teichoic acid from a particular bacterial species and can act as 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 mechanism. 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 4.
The glycosyl groups may abe N-acetylglucosaminyl more Accessory Wall Polymers In addition to the principal cell wall polymers, the walls of certain Gram-positive bacteria possess polysaccharide molecules linked to the peptidoglycan.
Sulfonamides Prontosil, a sulfonamide, was the first commercially available antibiotic, developed in In the present day, sulfonamides are rarely used, partially due to the development of bacterial resistance, but also due to concern about unwanted effects such as damage to the liver of patients.
Aminoglycosides Aminoglycosides inhibit the synthesis of proteins in bacteria, eventually leading to cell death. In the treatment of tuberculosis, streptomycin was the first drug found to be effective; however, due to issues with toxicity of aminoglycosides, their present day use is limited. Tetracyclines Tetracyclines are broad-spectrum antibiotics, active against both Gram-positive and Gram-negative bacteria.
Their use is decreasing to increasing instances of bacterial resistance; however, they still find use in treatment of acne, urinary tract, and respiratory tract infections, as well as chlamydia infections.
Chloramphenicol Another broad-spectrum antibiotic, chloramphenicol also acts by inhibiting protein synthesis, and thus growth and reproduction of bacteria. Due to the possibility of serious toxic effects, in developed countries it is generally only used in cases where infections are deemed to be life-threatening, although it is a much more common antibiotic in developing countries due to its low cost and availability.
Macrolides Macrolides' effectiveness is marginally broader than that of penicillins, and they have been shown to be effective against several species of bacteria that penicillins are not. Whilst some bacterial species have developed resistance to macrolides, they are still the second most commonly prescribed antibiotics in the NHS, with erythromycin being the most commonly prescribed in the class.
There are strict guidelines on the circumstances in which vancomycin can be used to treat infections, in order to delay the development of resistance. The bacteria against which glycopeptides are active are otherwise somewhat limited, and in most they inhibit growth and reproduction rather than killing bacteria directly. Oxazolidinones Oxazolidinones are active against Gram-positive bacteria, and act by inhibiting protein synthesis, and hence growth and reproduction.
Linezolid, approved for use inwas the first marketed antibiotic in the class, and resistance seems to be developing relatively slowly since its introduction.
Ansamycins This class of antibiotics are effective against Gram-positive bacteria, as well as some Gram-negative bacteria. A subclass of antibiotics, rifamycins, are used to treat tuberculosis and leprosy. Uncommonly, ansamycins can also demonstrate anti-viral activity.