Structure and Function 1&2

structure of gram negative cell walls

(1) They are more complex than Gram(+) cell walls (2) The peptidoglycan layer is thin and sparsely cross-linked; thus they are more susceptible to mechanical breakage than are Gram(+) cell walls. (3) The outer membrane is a very important component of the Gram(-) cell envelope. (a) It is made up of an asymmetric lipid bilayer of phospholipid and LPS. (b) Proteins and lipoproteins are embedded in the lipid bilayer. (c) Lipopolysaccharide (LPS) is the hallmark molecule of the outer membrane. It makes up most of the outer leaflet of the outer membrane lipid bilayer. This is the classical endotoxin. It consists of three parts.

describe the basic structure and synthesis of teichoic acid

(a) Teichoic acids may be composed mainly of ribitol or glycerol connected by phosphate bonds. (b) They are polymers which form long filaments. (c) There are two forms of teichoic acids, wall teichoic acids and lipoteichoic acids. (d) The wall teichoic acids are covalently attached to the peptidoglycan layer. (e) The lipoteichoic acids are linked to the cytoplasmic membrane and span the peptidoglycan layer. Functions of teichoic acids: (i) They have a net (-) charge. They bind cations and regulate their movement in/out of the cell. There is overlapping function between wall teichoic acids and lipoteichoic acids (ii) Lipoteichoic acids mediate adherence to mucosal cells, and wall teichoic acids also have a role in adherence and biofilm formation. (iii) Teichoic acids provide much of the Gram (+) cell wall's antigenic specificity. (iv) Lipoteichoic acid possesses an endotoxin-like activity which activates toll-like receptors and results in the release of cytokines.

describe the basic structure and synthesis of lipopolysaccharide (LPS)

- the hallmark molecule of the GRAM (-) outer membrane. It makes up most of the outer leaflet of the outer membrane lipid bilayer. This is the classical endotoxin. It consists of three parts. (i) Lipid A -is embedded in the membrane -TOXIC - a major virulence factor The Core oligosaccharide: connects the O antigen to the lipid A. O antigen: - repeating polysaccharide which extends into the environment -The tri-saccharide subunits of the O antigen polysaccharide vary between types of bacteria -very important antigenic determinant of Gram (-) bacteria. -If O antigen is missing = lipo-oligosaccharide or LOS. --Lipid A Core Oligosaccharide O Antigen Polysaccharide (1) The lipid A + R core oligo portion (same as LOS): -synthesized on the inner leaflet of the cytoplasmic membrane (inner membrane), -THEN FLIPPED to the outer leaflet of the inner membrane. Undecaprenol is not involved in this process. The repeating O-antigen chain of LPS is assembled on undecaprenol phosphate on the INNER SURFACE OF THE INNER MEMBRANE and is then flipped across the inner membrane to its outer surface in the same way as occurs during peptidoglycan biosynthesis. This long O-antigen chain is then transferred onto the waiting LOS, thus forming full-length LPS. The completed LPS is then transported across the PERIPLASMIC space on a series of carrier proteins to the outer membrane, and is finally inserted into the outer leaflet of the OUTER MEMBRANE by a β-barrel outer membrane protein complex called LptD. In strains where O-antigen is not produced, the LOS alone follows that same pathway across the periplasmic space to its ultimate destination in the outer leaflet of the outer membrane. In Gram(-) bacteria such as Escherichia coli, the inner and outer membranes appear to be fused to each other at several hundred sites. These sites may require small gaps in the peptidoglycan layer, and may be involved in the transport of lipopolysaccharide from the inner to the outer membrane.. PBP = Penicillin binding protein = transpeptidase that creates the crosslinks. When bound with penicillin, it cannot create the crosslinks and the cell will lyse.

describe the basic structure and synthesis of peptidoglycan

-The polysaccharide glycan chains of peptidoglycan are made up of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). -There are short peptide chains attached to the NAM residues. -Adjacent glycan chains are held together by CROSS-LINKS between peptides. - Some of the amino acids in the peptides are unique to bacterial cell walls. (a) D-amino acids (b) D, L-diaminopimelic acid (DAP). This molecule participates in the cross-link between two peptide side chains via its free amino group.

structure of gram positive cell walls

-relatively THICK -consist mainly of multiple layers of peptidoglycan, which is densely cross-linked. - may have added peptide bridges. - commonly contain teichoic acids. - Some Gram (+) bacteria have polysaccharides (Streptococcus) - some cases proteins in the outer part of their cell walls. These may also contribute to the antigenic specificity of the cell. Members of the genus Mycobacterium have waxy lipids in their cell walls which makes them more resistant to environmental insults

size of bacteria

Most bacterial pathogens are in the range of 0.5-1.0 μm in width and up to several μm in length. Most are resolvable by the light microscope, but some are better visualized by dark field.

shapes

coccus = spherical rod or bacillus = cylindrical spiral/spirochete = with 1 to many turns pleomorphic = variable in size and shape.

Eukarya vs. prokarya

see chart