15. Bacterial cell envelop. The composition and functions of Bacterial Envelope.

Bacteria cells are all around us. They cause disease and are essential for human life. Although several bacteria such as Streptococcus pneumoniae and Escherichia coli live symbiotically within human bodies, the bacteria structure and function is very different than human cells. Bacteria metabolize and produce some of the nutrients and ions that humans can't process, such as nitrogen and vitamin K. Bacteria cell structure might be different, but they have mutated over eons to survive environmental changes and reproduce. Cell Wall

Bacteria have cell walls made of a peptidoglycan layer that makes the cell rigid and gives it shape. It also serves as an anchor for the pili and flagella. It helps contain the organelles and keeps the cell from bursting under large changes from osmotic pressure. Cell walls are used by scientists to distinguish and categorize bacteria through gram-staining procedures.

Pili

Many bacteria have pili, which are tiny hair-like structures that branch out from the cell wall. These structures have two purposes. Pili are a part of a bacterium's virulence factors. They attach to host tissue and cells giving them the ability to cause infection. Additionally, special pili are used by bacteria for conjugation, a process in which one bacterium sends genes such as antibiotic resistance to neighboring bacteria cells.

Flagella

Flagella are also hair-like structures similar to pili, but flagella are used for locomotion by the bacteria cell. They are on one end of the bacteria, at both ends, or sometimes they surround the entire cell. Flagella help the bacteria move away from toxic chemicals or move toward nutrients. They work in a propeller-type movement, spinning around to move the bacteria to a location.

16.Cell Wall of Gram positive bacteria.

Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain (safranin) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of peptidoglycan in the cell wall. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.The following characteristics are generally present in a Gram-positive bacterium:

1)Cytoplasmic lipid membrane

2)Thick peptidoglycan layer

3)Teichoic acids and lipoteichoic acids are present, which serve to act as chelating agents, and also for certain types of adherence.

4)Capsule polysaccharides (only in some species)

5)Flagellum (only in some species)

6)If present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.

Classification

In the original bacterial phyla, the Gram-positive organisms made up the phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as Bacillus, Listeria,Staphylococcus, Streptococcus, Enterococcus, and Clostridium. It has also been expanded to include the Mollicutes, bacteria like Mycoplasma that lack cell walls and cannot be Gram stained, but are derived from such forms. Actinobacteria are the other major group of Gram-positive bacteria, which have a high guanosine and cytosine content in their genomes (high G+C group). This contrasts with the Firmicutes, which have a low G+C content. Both Gram-positive and Gram-negative bacteria may have a membrane called an S-layer. In Gram-negative bacteria, the S-layer is directly attached to the outer membrane. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of teichoic acids in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane. Gram-positive bacteria appear bluish-purple in the Gram stain, while Gram-Negative Bacteria are red. The crystal violet-iodine complex formed during Gram-staining is retained by the cell wall of Gram-positive bacteria. Gram-positive eubacteria (bacteria) are found in several taxonomic groups. Typical representatives are both endospore-forming and nonsporing rods of the genera Bacillus, Clostridium, Arthrobacter, Lactobacillus, Mycobacterium, Nocardia and Streptomyces. A few archaebacteria (Archaea), such as Methanobacterium formicicum, also stain Gram-positive. The Gram reaction depends on both the structural formats and the chemical composition of the cell envelope (see Gram-Negative Bacteria). Gram-positive bacteria have a broader range of cell-wall types than do Gram-negative bacteria .1. Type I Gram-positive cell wall This type of cell wall consists of a thick peptidoglycan layer mixed with teichoic acids. Peptidoglycan (murein) is a polymer of glycan strands of N-acetyl-D-glucosamine and N-acetylmuramic acid linked in alternating sequence by b-1,4 linkages. The glycan strands are cross-linked by peptide chains of varying complexity, which are bound in amide links to the carboxyl group of the lactic acid residue of muramic acid. In Gram-positive bacteria, the amino acid composition of the peptides cross-linking the glycan strands vary considerably. In these interpeptide bridges, D-amino acids dominate, while aromatic, branched-chain, and sulfur-containing amino acids, plus histidine, arginine, and proline, are absent. In the tetrapeptide of Gram-positive bacteria, diaminopimelic acid can be replaced by lysine. Teichoic acids are polymers consisting of glycerol phosphate or ribitol phosphate that are substituted with sugars and/or amino acids. They are covalently bound to the peptidoglycan and are exposed on the cell surface. Several Gram-positive bacteria produce, in addition to teichoic acids, lipoteichoic acids, which are not covalently bound to peptidoglycan but are associated with the cytoplasmic membrane through hydrophobic interactions. They consist of poly C1-3-glycerol phosphate linked to diglucosyl-1-3-diacylglycerol. Lipoteichoic acid penetrates the peptidoglycan and reaches the cell surface. Teichuronic acids of Gram-positive bacteria contain hexuronic acids (glucoronic acid or N-acetylmannosamine uronic acid) instead of polyol phosphate. Teichoic, teichuronic, and lipoteichoic acids are highly-charged polymers. Lipoproteins are present in both Gram-negative and Gram-positive cell walls. The terminal cysteine residue of the protein moiety is bound to fatty acids directly or via glycerol residues. Proteins are also constituents of the Gram-positive cell wall, eg porins and surface layer-forming proteins. Activated precursors of the cell wall polymers are transferred across the cytoplasmic membrane (CM) bound to undecaprenol phosphate (undec-P), which was synthesized from mevalonic acid. The undec-P is recycled via undec-P-P and undec. 2. Type II Gram-positive cell wall This type of cell wall is represented by the Corynebacterium-Mycobacterium-Nocardia group of bacteria. These bacteria have a thick peptidoglycan layer that is, however, penetrated by glycolipids. The muramic acid may be N-glycosylated. Long chains of lipoarabinomannan are anchored in the cytoplasmic membrane and penetrate the peptidoglycan layer. The external layer of the cell wall consists of lipooligosaccharides, phenolic glycolipids, glycopeptidolipids, and mycolic acids esterified to arabinogalactan, which is attached to the peptidoglycan. Mycolic acids are long-chain (40-90 C-atoms) alcohols and fatty acids partially unsaturated and substituted with keto-, methoxy-, epoxy- and OH-groups .The type II of Gram-positive cell wall stains Ziehl-Neelsen positive (acid-alcohol fastness). A mixture of the dye basic fuchsin and phenol is heated on the microscopic slides and penetrates the lipid layer. The slides are subsequently washed in distilled water and decolorized with acid-alcohol. By this procedure the fuchsin is removed from other bacteria and cells but is retained by the acid-fast bacteria. When the cells of these bacteria are treated with alkaline ethanol the lipid content of the cell wall is reduced and the cells became non-acid-alcohol-fast but remained Gram-positive. This result shows that the Gram-positive staining depends on a thick peptidoglycan layer. 3. Type III Gram-Positive Cell Wall This type of cell wall is present in few Archaea, eg Methanobacterium formicicum, containing pseudomurein . The glycan strands of pseudomurein are composed of alternating b-N-acetyl-L-talosamine-uronic acid b-1 ^ 3 N-acetyl-D-glucosamine and the tetrapeptide contains L-amino acids. Exceptions.The Deinococcus-Thermus bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria. Pathogenesis. Most pathogenic bacteria in humans are gram-negative organisms. Classically, six gram-positive organisms are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, arecocci (round bacteria). The remaining organisms are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are Corynebacterium andListeria, while Bacillus and Clostridium produce spores^.The spore-forming bacteria can again be divided based on their respiration: Bacillus is a facultative anaerobe, while Clostridium is an obligate anaerobe.