Bacteria, cell walls lipide

In LAB, the MOP superfamily (TC 2. A.66 Table 4.7 Hvomp et al. 2003a) shows low percentages of MATE family MDR pumps and Mouse Virulence Factor (MVF) family cell wall lipid flippases compared to other bacteria examined (Mohammad and Valvano 2014). U-MOP family members (of unknown function) are poorly represented in all groups of prokaryotes. However, LAB exhibit increased proportions of polysaccharide exporters of the PST family, and of U-MOPl family members relative to other types of bacteria (Table 4.7). It should be noted that in all of the superfamilies mentioned, several families of unknown specificity are present. Some of these are represented in LAB while others are not. This fact may be understood when the functions of these transporters are revealed. The underrepresentation of both DMT and MOP superfamilies relative to other firmicutes is also worthy of note. 

A lipopolysaccharide (LPS) is any compound consisting of covalently linked lipids and polysaccharides. The term is used more frequently to denote a cell wall component from Gram-negative bacteria. LPS has endotoxin activities and is a polyclonal stimulator of B-lymphocytes. 

Gram-negative cell walls are generally richer in lipides than Grampositive bacteria. 

Smithies, Gibbons, and Bayley reported a relatively high nitrogen content in the walls of several halophilic bacteria which indicated that the cell material was predominantly protein. They contained only small amounts of lipides. The cell walls were lipoprotein. 

In Gram-negative bacteria the cell wall is only about 3 nm thick, and located in the extended periplasmatic space between the inner membrane (IM) and an additional outer membrane (OM). The lipid monolayer in the outer leaflet of the OM contains about 90% lipopolysaccharides (LPS). LPS consist of Lipid A and an oligosaccharide component, which is highly specific for individual bacterial species and phenotypes [108, 114]. 

In Gram-negative bacteria which are characterised by a rather complex cell envelope, the CM is also referred to as inner membrane to distinguish it from a second lipid bilayer, termed outer membrane (OM). The space between these two layers is called the periplasm (PP). In the periplasmic space, many proteins are found with a variety of functions. Some are involved in biosynthesis and/or export of cell wall components and surface structures (e.g. pili, flagellae,... 

Recent results indicate that not only topogenic signals and membrane composition contribute to the proper topology of a membrane protein. The antimicrobial peptide nisin, produced by Lactococcus lactis, kills Gram-positive bacteria via pore formation, thus leading to the permeabilisation of the membrane. Nisin depends on the cell-wall precursor Lipid II, which functions as a docking molecule to support a perpendicular stable transmembrane orientation [43]. 

An inventory of known biomacromolecules is provided in Table 22.3. Many of these play essential metabolic roles in enabling growth and reproduction, such as the carbohydrates, lipids, proteins, and polynucleotides. Others are components of cell walls and exoskeletons. Some organisms, such as bacteria, plankton, plants, and lower invertebrates, synthesize biomolecules, called secondary metabolites, that are used to control ecological relationships, including predator/prey, host/symbiont, mating/spawning, and competition for food or space. 

Endotoxin. Endotoxin is the lipopolysaccharide that comprises a major portion of the cell wall of the gram-negative bacteria. The endotoxins from each species of bacteria are different but the lipid A moiety is similar for the Enterobacterlaceae, and has a similar series of biological actions regardless of its source. The lipid A material is different in some of the gram-negative rods present in cotton (i.e. in the Pseudomonas species). This variety of compounds makes quantitation of endotoxin difficult. Hence, it is usually measured by its biological activity as compared to a standard endotoxin (usually that of E. coll). (See Table VIII.)... 

The outer surfaces of bacteria are rich in specialized polysaccharides. These are often synthesized while attached to lipid membrane anchors as indicated in a general way in Eq. 20-20.136/296a One of the specific biosynthetic cycles (Fig. 20-9) that depends upon undeca-prenol phosphate is the formation of the peptidoglycan (murein) layer (Fig. 8-29) of both gram-negative and gram-positive bacterial cell walls. Synthesis begins with attachment of L-alanine to the OH of the lactyl... 

Lipid bilayers are flexible and they cannot resist significant mechanical stress. Cells which have to survive external forces such as bacteria or cells which have to provide mechanical stability have an outer cell wall in addition to the lipid membrane. 

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