Gram -negative bacteria outer membrane

Gram-negative bacteria Gram-positive bacteria Outer membrane No outer membrane peptidoglycan layer thicker peptidoglycan layer ... 

Gram-negative organisms produce ced-bound P-lactamases which reside in the periplasmic space. Thus, for gram-negative bacteria, the antibiotic must penetrate the outer ced membrane/wad before coming in contact with a P-lactamase (80,139,140). 

Gram-negative bacteria are surrounded by two membranes, an inner plasma membrane and an outer membrane. These are separated by a periplasmic space. Most plasma membrane proteins contain long, continuous sequences of about 20 hydrophobic residues that are typical of transmembrane a helices such as those found in bacteriorhodopsin. In contrast, most outer membrane proteins do not show such sequence patterns. 

As shown in Figure 9.24, the outer membrane of Gram-negative bacteria is coated with a highly complex lipopolysaccharide, which consists of a lipid group (anchored in the outer membrane) joined to a polysaccharide made up of long chains with many different and characteristic repeating structures... 

Currently, five different molecular classes of mdr efflux pumps are known [5], While pumps of the the ATP-binding cassette (ABC) transporter superfamily are driven by ATP hydrolysis, the other four superfamilies called resistance-nodulation-division (RND), major facilitator superfamily (MFS), multidrug and toxic compound extrusion (MATE), and small multidrag resistance transporter (SMR) are driven by the proton-motive force across the cytoplasmic membrane. Usually a single pump protein is located within the cytoplasmic membrane. However, the RND-type pumps which are restricted to Gram-negative bacteria consist of two additional components, a periplasmic membrane fusion protein (MFP) which connects the efflux pump to an outer... 

Endotoxins are the lipopolysaccharides (LPS) of the outer membrane of Gram-negative bacteria. They trigger inflammatory reactions in the infected organism, activate complement and cause fever or even a septic shock. They act on toll-like receptors. 

In Gram-negative bacteria, diffusion of (3-lactam antibiotics into the periplasm (where the activity of PBPs takes place) occurs via the channels that porins create in the outer membrane. The number and properties of the porin molecules are such that diffusion is relatively rapid in E. coli but much slower in Enterobacter and Pseudomonas. Mutants can be selected after the permeability of porin channels or their number has been decreased. A slow diffusion into... 

The outer membrane of gram-negative bacteria is a permeability barrier that allows the passive diffusion of small hydrophilic antibiotics only through aqueous channels, the porins. Drugs larger than 800 Da are... 

Gram-negative bacteria are intrinsically resistant to low levels of fusidic acid (a steroid) due to exclusion by the outer membrane. Nevertheless, acquired resistance does occur which has the effect of increasing the level of resistance to the antibiotic. Acquired resistance also occurs in Gram-positive bacteria normally susceptible to fusidic acid. 

A third resistance mechanism is akin to that described for the AGAC antibiotics and chloramphenicol, whereby changes in the outer membrane porins of Gram-negative bacteria reduce the penetration of /3-lactams resulting in low levels of resistance. 

A great deal of our current understanding of the structure and function of the outer membrane of Gram-negative bacteria has come from studies with Escherichia coli and Salmonella typhimurium. The permeability barrier function of the outer membrane can... 

The surface structure of gram-negative bacteria (these are not stained by Gram s method and must be stained red with carbol fuchsin) is more diversified. It consists of an outer membrane whose main building unit is a lipopolysaccharide together with phospholipids and proteins. The actual cell... 

The high space-time yields are the result of a doubling time of only 30 min and its applicability for high cell-density cultures. However, it is hardly possible to excrete overexpressed proteins into cultivation media. In addition, accumulation of pyrogenic lipopoly-saccharides in its outer membrane (a distinctive feature of Gram-negative bacteria) make additional purification steps necessary if pharmaceutical proteins are produced by E. coli [29]. 

E. coli, like other Gram-negative bacteria, has an outer membrane which hampers excretion of proteins to the culture media. Thus, expressed proteins can remain in the cytoplasm or can be directed into the periplasm employing the N-terminus fusion of a signal peptide (e.g. OmpA, pelB, OmpF, PhoA, Tat signal peptides) [30]. 

The Fur protein regulates iron uptake systems in many Gram-negative bacteria. The striking phenotype of the first fur mutants isolated was the overexpression of the outer membrane receptors for siderophore iron transport. In addition, excretion of siderophores under iron-rich growth conditions was observed in these mutants, indicating that the biosynthesis of siderophores is also regulated by Fur. 

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]. 

Figure 7.7 Structure of a generalized LPS molecule. LPS constitutes the major structural component of the outer membrane of Gram-negative bacteria. Although LPSs of different Gram-negative organisms differ in their chemical structure, each consists of a complex polysaccharide component, linked to a lipid component. Refer to text for specific details...

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