Gram-negative bacteria porins

Source Adapted, from Benz, R., 1984. Sti ucture and selecdvity of porin channels. Current Topics in Membrane TrawsjiJorf 21 199-219 and Benz, R., 1988. Sti ucture and function of porins from Gram-negative bacteria. Annual Review of Microbiology 42 359-393. 

Benz, R., 1980. Structure and function of porins from Gram-negative bacteria. Annual Review of Alicrobiology 42 359—393. 

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... 

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. 

The lipid membranes are a few nanometers thick. They contain proteins whose role is to actively transport particular target chemicals across these nonpolar barriers. The outer membranes of gram negative bacteria also have protein channels called porins that allow passage of small polar and charged substrates. 

Porins. The outer membranes of gram-negative bacteria contain several 34- to 38-kDa proteins known... 

The outer membranes of mitochondria can be removed from the inner membranes by osmotic rupture.13 Analyses on separated membrane fractions show that the outer membrane is less dense (density — 1.1 g / cm3) than the inner (density 1.2 g / cm3). It is highly permeable to most substances of molecular mass 10 kDa or less because of the presence of pores of 2 nm diameter. These are formed by mitochondrial porins,14-17 which are similar to the outer membrane porins of gram-negative bacteria (Fig. 8-20). The ratio of phospholipid to protein ( 0.82 on a weight basis) is much higher than in the inner membrane. Extraction of the phospholipids by acetone destroys the membrane. Of the lipids present, there is a low content of cardiolipin, a high content of phosphatidylinositol and cholesterol, and no ubiquinone. 

Some membranes contain relatively large pores which allow for the free passage of molecules with molecular weights up to about 600. For example the outer membranes of gram negative bacteria contain pores with diameters of about 10 A which are formed from proteins called porins. 

While gramicidin and other channel formers can show high transport rates, they do not show the high selectivity that characterizes natural channels. There is much interest at present in a class of proteins called porins, which form natural pores in the outer membranes of Gram-negative bacteria. Several different porin proteins have been isolated from Escherichia coli. These form water-filled channels of various sizes in membranes. Thus the proteins OmpC and OmpF seem to be cation-specific channels while other proteins give larger diameter channels that seem to be specific for anions.34,35... 

The outer membrane of Gram-negative bacteria is spanned by porins, trimeric proteins that form inlet channels between the outer membrane and the periplasmic space. The geometry of the porins defines the substrate uptake limit, which is on the order of 600 daltons (Weiss et al., 1991), approximately equivalent to a trisaccharide. Substrates larger than this limit (with few exceptions) must be hydrolyzed outside the outer membrane prior to uptake. Extracellular hydrolysis is carried out by means of extracellular en-... 

Some membranes contain pores. Examples of membranes that contain pores and allow quite hydrophilic compounds, generally of Mr less than 600, to pass freely include the outer membranes of mitochondria and those of Gram-negative bacteria. The pores of the membranes are formed by proteins called porins. 

Many metabolites and compounds such as antibiotics penetrate cells via transport proteins and, as a result, mutation in these proteins or decrease in their expression can decrease antibiotic uptake. In Gram-negative bacteria, the outer membrane porins are required for entry of many antibiotics such as fS-lactams and aminoglycosides, and antibiotic resistance can occur through modulation of these proteins. For example, imipenem resistance... 

A. Porins are found in outer membranes of Gram negative bacteriA. General porins sort molecules actpr ing to molecular size. 

Fluoroquinolones must penetrate bacteria to reach their target, DNA gyrase. The second mechanism of fluoroquinolone resistance is decreased cell wall permeability. The fluoroquinolones diffuse through porin channels in the outer membrane of Gram-negative bacteria. Mutation results in a decrease in porin channel proteins, resulting in decreased uptake of the fluoroquinolones into bacterial cells. Alterations in a wide range of outer membrane proteins in Pseudomonas spp. result in resistance. From these mutations, the increase in MIC of the fluoroquinolones is relatively low (2-to 32-fold). Flowever, there is cross-resistance with unrelated antibiotics, most frequently cefoxitin, chloramphenicol, trimethoprim and tetracycline. 

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