Membranes, bacterial, permeability

The synthetic and plasmid DNAs are mixed and join their sticky ends spontaneously. They are covalently bound together by DNA ligases, when the resulting hybrid plasmid is inserted into bacterial cells. Dilute calcium chloride solutions render the bacterial membranes permeable and allow the passage of ONA into the cells. 

Electroporation. When bacteria are exposed to an electric field a number of physical and biochemical changes occur. The bacterial membrane becomes polarized at low electric field. When the membrane potential reaches a critical value of 200—300 mV, areas of reversible local disorganization and transient breakdown occur resulting in a permeable membrane. This results in both molecular influx and efflux. The nature of the membrane disturbance is not clearly understood but bacteria, yeast, and fungi are capable of DNA uptake (see Yeasts). This method, called electroporation, has been used to transform a variety of bacterial and yeast strains that are recalcitrant to other methods (2). Apparatus for electroporation is commercially available, and constant improvements in the design are being made. 

The strategy for development of /3-lactamase-resistant /3-lactams has some limitations. Indeed, it has often been found that the more-resistant compounds are less-efficient antibiotics. Furthermore, the natural weapons wielded by bacteria mutation, gene transfer, and natural selection, combine to counter /3-lactamase resistance. Thus, /3-lactamase mutants have emerged that efficiently hydrolyze compounds that were previously considered /3-lactamase-resistant [37-41], The overproduction of enzymes - either PBPs or the original /3-lactamases - as well as a decrease in the permeability of the bacterial membrane to antibiotics - are other defense strategies of the bacteria [42] [43],... 

The late factors C5 to C9 are responsible for the development of the membrane attack complex (bottom). They create an ion-permeable pore in the bacterial membrane, which leads to lysis of the pathogen. This reaction is triggered by C5 convertase [2]. Depending on the type of complement activation, this enzyme has the structure C4b2o3b or C3bBb3b, and it cleaves C5 into C5a and C5b. The complex of C5b and C6 allows deposition of C7 in the bacterial membrane. C8 and numerous C9 molecules—which form the actual pore—then bind to this core. 

How do antibiotics act Some, like penicillin, block specific enzymes. Peptide antibiotics often form complexes with metal ions (Fig. 8-22) and disrupt the control of ion permeability in bacterial membranes. Polyene antibiotics interfere with proton and ion transport in fungal membranes. Tetracyclines and many other antibiotics interfere directly with protein synthesis (Box 29-B). Others intercalate into DNA molecules (Fig. 5-23 Box 28-A). There is no single mode of action. The search for suitable antibiotics for human use consists in finding compounds highly toxic to infective organisms but with low toxicity to human cells. 

Bacterial membranes have a much more complex construction than mammalian membranes. This enables bacteria to survive in the various environments of host organisms. Knowledge of the composition and functioning of bacterial membranes is therefore essential to the development of anti-infective drugs. In order to be effective, antibacterial agents not only have to have optimal pharmacokinetic properties such as uptake and distribution in the patient, but they must also be able to cross an additional barrier, the cell wall of the bacteria, so that they can reach the target site. This additional barrier is remarkable on account of its rigidity and permeability. The construction and structural uniqueness of this barrier is briefly described below. 

Polypeptide antibiotics, such as gramicidin A and polymyxin B, are capable of increasing the permeability of bacterial membranes. As is to be expected, they change the phase transition, much like cholesterol [130]. These substances induce a tightening of fluid membranes and an increase in the fluidity of rigid membranes. It has been shown that polymyxin B produces phase separation and forms a Dimyris-toylphosphatidylcholine (DMPG)-rich phase in DMPG/DMPC membranes [131]. 

In bacteria, a family of molecules with a striking chemical similarity to cholesterol, the hopanoids, insert into the membrane hemilayer and stabilize membrane structure (figure 7.28 bacteriohopanetetrol). The effects of these prokaryotic cholesterol analogs are similar to those of cholesterol they broaden the gel-fluid phase transition, condense the bilayer, and reduce bilayer permeability. Contents of hopanoids in bacterial membranes may rise with acclimation temperature (Poralla et ah, 1984). 

In 1979, Jette E. Kristiansen of Copenhagen, Denmark, performed extensive experiments to elaborate the effect of chlorpromazine on the permeability of the bacterial cell wall [55]. In vitro experiments were carried out with Staphylococcus aureus under the influence of chlorpromazine. Depigmentation and bacteriostatic/bactericidal effects of chlorpromazine on the microorganisms were observed. It has been shown that concentrations of chlorpromazine near the bacteriostatic value, in combination with bacterial haemolysins, could alter erythrocyte membranes of horse and rabbit blood in such a way that they become resistant to haemolysis. It was further realized that chlorpromazine in bacteriostatic concentration probably changed the transport of potassium through the bacterial membrane in the same manner as described for mammalian muscle tissue [54],... 

According to Ovchinnikov and coworkers, the antibiotic activity of valinomycin is due to impairment of alkali ion transport in bacterial membranes217 The main arguments for this theory are (i) None of the synthetic non-complexing analogs has antibiotic activity (ii) enantio-valinomycin has the same antibiotic activity as valinomycin, thus excluding interaction with a stereospecific receptor (iii) valinomycin increases the cation permeability of bacterial membranes (iv) the antimicrobial action of valinomycin depends on the cation composition of the medium. 

The P. B are cyclopeptides containing L-2,4-diamino-butanoic acid residues (Dab) and a o-amino acid col-istins A and B, which are also known as P. E, and E2, have the same basic structure. P. exert bactericidal effects on growing and resting cells by impairing the permeability of the bacterial membrane. On account of toxic side affects (nephro- and neurotoxicity) the therapeutic use of P. is limited it is mostly administered locally or orally but rarely parenterally Lit. Experientia 22,354 (1966). J. Antibiot. 30,767 (1977). Antibiotiki 16, 250 (1971). j. Antibiot. 30,427, 1029,1039 (1977). Hager (5.) 1,749f. 7,1091 9,286 ff. Kleemann-En-gel, p. 1556. 

Should the above result be taken to mean that thymidine as a whole is permeable to the bacterial membrane Not necessarily. It could also have been that the bacteria were breaking down thymidine into thjnnine and phosphate and then taking up both the components. How does one test this possibility ... 

The lipid composition of membranes is a sensitive indicator of changes in environmental temperature. The fluidity of a membrane is critical to its functioning as a semi-permeable barrier, and is directly related to the fatty acid composition of the membrane. In artificial lipid membrane the liquid to crystalline transition occurs at lower temperatures for phospholipids containing higher proportions of shorter chain fatty acids or increased degree of unsaturation. Bacterial membranes with a greater proportion of unsaturated fatty acids are better able to function at low temperatures. 

The integration of imsaturated fatty acids into the bacterial membrane also causes an increase of its fluidity and permeability which leads to growth inhibition or bacterial cell death (Boyaval et al., 1995 Shin et al., 2007). 

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