Microbial cell-containing membranes

Microbial Cell-containing Membranes for Molecular Recognition. Suzuki et al. (87) have proposed a miocrobial sensor which consists of membrane-bound microbial cells and an electrochemical device. The assemblies of microbial sensors are similar to enzyme sensors. Two types of microbial sensors have been developed as presented In Figure 9. The first monitors the respiration activity of membrane-bound microbial cells with a Clark-type oxygen electrode. The... 

Unimmobilized Corynebacterium propinquum (CGMCC No. 0886) cells containing a cobalt-dependent NHase were employed in either batch or continuous reactions for the production of nicotinamide from 3-cyanopyridine [24]. In the continuous process, membrane filtration separated precipitated product (>5 wt%) and the microbial cell catalyst from the reaction mixture, where the catalyst was then recovered and returned to the reactor using a continuous addition of aqueous 3-cyanpyridine to maintain substrate concentration at <20% (w/v), a final conversion of >99% was obtained. 

Microbes also have a plasma membrane that resides adjacent to their cell wall. Polymyxins are amphipathic agents (containing both nonpolar, lipophilic and polar, lipophobic groups) that interact with phospholipids in microbial cell membranes. The result is disruption of the membrane and increased permeability. However, because microbial and mammalian cell membranes are not exceedingly dissimilar, polymixins can produce significant toxicity in humans (i.e., they have low selective toxicity). This is also true for the related drug nystatin. This is why these particular antibiotics are not generally used systemically and are usually restricted to topical application. 

Many pharmaceutical preparations containing oil-water systems (creams, ointments, or suspensions) are subject to microbial contamination. Bacteria in these heterogenous systems are usually grown in the aqueous phase and at the oil-water interface. To preserve the shelf-life of these preparations, benzoic acid or other organic acids are added as preservatives. Because the microbial cell membrane is lipophilic in nature, the bacteriostatic actions of the acidic preservative are attributable almost entirely to the undissociated acid and not to the ionized form. A good understanding of the partition coefficient and the degree of ionization allows accurate calculation of the free un-ionized acid in the aqueous phase, which provides the bacteriostatic concentration. 

Microbial cell structure is varied with a tremendous diversity in size and shape. Prokaryotic cells typically contain a cell wall, 70s ribosomes, a chromosome that is not membrane bound, various inclusions and vacuoles, and extrachromosomal DNA or plasmids. Eukaryotic microorganisms are equally varied with a variety of forms many are photosynthetic or harbor photosynthetic symbionts. Many eukaryotic cells contain prokaryotic endosymbionts, some of which contain their own set of plasmids. Given the variety of eukaryotic microorganisms, they have been labeled protists, since they are often a mixing of algal and protozoan characteristics within apparently related groups. 

Surfactant Effects on Microbial Membranes and Proteins. Two major factors in the consideration of surfactant toxicity or inhibition of microbial processes are the disruption of cellular membranes b) interaction with lipid structural components and reaction of the surfactant with the enzymes and other proteins essential to the proper functioning of the bacterial cell (61). The basic structural unit of virtually all biological membranes is the phospholipid bilayer (62, 63). Phospholipids are amphiphilic and resemble the simpler nonbiological molecules of commercially available surfactants (i.e., they contain a strongly hydrophilic head group, whereas two hydrocarbon chains constitute their hydrophobic moieties). Phospholipid molecules form micellar double layers. Biological membranes also contain membrane-associated proteins that may be involved in transport mechanisms across cell membranes. 

Extensive research and development of microbial sensors has been carried out by Suzuki et al. (89-94) and Rechnitz et al. (95-97) (see Table III). Microbial sensors consisting of membrane-bound whole cells and an oxygen electrode were constructed for the determination of substrates such as assimilable sugars, acetic acid, alcohols and ammonia, and for the estimation of biochemical oxygen demand (BOD) (98-104). Glutamic acid was determined with a microbial sensor which consists of membrane-bound whole cells containing glutamate decarboxylase and a carbon dioxide gas electrode. These microbial sensors have been applied and evaluated for on-line measurements in fermentation processes (105,106). 

The polypyrrole (Ppy)/dextrin nanocomposite is synthesised via in situ polymerisation and the preparation of this nanocomposite is shown in Figure 5.4. The backbone chain of this nanocomposite polymer contains hydrophobic side chains, which disrupt the microbial cell membrane leading to leakage of the cytoplasm in bacteria including Escherichia coli. Pseudomonas aeruginosa. Staphylococcus aureus and Bacillus subtilis. This material can be implemented in the fields of biomedicine, biosensors and food packaging due to the biodegradable property of dextrin as well as the antibacterial properties of the Ppy [79]. 

Infrared spectroscopy has proved to be a valuable tool for characterizing and differentiating microbial cells [4, 6, 45-48]. Given the complex nature of such micro-organisms, the spectra can contain a superposition of hundreds of infrared modes. However, the FTIR spectra of bacteria show bands predominantly due to the protein component [6]. Although the cells also contain DNA and RNA structures, carbohydrates and lipids, the various cell and membrane proteins form the major part of the cell mass. Figure 7.15 shows parts of the infrared spectra of Escherichia coli and a typical protein, i.e. ribonuclease A, illustrating their similarity. Bands due to carbohydrates and lipids may also be observed in bacterial infrared spectra to a lesser extent. 

As the microbial mass synthesised in the rumen provides about 20 per cent of the nutrients absorbed by the host animal, the composition of microorganisms is important. The bacterial dry matter contains about 100 g nitrogen/kg, but only 80 per cent of this is in the form of amino acids, the remaining 20 per cent being present as nucleic acid nitrogen. Moreover, some of the amino acids are contained in the pepti-doglycan of the cell wall membrane and are not digested by the host animal. 

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