Microbial cell membrane

The principal stages in complement activation. Complement activation occurs exclusively on the microbial cell membrane, where it is triggered by bound antibody or microbial envelope polysaccharides, both of which activate early complement components. Two sets of early components belong to two distinct pathways of complement activation. Activation of each complement system involves a cascade of proteolytic reactions. Each component of the complement system is a proenzyme that is activated by the preceding component of the chain by a limited proteolytic cleavage. The ultimate result of this chain reaction is the development of a complex that attacks the cell membrane. 

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. 

Differences between host cell membranes and microbial cell membranes mean that the cascade is only activated in the presence of microorganisms, so C3 tickover cannot give rise to full activation of the alternative pathway in the absence of microbial membrane. Stable deposition of a functional C3 convertase only occurs on the microbial cell surface. The differences that exist include, for example ... 

Organic acids can enter the microbial cell only in their undissociated forms, which diffuse across the microbial cell membrane. This entrance of the acid molecule then lowers the intracellular pH (pH,) of the cell (Carrasco et al., 2006). The concentration of the undissociated form of an organic acid and the pH of the environment are interdependent variables, linked by the Henderson-Hasselbach equation (Breidt, Jr., Hayes, and McFeeters, 2004). As the extracellular pH decreases, the number of undissociated organic acids increases, and so do their activities toward the microbial cells (Kwon and Ricke, 1998). This undissociated state of the acid molecule is primarily responsible for any antimicrobial activity and effectiveness is dependent on the dissociation constants (pKa) of the acid (Barbosa-Canovas et al., 2003). This undissociated state of the organic acid is extremely important in the capacity to inhibit a microbial cell (Gauthier,... 

Optimum growth of bacteria is generally at pH 6-7. A decrease in pH will inevitably cause a fall in growth rate, until it eventually reaches zero. In acidic foodstuffs it is important that the following factors are kept in mind (1) weak carboxylic acids will only naturally dissociate in aqueous solution, and (2) the undissociated form is very lipid soluble which allows them to diffuse freely through the microbial cell membrane into the cytoplasm. It is, however, also possible that the low pH of acidic food will cause an increase in the amount of undissociated acid (Adams and Nicolaides, 1997). 

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]. 

Antibiotics are generally toxic to microbes (although the general term antibiotic can be used as well for substances to combat nonmicrobial pathogens and others). Antibiotics are often based upon naturally occurring substances produced by some BU to combat other BU, and often disrupt microbial cell membranes or metabolism. Microbial immunity to antibiotics can be built by the following mechanisms ... 

Chlorinated derivatives of phenols, cresol, and xylenol elicit the release of [ C]-glutamate from E. colt [92]. These compounds are effective bactericides and are used in crude disinfectant and antiseptic preparations such as LysoT and Det-tol . Their abUity to denature protein is probably responsible for their disruptive action upon microbial cell membranes. In addition to damaging the membrane [92], phenol exerts a lytic action upon the cell wall lysis occurs in growing cultures of E. coll [93] and is particularly noted in synchronous cultures at the point of cell division [94]. 

Sorbic acid is a naturally occurring compound it may be characterized as one of the least toxic of all the preservative agents known. It is able to penetrate the semipermeable microbial cell membrane inhibiting nutrient transport and enzymes. 

Membrane-active molecules act via non-specific adsorption on the microbial cell membrane and disturbance of the embedded proteins by influencing the penetration of ions and organic molecules and by inhibition of the ATP-synthesis. 

Sorbic acid is a naturally occurring compound, its lactone ( sorbic oil ) is found in Sorbus acuparia. It is justified to characterize sorbic acid as one of the least toxic of all the preservative agents known. In its undissociated form it is a membrane active agent which due to its unsaturated character additionally may exhibit electrophilic activity. Therefore sorbic acid is able to penetrate the microbial cell membrane and to inhibit nutrient transport... 

Various mechanisms of electron transfer and linking species have been identified and exploited. Basically there are two major types of linking species (i) soluble compounds (artificial or self-produced mediators), and (ii) compounds bonded to the microbial cell membrane (membrane-bond proteins or nanowires). Accordingly, electron transfer mechanisms from microorganisms to the electrode can be divided into five primary types, which will be discussed in the following section (1) direct cell-surface electron transfer, (2) direct electron transfer via nanowires, (3) electron transfer via exogenous redox mediators, (4) endogenous redox mediators and (5) reduced metabolic products. 

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