Intrinsic natural resistance

Where a = amount of undissociated acid [H+] = Hydrogen concentration 

Although much of the inhibition by organic acids is accounted for by pH, they vary considerably in inhibitory effects (Hsiao and Siebert, 1999). Different organisms have also shown variations in susceptibility to inhibition by organic acids (Matsuda et al., 1994). For example, lactobacilli may be much more resistant to acetic, benzoic, butyric, and lactic acids, whereas E. coli has been found to be most resistant toward citric, malic, and tartaric acids (Hsiao and Siebert, 1999). 

The content of undissociated acid declines with an increase in pH value. 

Acetic acid bacteria are naturally highly resistant to acetic acid, with notable differences in tolerance between species (Trcek et al., 2006). The enzyme AatA plays an important role in acetic acid resistance in these 

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Microbial Resistance to Drugs. Figure 1 MlC-distribution showing the number of isolates of one species with a certain MIC. Some bacterial species are naturally resistant (intrinsic resistance) to drugs because their natural MIC is above the breakpoint. Naturally sensitive isolates as well as naturally resistant ones can acquire resistance and with that increase their MIC (acquired or secondary resistance). 

Intrinsic (natural, innate) resistance. In one form of intrinsic resistance, the fungal cell wall (see Chapter 2) is considered to present a barrier to exclude or, more likely, to reduce the penetration by biocide molecules. The evidence to date is sketchy but the available information tentatively links cell wall glucan, wall thickness and consequent relative porosity to the sensitivity of Saccharomyces cerevisiae to chlorhexidine. 

All fiber applications derive from the intrinsic nature of the material from which the hbers are formed. Each material, and each molecular variation of a material, produces fibers with unique characteristics and properties, even though the basic molecular formulas of different materials are very similar. As well, the physical structure of the fibers and the manner in which they were processed work to determine the properties of those fibers. The diameter of the fibers is a very important consideration. Other considerations are the temperature of the melt from which fibers of a material were drawn whether the fibers were stretched or not, and the degree by which they were stretched whether the fibers are hollow, filled, or solid and the resistance of the fiber material to such environmental influences as exposure to light and other materials. 

Two major types of microbial resistance can be distinguished intrinsic and acquired resistance. Intrinsic (innate) resistance refers to a natural chromosomally controlled property, including physiological adaptation. 

Some ten years after the structure of cephalosporin C had been established, the isolation of two naturally occurring cephalosporins possessing a 7(a)-methoxy group was reported (149). These were identified as 7-methoxycephalosporin C (202) and the C(3)-carbamate (203). Further examples of this type of natural product have been reported and are listed in Table 7. All possess the a-aminoadipic acid side chain, but vary in the substitution pattern at C(3). Collectively they are known as the cepha-mycins (174). A major point of interest with the cephamycin type of antibiotic is their intrinsically higher resistance to hydrolysis by 3-... 

HP-LCP is a promising matrix polymer for tribological applications because of its low melt viscosity, which is the typical namre of TLCPs, as well as high heat resistance and high strength and modulus. However, TLCPs have an intrinsic nature that results from the orientation of molecular chains in the skin layer of a molded article along MD, which generates fibrils easily from the surface. 

As the film has a domain structure, the intrinsic nature of electrical conduction in the film may not be observed by d.c. measurements. Indeed, the distribution of the resistance in the film was investigated by electric field mapping where the grain boundaries in the film were more resistive than the insides of the grains [45]. 

Viscoelastic polymers essentially dominate the multi-billion dollar adhesives market, therefore an understanding of their adhesion behavior is very important. Adhesion of these materials involves quite a few chemical and physical phenomena. As with elastic materials, the chemical interactions and affinities in the interface provide the fundamental link for transmission of stress between the contacting bodies. This intrinsic resistance to detachment is usually augmented several folds by dissipation processes available to the viscoelastic media. The dissipation processes can have either a thermodynamic origin such as recoiling of the stretched polymeric chains upon detachment, or a dynamic and rate-sensitive nature as in chain pull-out, chain disentanglement and deformation-related rheological losses in the bulk of materials and in the vicinity of interface. 

Bacterial resistance to biocides (Table 13.2) is usually considered as being of two types (a) intrinsic (innate, natural), a natural property of an organism, or (b) acquired, either by chromosomal mutation or by the acquisition of plasmids or transposons. Intrinsic resistance to biocides is usually demonstrated by Gram-negative bacteria, mycobacteria and bacterial spores whereas acquired resistance can result by mutation or, more frequently, by the acquisition of genetic elements, e.g. plasmid- (or transposon-) mediated resistance to mercury compounds. Intrinsic resistance may also be exemplified by physiological (phenotypic) adaptation, a classical example of which is biofilm production. 

Intrinsic resistance may than be defined as a natural, chromosomally controlled property of a bacterial cell that enables it to circumvent the action of a biocide (see Table 13.2). A summary of intrinsic resistance mechanisms is provided in Table 13.4. 

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