Bacterial resistance, acquired intrinsic

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. 

Table 13.2 Intrinsic and acquired bacterial resistance to biocides... 

Bacterial resistance to antibiotics and biocides is essentially of two types, intrinsic and acquired. Whilst the latter is of greater significance clinically with antibiotics, specific examples of intrinsic resistance to both antibiotics, e.g. mycobacteria, and biocides (e.g. mycobacteria, Gram-negative bacteria, spores) are also of importance. 

In general terms, bacterial resistance to antibacterial agents can be considered as being either intrinsic (innate, a natural property) or acquired, for example, by mutation or by the acquisition of a plasmid or transposon (jumping gene) [105]. Resistance to mercury [103, 122-125] and to other cations and anions [103, 122, 123, 125] is well characterized. The mechanisms involved in resistance to some metals, including silver, are summarized in Table 7.1. 

Antimicrobial resistance traits are genetically coded and can either be intrinsic or acquired. Intrinsic resistance is due to innately coded genes which create natural insensitivity to a particular antibiotic. Innate resistance is normally expressed by virtually all strains of that particular bacterial species. Acquired resistance is gained by previously susceptible bacteria either through mutation or horizontally obtained from other bacteria possessing such resistance via transformation, transduction, or conjugation. Acquired resistance is limited to subpopulations of a particular bacterial species and may result from selective pressure exerted by antibiotic usage. 

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). 

Antibiotics have been used for the treatment of bacterial infections for more than 50 years. However, since that time, we have learned that antibiotics did not kill certain bacteria. It is generally accepted that there are two major classes of antibiotic resistance intrinsic resistance and acquired resistance. 

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