Resistance chromosomal mutations

Plasmids have the ability to transfer within and between species and can therefore be acquired from other bacteria as well as a consequence of cell division. This property makes plasmid-acquired resistance much more threatening in terms ofthe spread of antibiotic resistance than resistance acquired due to chromosomal mutation. Plasmids also harbour transposons (section 2.1.3), which enhances their ability to transfer antibiotic resistance genes. 

Two mechanisms of chromosomal resistance have been identified. A mutation of dihydropteroate synthetase (DHPS) in Strep, pneumoniae produces an altered enzyme with reduced affinity for sulphonamides. Hyperproduction of p-aminobenzoic acid (PABA) overcomes the block imposed by inhibition ofDHPS. The specific cause of PABA hyperproduction is unknown, though chromosomal mutation is the probable cause. 

Chromosomal mutations in E. coli result in overproduction of dihydrofolate reductase (DHFR). Higher concentrations of trimethoprim, which may not be therapeutically achievable, are therefore required to inhibit nucleotide metabolism. Other mutations lower the affinity of DHFR for trimethoprim. These two mechanisms of resistance may coexist in a single strain, effectively increasing the level of resistance to the antibiotic. 

Rifampicin is the semisynthetic derivative used widely in the UK. Resistance to rifampicin is primarily due to chromosomal mutations resulting in an altered RNA polymerase which is less well inhibited by the drug. The mutations tend to be clustered within short conserved regions of the J3 subunit gene of RNA polymerase. Similar mutations have been found in all bacterial species studied thus far. 

A second mechanism of acquired resistance to fosfomycin involves chromosomal mutations in sugar phosphate uptake pathways which are responsible for transporting fosfomycin into the cell. The alterations decrease accumulation of the antibiotic to levels below those required for inhibition. 

Acquired resistance to polymyxins in E. coli occurs because of chromosomal mutations which cause incorporation of aminoethanol and aminocarabinose in lipo-polysaccharide (LPS) in place of phosphate groups. The altered LPS has a decreased ionic charge which results in lowered binding of polymyxin and thus an increase in resistance to this group of antibiotics. 

The mechanism of acquired resistance in Pseudomonas aeruginosa is different. Chromosomal mutations result in the increase of a specific outer membrane protein with a concomitant reduction in divalent cations. Polymyxins bind to the outer membrane at sites normally occupied by divalent cations, and therefore it is thought that a reduction in these sites will lead to decreased binding of the antibiotic with a consequent decreased susceptibility of the cell. 

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. 

When the fluoroquinolones were first introduced, there was optimism that resistance would not develop. Although no plasmid-mediated resistance has been reported, resistance of MRSA, pseudomonas, coagulase-negative staphylococci and enterococci has unfortunately emerged due to chromosomal mutations. Crossresistance exists among the quinolones. The mechanisms responsible for this resistance include ... 

Spontaneous chromosomal mutation of the target enzymes DNA gyrase (GyrA and GyrB) and topoisomerase IV (ParC and ParE), which can gradually produce an increase in resistance to quinolones. 

Unlike intrinsic resistance, which is usually expressed by chromosomal genes, acquired resistance arises as a consequence of mutations in chromosomal genes or by the acquisition of plasmids or transposons [6, 7, 157, 158]. Chromosomal mutations are associated with changes in the base se-... 

Plasmid- and transposon-mediated resistance to trimethoprim involves a by-pass of the sensitive step by duplication of the chromosomally-encoded dihydrofolate reductase (DHFR) target enzyme [203]. Several trimethoprim-resistant bacterial DHFRs have been identified, resistance ensuing because of altered enzyme target sites [204], Low-level resistance to tetracyclines arises in E. coli as a result of chromosomal mutations leading to loss of the outer membrane porin OmpF through which these drugs normally pass [6, 193],... 

To prevent the rapid development of bacterial resistance. Erythromycin and rifampin are used in combination in the treatment of foals with Rhodococcus (R.) equi infections. Each drug has a completely different mechanism of antimicrobial action their combination reduces the chance of chromosomal mutations conferring bacterial resistance. 

Resistance to the sulfonamides occurs via chromosomal mutation or is plasmid mediated. Chromosomal mutation results in hyperproduction of PABA in bacteria, which overcomes the competitive substitution of the sulfonamides. These mutations are of minor clinical significance. The most common form of bacterial resistance to sulfonamides is via the plasmid-encoded production of altered forms of DPS. More than 50 years of widespread use of the sulfonamides in animal health has resulted in widespread resistance. 

Chromosomal mutation develops readily in most bacteria exposed to rifampin and leads to a high level of resistance. These mutants show stable changes in RNA polymerase that prevent binding. Resistance to rifampin is not transferable and there is no cross-resistance with other antibiotics. 

Knowledge of the mechanisms of action of antimicrobial agents is required for understanding resistance acquired through chromosomal mutation and selection, and forms the basis of selecting antimicrobials for concurrent use, either as combination preparations or separately. 

Lincomycin, which resembles erythromycin, in a dose of 500 mg t.i.d., is indicated in the treatment of serious infections due to susceptible strains of streptococci, pneumococci, and staphylococci resistant to other antibiotics. Lincomycin inhibits protein synthesis by interfering with the formation of initiation complexes and with aminoacyl translocation reactions. The receptor for lincomycins on the 50S subunit of the bacterial ribosome is a 23S rRNA, perhaps identical to the receptor for erythromycins (see also Figure 88). Thus, these two drug classes may block each other s attachment and may interfere with each other. Resistance to lincomycin appears slowly, perhaps as a result of chromosomal mutation. Plasmid-mediated resistance has not been established with certainty. Resistance to lincomycin is not rare among streptococci, pneumococci, and staphylococci. C. difficile strains are regularly resistant. 

Resistance to macrolides can result from (1) drug efflux by an active pump mechanism (2) ribosomal protection by inducible or constitutive production of methylase enzymes that modify the ribosomal target and decrease drug binding (3) macrolide hydrolysis by esterases produced by Enterobacteriaceae and (4) chromosomal mutations that alter a SOS ribosomal protein (found in B. subtilis, Campylobacter spp., mycobacteria, and gram-positive cocci). 

Resistance to sulfonamides is widespread in bacteria isolated from animals, and may involve chromosomal mutations or plasmid-mediated mechanisms. Chromosomal mutations cause impaired drug penetration, production of altered forms of dihydropteroate synthetase for which sulfonamides have a lowered affinity, or production of excessive PABA that overcomes the metabolic block imposed by the inhibition of dihydropteroate synthetase. A more common cause of bacterial resistance to sulfonamides is plasmid-mediated mechanisms, which may result in impaired drug penetration or the synthesis of sulfonamide-resistant dihydropteroate synthetase. There is cross-resistance among sulfonamides. 

Resistance to bacterial diaminopyrimidines results from chromosomal mutations or plasmid-mediated mechanisms and develops very rapidly. Resistance conferred by chromosomal mutations allows bacteria to utilize exogenous sources of folinic acid or thymidine, thereby overcoming the drug-imposed blockade. Plasmid-mediated mechanisms result in the synthesis of dihydrofolate reductase characterized by a reduced affinity for antibacterial diaminopyrimidines. 

Furthermore, it may be likely that highly resistant strains with chromosomal mutations may also have genes coding for enzymes that modify streptomydn. 

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