Dihydrofolate reductase trimethoprim resistance

Detecting evolutionary hot spots of antibiotic resistance in Europe Disc diffusion method Genes encoding dihydrofolate reductases confers resistance to trimethoprim... 

Resistance can emerge by mutation, though more commonly it is due to plasmid-encoded trimethoprim-resistant dihydrofolate reductases. These resistant enzymes may be located within transposons on conjugative plasmids that exhibit a broad host range, accounting for rapid and widespread dissemination of trimethoprim resistance among numerous bacterial species. 

Overproduction of the chromosomal genes for the dihydrofolate reductase (DHFR) and the dihydroptero-ate synthase (DHPS) leads to a decreased susceptibility to trimethoprim and sulfamethoxazol, respectively. This is thought to be the effect of titrating out the antibiotics. However, clinically significant resistance is always associated with amino acid changes within the target enzymes leading to a decreased affinity of the antibiotics. 

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. 

VI.a.2.4. Diaminopyrimidines. Pyrimethamine is a dihydrofolate reductase inhibitor, like the biguanides, and is structurally related to trimethoprim. It is seldom used alone. Pyrimethamine in fixed combinations with dapsone or sulfadoxine is used for treatment and prophylaxis of chloroquine-resistant falciparum malaria. The synergistic activities of pyrimethamine and sulfonamides are similar to those of trimethoprim/sulfonamide combinations. Resistant strains of Plasmodium falciparum have appeared world wide. Prophylaxis against falciparum... 

Resistance can develop from alterations in dihydrofolate reductase, bacterial impermeability to the drug, and by overproduction of the dihydrofolate reductase. The most important mechanism of bacterial resistance to trimethoprim clinically is the production of plasmid-encoded trimethoprim-resistant forms of dihydrofolate reductase. 

Resistance to trimethoprim can result from reduced cell permeability, overproduction of dihydrofolate reductase, or production of an altered reductase with reduced drug binding. 

Modification of target sites Alteration of the target site through mutation can confer resistance as occurs with the penicillin binding proteins in methicillin-resistant aureus, or the enzyme dihydrofolate reductase, which is less sensitive to inhibition in organisms resistant to trimethoprim. 

Correct answer = C. Trimethoprim is 20 to 50 times more potent than sulfamethoxazole. It inhibits the enzyme dihydrofolate reductase, thus preventing both purine and pyrimidine synthesis. Trimethoprim resistance has been observed in gram-negative bacteria caused by the presence of a plasmid that codes for an altered dihydrofolate reductase with a lower affinity for the drug. 

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],... 

Haloferax volcanii mutants resistant to the competitive dihydrofolate-reductase inhibitor trimethoprim contain amplifications of various lengths of DNA from a particular region and overproduce a particular protein [166], which proved to be dihydrofolate reductase. The dhf gene was cloned from one of the amplification mutants, and its sequence can... 

Trimethoprim competitively inhibits dihydrofolate reductase (DHFR) and resistance can be caused by overproduction of host DHFR, mutation in the structural gene for DHFR and acquisition of the dfr gene encoding a resistant form. There are at least 15 DHFR enzyme types based on sequence homology and acquisition of dfr genes encoding alternative DHFR of type I, II or V is the most common mechanism of trimethoprim resistance among the Enterobacteriaceae. 

Despite its apparent simplicity, on the basis of an extraordinarily large amount of experimental data, the reaction catalyzed by dihydrofolate reductase has been dissected into a complex series of reaction steps. A comparison between the E. coli enzyme and an R-plasmid-coded dihydrofolate reductase involved in trimethoprim resistance indicates that the chromosomal enzyme has evolved to a virtually perfect catalytic process. ... 

In order to counteract resistance development and to improve antibacterial activity, sulfonamides are typically combined with an inhibitor of dihydrofolate reductase (Figure 2, i). Dihydrofolate reductase is a core enzyme of human central intermediary metabolism. Although the bacterial and mammalian dihydrofolate reductases are orthologues with considerable sequence similarity, trimethoprim (35) has sufficient selectivity to enable the selective inhibition of the parasite enzyme. 

Bacterial resistance increasingly complicates treatment and often results from the acquisition of a plasmid that encodes an altered dihydrofolate reductase. Emergence of trimethoprim-sulfamethoxazole-resistant Staphylococcus aureus and Enterobacteriaceae is a special problem in AIDS patients receiving the drug for prophylaxis of Pneumocystis jiroveci (formerly called Pneumocystis carinii pneumonia. 

Trimethoprim (TMP), a folate analog and inhibitor of dihydrofolate reductase (Figure V-1-3), is usually used together with sulfamethoxazole (SMX). The simultaneous inhibition of the tetrahydrofolate synthesis pathway at two steps has a synergistic effect and prevents the rapid generation of resistance. The clinical uses and side effects of TMP-SMX are discussed. 

C. Resistance Bacterial resistance to sulfonamides is common and may be plasmid-mediated. It can result from decreased intracellular accumulation of the dmgs, increased production of PABA by bacteria, or a decrease in the sensitivity of dihydropteroate synthase to the sulfonamides. Clinical resistance to trimethoprim most commonly results from the production of dihydrofolate reductase that has a reduced affinity for the dmg. 

Bacterial resistance to trimethoprim is increasingly common. In pneumococcal infections, it can result from a single amino acid mutation (lie-100 to Leu) in the dihydrofolate reductase enzyme. Overexpression of dihydrofolate reductase by Staphylococcus aureus has been reported in resistant strains as well. 

Development of allemalivc metabolic pathways. Bacteria can become resistant to sulphoaamides and trimethoprim because they produce modified dihydropieroate. synthetase and dihydrofolate reductase enzymes, respectively, which have little or no affinity for the drugs. 

Two other validated targets for antimicrobial chemotherapy will be mentioned here, both of which are defined by pathway inhibition. The earliest antibiotics, the sulfonamides, are inhibitors of the folate biosynthetic pathway (24). Sulfamethoxazole and trimethoprim, targeting dihydropteroate synthase and dihydrofolate reductase, respectively, are each susceptible to rapid emergence of resistance but have been used successfully in combination. Recently, other enzymes in the pathway have begun to engender interest for target-based screening. Fatty acid biosynthesis is another pathway that has previously been... 

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