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Biomedical subjects

I Chopra

Publications and source records attributed to I Chopra.

At least 19 recordsLinked to original sources
PubMed2002

Antimicrobial activity and mechanisms of resistance to cephalosporin P1, an antibiotic related to fusidic acid.

The antimicrobial properties of cephalosporin P1, an antibiotic structurally related to fusidic acid, were examined. Cephalosporin P1 exhibited potent activity against methicillin-sensitive Staphylococcus aureus, methicillin-resistant S. aureus and vancomycin-intermediate S. aureus. Mutants of S. aureus resistant to cephalosporin P1 arose with a frequency of 1.6 x 10(-6) for selections at 4 x MIC, a frequency similar to that for fusidic acid. The mutations conferred cross-resistance to fusidic acid and mapped in fusA, the gene encoding elongation factor G. Cross-resistance between cephalosporin P1 and fusidic acid also occurred for S. aureus fusA mutants selected with fusidic acid, and in fusidic acid-resistant clinical isolates. Plasmid pUB101, which mediates resistance to fusidic acid in S. aureus, also conferred resistance to cephalosporin P1. Escherichia coli was intrinsically resistant to both fusidic acid and cephalosporin P1, but deletion of the AcrAB efflux pump resulted in susceptibility to both antibiotics. Although complete cross-resistance between fusidic acid and cephalosporin P1 was demonstrated, the nature and location of fusA mutations in S. aureus when cephalosporin P1 was the selective agent frequently differed from those selected with fusidic acid. This may reflect differences in the interaction of the two antibiotics with the translational apparatus, which results in the selection of separate mutation classes for each antibiotic. Furthermore, in three of 14 mutants selected with fusidic acid, resistance was attributed to mutations lying outside fusA. In contrast, mutations in 10 mutants selected with cephalosporin P1 were all located in fusA.

Amino Acid Sequence↗
PubMed2002

Exploiting current understanding of antibiotic action for discovery of new drugs.

The introduction of antibiotics for the chemotherapy of bacterial infections has been one of the most important medical achievements of the past 50 years. However, the emergence of bacterial resistance to antibiotics undermines the therapeutic utility of existing agents, creating a requirement for the discovery of new antibacterial drugs. Several drug discovery strategies have emerged, including incremental improvements to existing antibiotics by chemical manipulation and the search for novel drug targets based on genomic approaches. An alternative strategy seeks to exploit opportunities for drug discovery arising from an understanding of the mode of action of existing antibiotics. Thus biochemical pathways or processes inhibited by antibiotics already in clinical use may nevertheless contain key functions that represent unexploited targets for further drug discovery. A major benefit of employing pathways or processes that are already known to contain drug targets is that proof of principle for drug intervention is already established. This approach to drug discovery is illustrated by reviewing target sites for existing antibiotics and considering how this information might be applied for the discovery of new agents inhibiting peptidoglycan synthesis, tRNA synthesis, transcription and DNA replication

Anti-Bacterial Agents↗
PubMed2002

Exploiting current understanding of antibiotic action for discovery of new drugs.

The introduction of antibiotics for the chemotherapy of bacterial infections has been one of the most important medical achievements of the past 50 years. However, the emergence of bacterial resistance to antibiotics undermines the therapeutic utility of existing agents, creating a requirement for the discovery of new antibacterial drugs. Several drug discovery strategies have emerged, including incremental improvements to existing antibiotics by chemical manipulation and the search for novel drug targets based on genomic approaches. An alternative strategy seeks to exploit opportunities for drug discovery arising from an understanding of the mode of action of existing antibiotics. Thus biochemical pathways or processes inhibited by antibiotics already in clinical use may nevertheless contain key functions that represent unexploited targets for further drug discovery. A major benefit of employing pathways or processes that are already known to contain drug targets is that proof of principle for drug intervention is already established. This approach to drug discovery is illustrated by reviewing target sites for existing antibiotics and considering how this information might be applied for the discovery of new agents inhibiting peptidoglycan synthesis, tRNA synthesis, transcription and DNA replication.

Anti-Bacterial Agents↗
PubMed2001

Glycylcyclines: third-generation tetracycline antibiotics.

Although tetracycline antibiotics have some roles in human and veterinary medicine, the widespread emergence of microbial resistance has severely limited their effectiveness. A new generation of tetracyclines, the glycylcyclines, is being specifically developed to overcome problems of resistance to earlier tetracyclines. One of the glycylcyclines, 9-t-butylglycylamido-minocycline (GAR-936, tigilcycline), is currently undergoing clinical trials and microbiological, pharmacodynamic and pharmacokinetic data have recently been presented for several glycylcyclines, including GAR-936. An ongoing concern is whether resistance to glycylcyclines might arise in the future.

Animals↗
PubMed2001

Mutation frequencies for resistance to fusidic acid and rifampicin in Staphylococcus aureus.

Frequencies at which mutants resistant to fusidic acid and/or rifampicin arose in vitro were determined in Staphylococcus aureus strains including methicillin-susceptible S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), vancomycin-intermediate resistant S. aureus (VISA) and hetero-VISA. The concentrations of fusidic acid (30 and 15 mg/L) and rifampicin (16 and 1 mg/L) used for selection were equal to the expected maximum and minimum serum concentrations after an oral regimen of rifampicin 900 mg od, together with fusidic acid 500 mg tds. Resistant mutants arose at a frequency of around 10(-8) for selections with rifampicin, but were undetectable (frequency <10(-11)) for selections with fusidic acid. Mutants were not recovered (frequency <10(-11)) after selections in the presence of both fusidic acid and rifampicin at 30/16 and 15/1 mg/L. Our results suggest that these antibiotics, when used in combination, could have a wider role in the management of staphylococcal infections.

Anti-Bacterial Agents↗
PubMed2001

Affinities of beta-lactams for penicillin binding proteins of Chlamydia trachomatis and their antichlamydial activities.

Binding affinities of beta-lactam antibiotics for the three penicillin binding proteins (PBPs) from Chlamydia trachomatis were determined in vitro and compared with their antichlamydial activities. Mecillinam selectively inhibited PBP1, with a 50% inhibitory concentration for PBP1 binding (0.2 microg/ml) similar to the MIC (0.1 microg/ml) and minimum bactericidal concentration (0.25 microg/ml). Although the other beta-lactams inhibited a wider range of PBPs than mecillinam, their antichlamydial activities were inferior to that of mecillinam.

Amdinocillin↗
PubMed2001

Antimicrobial properties and mode of action of the pyrrothine holomycin.

Holomycin, a member of the pyrrothine class of antibiotics, displayed broad-spectrum antibacterial activity, inhibiting a variety of gram-positive and gram-negative bacteria, with the exception of Enterobacter cloacae, Morganella morganii, and Pseudomonas aeruginosa. The antibiotic lacked activity against the eukaryotic microorganisms Saccharomyces cerevisiae and Candida kefyr. Holomycin exhibited a bacteriostatic response against Escherichia coli that was associated with rapid inhibition of RNA synthesis in whole cells. Inhibition of RNA synthesis could have been a secondary consequence of inhibiting tRNA aminoacylation, thereby inducing the stringent response. However, the levels of inhibition of RNA synthesis by holomycin were similar in a stringent and relaxed pair of E. coli strains that were isogenic except for the deletion of the relA gene. This suggests that inhibition of RNA synthesis by holomycin could reflect direct inhibition of DNA-dependent RNA polymerase. Examination of the effects of holomycin on the kinetics of the appearance of beta-galactosidase in induced E. coli cells was also consistent with inhibition of RNA polymerase at the level of RNA chain elongation. However, holomycin only weakly inhibited E. coli RNA polymerase in assays using synthetic poly(dA-dT) and plasmid templates. Furthermore, inhibition of RNA polymerase was observed only at holomycin concentrations in excess of those required to inhibit the growth of E. coli. It is possible that holomycin is a prodrug, requiring conversion in the cell to an active species that inhibits RNA polymerase.

Anti-Bacterial Agents↗
PubMed2001

Tetracycline antibiotics: mode of action, applications, molecular biology, and epidemiology of bacterial resistance.

Tetracyclines were discovered in the 1940s and exhibited activity against a wide range of microorganisms including gram-positive and gram-negative bacteria, chlamydiae, mycoplasmas, rickettsiae, and protozoan parasites. They are inexpensive antibiotics, which have been used extensively in the prophlylaxis and therapy of human and animal infections and also at subtherapeutic levels in animal feed as growth promoters. The first tetracycline-resistant bacterium, Shigella dysenteriae, was isolated in 1953. Tetracycline resistance now occurs in an increasing number of pathogenic, opportunistic, and commensal bacteria. The presence of tetracycline-resistant pathogens limits the use of these agents in treatment of disease. Tetracycline resistance is often due to the acquisition of new genes, which code for energy-dependent efflux of tetracyclines or for a protein that protects bacterial ribosomes from the action of tetracyclines. Many of these genes are associated with mobile plasmids or transposons and can be distinguished from each other using molecular methods including DNA-DNA hybridization with oligonucleotide probes and DNA sequencing. A limited number of bacteria acquire resistance by mutations, which alter the permeability of the outer membrane porins and/or lipopolysaccharides in the outer membrane, change the regulation of innate efflux systems, or alter the 16S rRNA. New tetracycline derivatives are being examined, although their role in treatment is not clear. Changing the use of tetracyclines in human and animal health as well as in food production is needed if we are to continue to use this class of broad-spectrum antimicrobials through the present century.

Animal Husbandry↗
PubMed2000

RNA polymerase inhibitors with activity against rifampin-resistant mutants of Staphylococcus aureus.

A collection of rifampin-resistant mutants of Staphylococcus aureus with characterized RNA polymerase beta-subunit (rpoB) gene mutations was cross-screened against a number of other RNA polymerase inhibitors to correlate susceptibility with specific rpoB genotypes. The rpoB mutants were cross-resistant to streptolydigin and sorangicin A. In contrast, thiolutin, holomycin, corallopyronin A, and ripostatin A retained activity against the rpoB mutants. The second group of inhibitors may be of interest as drug development candidates.

Anti-Bacterial Agents↗
PubMed1999

Expression of the Staphylococcus aureus UDP-N-acetylmuramoyl- L-alanyl-D-glutamate:L-lysine ligase in Escherichia coli and effects on peptidoglycan biosynthesis and cell growth.

The monomer units in the Escherichia coli and Staphylococcus aureus cell wall peptidoglycans differ in the nature of the third amino acid in the L-alanyl-gamma-D-glutamyl-X-D-alanyl-D-alanine side chain, where X is meso-diaminopimelic acid or L-lysine, respectively. The murE gene from S. aureus encoding the UDP-N-acetylmuramoyl-L-alanyl-D-glutamate: L-lysine ligase was identified and cloned into plasmid vectors. Induction of its overexpression in E. coli rapidly results in abnormal morphological changes and subsequent cell lysis. A reduction of 28% in the peptidoglycan content was observed in induced cells, and analysis of the peptidoglycan composition and structure showed that ca. 50% of the meso-diaminopimelic acid residues were replaced by L-lysine. Lysine was detected in both monomer and dimer fragments, but the acceptor units from the latter contained exclusively meso-diaminopimelic acid, suggesting that no transpeptidation could occur between the epsilon-amino group of L-lysine and the alpha-carboxyl group of D-alanine. The overall cross-linking of the macromolecule was only slightly decreased. Detection and analysis of meso-diaminopimelic acid- and L-lysine-containing peptidoglycan precursors confirmed the presence of L-lysine in precursors containing amino acids added after the reaction catalyzed by the MurE ligase and provided additional information about the specificity of the enzymes involved in these latter processes.

Bacteriolysis↗
PubMed1998

Research and development of antibacterial agents.

Several new antibacterial agents are currently being developed in response to the emergence of bacterial resistance to existing drugs. The new agents include compounds that inhibit macromolecular synthesis or interfere with bacterial membrane function. Apart from the oxazolidinones and cationic peptides, the remainder of these new compounds are analogues of earlier antibiotic classes; therefore, it is probable that existing resistance mechanisms will adapt to accommodate the new derivatives. To minimise the potential for emergence of resistance to new agents, research strategies should be chosen that not only enhance the discovery of structurally novel drugs, but also direct these to new molecular targets that may themselves have decreased potential to give rise to drug-resistant variants.

Anti-Bacterial Agents↗
PubMed1998

Antibacterial spectra of drugs used for chemotherapy of mycobacterial infections.

The mechanism of action of many antimycobacterial agents is poorly understood. To obtain preliminary information on whether the targets for some of these drugs might also occur in other bacteria, the in vitro activities of selected agents against Escherichia coli, Bacillus subtilis and Staphylococcus aureus were determined. Dapsone, p-aminosalicylic acid and thiacetazone failed to inhibit the above organisms (MIC values > 100 micrograms/ml) that may therefore lack targets for these drugs. Capreomycin, viomycin and clofazimine demonstrated activity against some of the organisms (MIC values < 100 micrograms/ml) suggesting that the targets of these drugs may not be restricted to mycobacterial species. The agents were all potent inhibitors of Mycobacterium bovis bacille Calmette-Guérin (MIC values 0.08-0.5 microgram/ml).

Aminosalicylic Acid↗
PubMed1998

Joint tolerance to beta-lactam and fluoroquinolone antibiotics in Escherichia coli results from overexpression of hipA.

The basis of joint tolerance to beta-lactam and fluoroquinolone antibiotics in Escherichia coli mediated by hipA was examined. An antibiotic tolerance phenotype was produced by overexpression of hipA under conditions that did not affect the growth rate of the organism. Overexpressing hipA probably decreases the period in which bacteria are susceptible to the antibiotics by temporarily affecting some aspect of chromosome replication or cell division.

Anti-Bacterial Agents↗
PubMed1997

Molecular action of anti-mycobacterial agents.

In terms of the paradigms for antibacterial action presented in the introduction, there is good evidence that broad spectrum agents exert their anti-mycobacterial activity by interaction with classical targets occurring in a wide range of organisms including the mycobacteria. This is supported either by direct evidence (e.g., inhibition by rifampicin of mycobacterial RNA polymerase), or indirectly by the characterization of drug-resistant mycobacteria where mutations conferring resistance have been mapped to target sites homologous to those found in other bacteria (fluoroquinolones, macrolides, rifampicin, streptomycin). On the other hand, although the mode of action of some of the agents with an anti-mycobacterial spectrum is not fully understood, it is evident that the restricted spectrum is likely to arise from the possession of unique targets, or specific pro-drug conversion systems, or to a combination of both mechanisms. In several cases the narrow spectrum of the agents can be attributed to inhibition of molecular targets involved in the biosynthesis of the mycobacterial cell envelope that contains many unique polymers. The recent re-emergence of tuberculosis as an important human pathogen has led to improved methods for exploring the structure, biochemistry and genetics of the mycobacteria. These technical advances can now be used to gain a better understanding of the molecular basis of drug action in mycobacteria.

Antibiotics, Antitubercular↗