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Biomedical subjects

I Chopra

Publications and source records attributed to I Chopra.

At least 55 records · Page 3Linked to original sources
PubMed1994

Bacterial resistance mechanisms as therapeutic targets.

In the 50 years since antimicrobial agents were first introduced, bacteria have acquired a wide variety of mechanisms which have enabled them to resist the effects of these drugs. One way of overcoming this problem is to administer an antibiotic with an agent which counteracts the mechanism of resistance to that antibiotic; an example of such an approach which has already been successfully implemented is the combination of a beta-lactam antibiotic with a beta-lactamase inhibitor. This review describes antibiotic resistance mechanisms which might lend themselves to an inhibitor approach and the potential therapeutic applications of such a strategy.

Bacteria↗
PubMed1993

Mode of action of the cyclic depsipeptide antibiotic LL-AO341 beta 1 and partial characterization of a Staphylococcus aureus mutant resistant to the antibiotic.

The antibacterial activity of the cyclic antibiotic LL-AO341 beta 1 was examined. The antibiotic was a narrow spectrum agent, effective principally against Gram-positive organisms. The intrinsic insusceptibility of Escherichia coli was due to exclusion of the drug by the outer membrane. The antibiotic was bactericidal against Staphylococcus aureus, and cell death was associated with lysis of the bacteria. The antibiotic did not specifically inhibit the synthesis of DNA, RNA, protein, lipid or peptidoglycan since these synthetic activities continued for several minutes after exposure to lethal concentrations of the antibiotic and then all abruptly ceased between about 8 and 15 minutes post antibiotic exposure. These results are consistent with the cytoplasmic membrane being the primary target for LL-AO341 beta 1. Mutants of S. aureus 8325-4 selected on 10- or 20-times the MIC of LL-AO341 beta 1 occurred spontaneously with a frequency of about 3 x 10(-6). A mutant expressing a 160-fold increase in the MIC of LL-AO341 beta 1 was obtained by exposing cultures to progressively increasing concentrations of the antibiotic. This mutant displayed no cross-resistance to other agents apart from telomycin (a structural analogue of LL-AO341 beta 1), apparently did not modify or degrade LL-AO341 beta 1 and had only a slightly longer doubling time than the parent strain.

Anti-Bacterial Agents↗
PubMed1992

Uptake of minocycline by Escherichia coli.

Uptake of tetracyclines into Escherichia coli was assessed with a strain carrying a tetA-lacZ translational fusion, in which expression of the enzyme is controlled by the pSC101 tetR repressor gene, by examining beta-galactosidase induction. The ability of tetracycline analogues to induce beta-galactosidase synthesis was correlated with their hydrophobicity, such that hydrophobic analogues were poor enzyme inducers. Treatment of E. coli with polymyxin B nonapeptide (PMBN) rendered cells more permeable to minocycline, but not to tetracycline.

Enzyme Induction↗
PubMed1992

Tet determinants provide poor protection against some tetracyclines: further evidence for division of tetracyclines into two classes.

Atypical tetracyclines were active against Escherichia coli and Staphylococcus aureus strains containing determinants that mediate resistance to typical tetracyclines by efflux (Tet B and Tet K) or ribosomal protection (Tet M) mechanisms. The results support recently published data that tetracyclines are divisible into at least two classes on the basis of their modes of action.

Escherichia coli↗
PubMed1992

Resistance to beta-lactams in Mycobacterium fortuitum.

It is widely assumed that the high level of intrinsic resistance to beta-lactam antibiotics exhibited by mycobacteria results from the combination of factors including permeability to the drugs, beta-lactamase production, and affinity for penicillin-binding proteins (PBPs). We conducted an evaluation of the second and third factors by isolating nitrosoguanidine-induced mutants from the beta-lactamase-producing strain Mycobacterium fortuitum ATCC 19542 that displayed either elevated or reduced resistance to various beta-lactam antibiotics. The mutants studied included D1 (a beta-lactamase producer with high penicillin resistance), gamma 27 (a low-level beta-lactamase producer with low penicillin resistance), and D316 (a high-level beta-lactamase producer with high penicillin resistance). In all strains examined, four major PBPs, named 1, 2a, 2b, and 3, with apparent molecular weights of 102,000, 90,000, 87,000, and 50,000, respectively, were found. The MICs of various beta-lactams toward ATCC 19542 and its mutants were considered in the context of beta-lactamase production, the quantity of PBPs synthesized, and their affinities for beta-lactam antibiotics. The data obtained show that beta-lactamase production is likely to be an important factor in the expression of resistance by clinical isolates and that PBP alterations can contribute to resistance at least in laboratory-derived mutants.

Anti-Bacterial Agents↗
PubMed1992

Evidence that tetracycline analogs whose primary target is not the bacterial ribosome cause lysis of Escherichia coli.

The modes of action of atypical tetracyclines that do not directly inhibit bacterial protein synthesis were investigated. The analogs tested, chelocardin, anhydrotetracycline, 6-thiatetracycline, anhydrochlortetracycline, and 4-epi-anhydrochlortetracycline, were bactericidal and caused the lysis of Escherichia coli accompanied by the release of the cytoplasmic enzyme beta-galactosidase into the supernatant. Examination by electron microscopy demonstrated that cells exposed to these analogs underwent marked morphological alterations that included the formation of numerous ghosts and the appearance of cellular debris in the culture medium. Although atypical tetracyclines promoted lysis in intact organisms, they did not cause lysis of E. coli spheroplasts, indicating that the analogs do not directly destroy the cytoplasmic membrane. These agents may promote cell lysis and death by interfering with the membrane's electrochemical gradient, which in turn leads to stimulation of autolytic enzyme activity and cellular lysis. The results support recently published data which indicate that tetracyclines are divisible into two classes on the basis of their modes of action.

Bacterial Proteins↗
PubMed1992

Genetic analysis of the tetA(C) gene on plasmid pBR322.

The TetA(C) protein, encoded by the tetA(C) gene of plasmid pBR322, is a member of a family of membrane-bound proteins that mediate energy-dependent efflux of tetracycline from the bacterial cell. The tetA(C) gene was mutagenized with hydroxylamine, and missense mutations causing the loss of tetracycline resistance were identified at 30 distinct codons. Mutations that encoded substitutions within putative membrane-spanning alpha-helical regions were scattered throughout the gene. In contrast, mutations outside the alpha-helical regions were clustered in two cytoplasmic loops, between helices 2 and 3 and helices 10 and 11, suggesting that these regions play a critical role in the recognition of tetracycline and/or energy transduction. All of the missense mutations encoded a protein that retained the ability to rescue an Escherichia coli strain defective in potassium uptake, suggesting that the loss of tetracycline resistance was not due to an unstable TetA(C) protein or to the failure of the protein to be inserted in the membrane. We postulate that the mutations encode residues that are critical for the active efflux of tetracycline, except for mutations that result in the introduction of charged residues within hydrophobic regions of the TetA(C) protein.

Amino Acid Sequence↗
PubMed1991

Inhibition of K88ab-mediated haemagglutination by polymyxin B nonapeptide.

The ability of the cyclic peptide polymyxin B nonapeptide (PMBN) to inhibit haemagglutination of erythrocytes by Escherichia coli bearing K88ab, K99 or F41 fimbriae was examined. The agent strongly inhibited K88ab-mediated haemagglutination, but had little or no effect on haemagglutination mediated by K99 or F41 fimbriae. Inhibition of K88ab-mediated haemagglutination did not result from release of fimbrial adhesins from the bacterial cell surface, nor from solubilization of K88ab receptors in erythrocytes. Since PMBN also prevented haemagglutination mediated by partially-purified K88ab fimbriae, the agent may directly obstruct access of fimbriae to their mammalian receptor binding sites.

Adhesins, Escherichia coli↗
PubMed1991

Inhibition of Escherichia coli K12 by short-chain organic acids: lack of evidence for induction of the SOS response.

Sublethal concentrations of formic acid (10 mmol/l) and propionic acid (5 mmol/l) at pH 5.0 preferentially inhibit DNA synthesis and stop cell multiplication in the absence of a corresponding cessation in the increase of culture turbidity. The possibility that the acids induce the SOS response by starving cells of thymine or by causing physical damage to the DNA molecule has now been investigated. Accumulation of thymine into the cytoplasm of whole cells was not inhibited by either acid. Mutants defective in excision repair (uvr A6), recombination repair (rec A56) and polymerase activity (pol A1) were not more sensitive to the acids than their isogenic parent. No significant increase in cell length was observed from measurements of transmission electron microscope images of acid-treated cells. It is concluded, therefore, that sublethal concentrations of formic and propionic acid inhibit DNA synthesis without physically damaging DNA molecule, or starving the cell of essential thymine or otherwise inducing an SOS response.

DNA Repair↗
PubMed1991

Short-chain organic acids at ph 5.0 kill Escherichia coli and Salmonella spp. without causing membrane perturbation.

When strains of Escherichia coli K12 and Salmonella spp. were incubated with 0.5-0.7 mol/l formic or propionic acid at pH 5.0, propionic acid was more active than formic acid. It killed 90% of the cell population within 60 min compared with over 3 h for formic acid. Cell death was not associated with a reduction in culture turbidity or a loss of membrane integrity since morphologically normal membranes were observed by electron microscopy and only a small proportion of the cytoplasmic enzyme beta-galactosidase leaked into the supernatant fluid of acid-treated E. coli K12 cultures.

Cell Membrane↗