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Biomedical subjects

I Chopra

Publications and source records attributed to I Chopra.

At least 109 records · Page 6Linked to original sources
PubMed1981

Reduced expression of Tn 10-mediated tetracycline resistance in Escherichia coli containing more than one copy of the transposon.

In Escherichia coli a single copy of Tn10 confers high-level resistance to tetracycline. Resistance itself results from expression of three distinct mechanisms which normally act together (Shales et al., 1980). In cells containing two copies of Tn10, the level of resistance to tetracycline was reduced. This was not due to overproduction of the repressor which controls the resistance genes, because strains diploid for an operator-constitutive allele of Tn10 also exhibited reduced expression of resistance. The negative gene dosage effect resulted from decreased expression of two mechanisms (1 and 2) consequent on enhanced expression of the third mechanism. The net result of increasing the copy number was a decrease in resistance because mechanism 3 was less efficient than mechanisms 1 and 2 in protecting the cell against tetracycline. The DNA sequence responsible for the reduced expression of resistance was contained in the internal BglII fragment of Tn10. This sequence, which is probably unique to Tn10, may encode the protein which mediates mechanism 3. Elevated levels of this protein probably cause decreased expression of mechanisms 1 and 2.

DNA Transposable Elements↗
PubMed1981

Tetracycline resistance genes from Bacillus plasmid pAB124 confer decreased accumulation of the antibiotic in Bacillus subtilis but not in Escherichia coli.

Expression of tetracycline resistance by genes originating in the Bacillus plasmid pAB124 was examined in both Bacillus subtilis and Escherichia coli host cells. Expression of resistance in B. subtilis by genes from pAB124 was inducible and associated with decreased accumulation of the antibiotic. A fragment of pAB124 carrying the genes coding for tetracycline resistance was cloned into the E. coli plasmid RSF2124. The cloned fragment conferred a low level of resistance in E. coli, but this was not associated with decreased uptake of tetracycline and was not inducible.

Bacillus subtilis↗
PubMed1980

Evidence for more than one mechanism of plasmid-determined tetracycline resistance in Escherichia coli.

The basis of tetracycline resistance mediated by TetA determinants and joint resistance to tetracycline and minocycline coded by TetB determinants was investigated. The TetA class of determinants was represented by those carried on plasmids pSC101, RP1 and pIP7 and TetB by Tn10. The relationships between expression of tetracycline and minocycline resistance and accumulation of these antibiotics suggest that there are three mechanisms of plasmid-determined resistance conferring (1) about a 10- to 20-fold increase in resistance to tetracycline that is not associated with decreased antibiotic accumulation, (2) a 4- to 7-fold increase in resistance to tetracycline that is associated with decreased drug accumulation, and (3) about a 2- to 3-fold increase in resistance to both tetracycline and minocycline that is not associated with decreased accumulation of either antibiotic. Mechanism 1 was coded by the tetracycline resistance determinant of pSC101 (TetA), mechanisms 1 and 2 by the determinants in RP1 and pIP7 (TetA) and all three mechanisms by Tn10 (TetB).

Drug Resistance, Microbial↗
PubMed1980

Comparison of the polypeptide composition of Escherichia coli outer membranes prepared by two methods.

Escherichia coli outer membranes were prepared by centrifugation to equilibrium in sucrose gradients and then treated with Sarkosyl in the presence of ethylenediaminetetraacetate. The polypeptide profiles of the two outer membrane preparations were compared by two-dimensional polyacrylamide gel electrophoresis. The patterns obtained were not identical, and Sarkosyl removed several minor proteins from the outer membrane.

Bacterial Proteins↗
PubMed1977

Lysozyme-promoted association of protein I molecules in the outer membrane of Escherichia coli.

Incubation of whole envelopes prepared from sonically oscillated Escherichia coli K-12 cultures with lysozyme in vitro resulted in the appearance of a protein species with an apparent molecular weight double that of outer membrane protein I. Similar dimers were also detected in purified outer membranes and whole envelopes from lysozyme-induced spheroplasts of E. coli K-12. This was confirmed by two-dimensional electrophoresis in which the dimers were resolved in the second dimension to run as single polypeptides of protein I. Formation of dimers was correlated with peptidoglycan degradation, but the ability of protein I molecules to associate may vary between strains of E. coli, since dimers were found only in outer membranes from E. coli W7. We suggest that extensive degradation of peptidoglycan leads to nonspecific formation of protein I aggregates, but that these aggregates do not occur in vivo.

Bacterial Proteins↗
PubMed1976

Mechanisms of resistance to fusidic acid in Staphylococcus aureus.

The biochemical mechanisms of resistance to fusidic acid in Staphylococcus aureus were investigated. Organisms possessing plasmid genes for resistance showed a high basal level of resistance, but could be induced to higher levels after pre-incubation with fusidic acid. This induction occurred rapidly and probably did not depend on gene dosage effects. Mutants resistant to fusidic acid, obtained from plasmid-negative cultures, expressed resistance constitutively. Protein synthesis in cell-free extracts from staphylococci with plasmid-mediated resistance to fusidic acid was as sensitive to fusidic acid as was synthesis in preparations from sensitive organisms; whereas protein synthesis in preparations from a spontaneous fusidic acid resistant mutant was resistant to the antibiotic. None of the resistant strains caused detectable inactivation of fusidic acid and no new derivative of fusidic acid was found in culture extracts of plasmid-possessing organisms grown in the presence of radioactive antibiotic. Expression of plasmid-mediated resistance to fusidic acid was associated with a decrease in the molar ratio of phosphatidylglycerol to lysylphosphatidylglycerol, but the cardiolipin content remained constant.

Bacterial Proteins↗
PubMed1975

Effect of plasmid carriage on the virulence of staphylococcus aureus.

The possession of any of eight different plasmids by Staphylococcus aureus strain 649--either singly or simultaneously (in no. 649MR)--caused changes in growth kinetics. Six of the plasmids caused an increase in exponential doubling time (by 8-25%), and most also altered the duration of the lag period. Strain 649MR was significantly less virulent for 10-day chick embryos than the corresponding plasmid-negative culture (no. 649N). The avirulence persisted even after loss of the plasmids from no. 649MR. The presence of a single plasmid specifying tetracycline resistance produced a moderate reduction in virulence, but chromosomal tetracycline resistance had an insignificant effect on it. The decrease in virulence could not be attributed to reduced formation of soluble products. It probably resulted from alterations in the cell surface, but membrane-polypeptide profiles of virulent and avirulent cells lacking plasmids were similar. Survival of strains 649MR and 649N on glass was identical. Therefore, reduction in the incidence of staphylococcal sepsis may be due in part to loss of virulence that has resulted from plasmid carriage.

Animals↗
PubMed1975

Mechanism of plasmic-mediated resistance to cadmium in Staphylococcus aureus.

The mechanism of plasmid-mediated resistance to cadmium in Staphylococcus aureus was investigated. Protein synthesis in cell-free extracts from resistant or susceptible bacteria was equally susceptible to inhibition by Cd(2+), but spheroplasts from resistant bacteria retained their resistance. Resistant bacteria did not have a decreased affinity for cations in general, nor was active metabolism required for exclusion of Cd(2+). The kinetics of Cd(2+) uptake into susceptible and resistant bacteria suggested that the conformation of membrane proteins in resistant bacteria may be important in the exclusion of Cd(2+).

Cadmium↗
PubMed1974

Biochemical and genetic basis of tetracycline resistance in Staphylococcus aureus.

Both the genetic and biochemical basis of tetracycline resistance in a number of staphylococcal strains was investigated. The strains examined could be classified into three groups: (i) those possessing a high basal level of resistance and in which resistance could be induced to higher levels (macro-inducible); (ii) those which had a high uninduced level of resistance, but which were virtually uninducible (macro-constitutive); (iii) one derivative which had a low basal level of resistance and was also uninducible (micro-constitutive). Resistance in macro-constitutive strains was plasmid mediated and typical of organisms possessing wild-type plasmids. The macro-constitutive pattern of resistance appeared to be correlated with a chromosomal location for the resistance genes, whereas the micro-constitutive pattern was correlated with loss of a region from the wild-type plasmid. Analysis of membrane proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis suggested that a number of membrane polypeptides became unstable in staphylococci possessing high-level tetracycline resistance. In particular, the absence of a polypeptide of 22,000 daltons was always associated with high-level resistance. There was no evidence that multiple gene copies are required for expression of tetracycline resistance in Staphylococcus aureus.

Gene Expression Regulation, Bacterial↗