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I Chopra

Publications and source records attributed to I Chopra.

At least 91 records · Page 5Linked to original sources
PubMed1986

Inhibition of K88-mediated adhesion of Escherichia coli to mammalian receptors by antibiotics that affect bacterial protein synthesis.

The ability of ten inhibitors of bacterial protein synthesis to decrease adhesion of Escherichia coli bearing K88ac fimbriae was examined. In the presence of the antibiotics at concentrations below the MIC values neomycin was the least effective inhibitor of adhesion and minocycline the most active. The effect of minocycline on the synthesis of individual polypeptides encoded by the K88ac determinant was examined in detail. The rate of synthesis of K88ac pilus protein in the presence of minocycline 0.75 mg/l (0.5 MIC) was less than that of total cell protein synthesis, suggesting that pilus protein becomes progressively 'diluted' in the outer membrane during exposure to this antibiotic concentration. Furthermore, the synthesis of two 'helper' polypeptides (molecular weights of 27.5 K and 27 K) which are probably involved in secretion of K88ac pilus protein through the cell envelope, was particularly sensitive to minocycline. Our observations suggest that the ability of translational inhibitors to decrease K88ac mediated adhesion probably results from direct inhibition of synthesis of fimbrial protein itself, together with inhibition of 'helper' polypeptide synthesis.

Animals↗
PubMed1986

Polymyxin B and polymyxin B nonapeptide alter cytoplasmic membrane permeability in Escherichia coli.

The effects of polymyxin B and polymyxin B nonapeptide (PMBN) on the permeability of the Escherichia coli cytoplasmic membrane were investigated. Both compounds caused loss of free amino acids, uracil and K+ from E. coli. The rates of loss promoted by polymyxin B were one and a half to two-fold greater than those caused by PMBN. Although PMBN mediated loss of low molecular weight substances from E. coli, it was not bactericidal. In contrast, polymyxin B treated E. coli lysed and rapidly lost viability. We suggest that the bactericidal activity of polymyxin B may be related to its previously reported ability to release cytoplasmic proteins from bacteria.

Amino Acids↗
PubMed1986

Transport of tetracyclines into Escherichia coli requires a carboxamide group at the C2 position of the molecule.

Structural features of the tetracycline molecule required for transport into Escherichia coli were investigated by examining the uptake of various tetracycline analogues. Uptake was assessed by determining the ability of analogues to induce Tn10 encoded resistance to tetracycline and by direct transport assays using tetracycline susceptible bacteria. Seven tetracyclines (including tetracycline itself) were studied. Two of these compounds, chlorotetracyclinonitrile and tetracyclinonitrile were not accumulated by E. coli, which suggests that the carboxamide group at the C2 position of the tetracycline molecule is required for transport into bacteria.

Amino Acids↗
PubMed1986

Genetic and biochemical basis of tetracycline resistance.

Properties of several, well characterized, tetracycline resistance determinants were compared. The determinants in Tn1721 and Tn10 (both from Gram-negative bacteria) each contain two genes; one encodes a repressor that regulates both its own transcription and that of a membrane protein that confers resistance by promoting efflux of the drug. Determinants from Gram-positive bacteria also encode efflux proteins, but expression of resistance is probably regulated by translational attenuation. The likely tetracycline binding site (a common dipeptide) in each efflux protein was predicted. The presence of the common binding site is consistent with the ability of an efflux protein originating in Bacillus species to be expressed in Escherichia coli.

Bacterial Proteins↗
PubMed1986

Leakage of periplasmic proteins from Escherichia coli mediated by polymyxin B nonapeptide.

The effects of polymyxin B and polymyxin B nonapeptide (PMBN) on cell envelope integrity in Escherichia coli were compared. Both compounds caused loss of proteins from E. coli K-12 3300(pBR322), although PMBN released less protein than did polymyxin B. The origin of the released protein was determined both by polyacrylamide gel electrophoresis and by using specific enzyme markers (beta-lactamase in periplasm, beta-galactosidase in cytoplasm). The proteins released by both compounds were derived principally from the periplasm, accompanied in the case of polymyxin B by a low level of cytoplasmic proteins. Although polymyxin B and PMBN both caused release of periplasmic proteins, the individual proteins released by the compounds differed. The periplasmic fraction contained six principal polypeptides with molecular weights between 62,000 (polypeptide 1) and 29,000 (polypeptide 6). Polypeptide 6 was identified as the pBR322-encoded beta-lactamase, but the other proteins were not specifically identified. Polymyxin B caused considerable release of polypeptides 1, 2, and 5 with some release of polypeptides 4 and 6. PMBN released polypeptide 1 (trace), 3, 4, and 6 (trace). Scanning electron microscopy showed that polymyxin B and PMBN both caused surface damage in E. coli. However, polymyxin B produced greater morphological changes than PMBN.

Bacterial Proteins↗
PubMed1985

Inhibition of beta-lactamase synthesis in Staphylococcus aureus by minocycline.

The effect of minocycline on beta-lactamase synthesis in Staphylococcus aureus was examined. Some inhibition of synthesis was detectable in organisms exposed to 0.015 mg minocycline/litre (0.15 of the single cell MIC) and complete inhibition was observed at 0.05 mg drug/litre (0.5 MIC). In contrast, total cell protein synthesis was less susceptible to inhibition by minocycline. Although synthesis of beta-lactamase was inhibited by low concentrations of minocycline, benzylpenicillin and minocycline failed to show synergy when tested in a variety of combinations against penicillinase producing staphylococci. In fact, benzyl-penicillin did not influence the minocycline MIC, and in the majority of cases the same pattern was exhibited by minocycline i.e. it did not generally alter the penicillin MIC. This is an example of true indifference whereby each antibiotic behaves as if the other were not present. The results presented here are discussed in relation to the possibility of therapeutic control of beta-lactamase producing organisms by suppressing the level of enzyme produced.

Drug Synergism↗
PubMed1984

Energetics of tetracycline transport into Escherichia coli.

The nature of energy coupling for the active transport of tetracycline into Escherichia coli was examined under conditions in which antibiotic uptake was directly compared with transport of proline (proton motive force dependent) and glutamine (phosphate bond dependent). Tetracycline transport was partially inhibited by osmotic shock and by exposure of bacteria to arsenate, two procedures which substantially reduced glutamine transport. Tetracycline transport was also partially inhibited in an uncB mutant (AN283) exposed to the uncoupler carbonyl cyanide-m-chlorophenyl hydrazone (CCCP) under conditions that inhibited proline transport. Taken together, these data indicate involvement of both phosphate bond hydrolysis and the proton motive force for the active transport of tetracycline into E. coli.

Arsenates↗
PubMed1984

Biochemical and genetic characterization of the tet determinant of Bacillus plasmid pAB124.

A fragment of Bacillus plasmid pAB124 carrying the genes encoding tetracycline resistance was previously cloned into Escherichia coli plasmid pSF2124 (S.J. Eccles, A. Docherty, I. Chopra, S. Shales, and P. Ball, J. Bacteriol. 145:1417-1420, 1981). The cloned pAB124 tet fragment conferred low-level resistance in E. coli, but exposure of this strain to a subinhibitory level of tetracycline led to selection of a mutant plasmid in which high-level resistance, associated with decreased drug accumulation, was expressed constitutively. In this plasmid, the Bacillus tet determinant appeared to be transcribed from a promoter on the vector. Construction of tetracycline-sensitive derivatives of this plasmid by transposon insertion mutagenesis allowed identification of a 32,000-dalton membrane-located protein, which apparently promoted decreased accumulation of tetracycline. This protein was also synthesized as a 32,000-dalton polypeptide in a coupled, in vitro transcription-translation system directed by plasmid DNA. The pAB124 tet determinant differed from the tetA through tetD determinants found in gram-negative bacteria in DNA-DNA hybridization and in the ability to prevent accumulation of different tetracycline derivatives, but was closely related to the tet determinant of another plasmid isolated from Bacillus species, pBC16.

Bacillus subtilis↗
PubMed1983

Limitations of a fluorescence assay for studies on tetracycline transport into Escherichia coli.

Transport of tetracycline into Escherichia coli was studied by two methods, one involving an absolute determination of accumulated drug and the other a fluorescence assay. Tetracycline uptake was nonsaturable when assayed by the absolute method, but fluorescence enhancement was maximal at an initial external tetracycline concentration of about 200 microM. The two transport assays also gave different results for the pH optimum of tetracycline transport. The absolute method indicated a pH optimum of 7.0 to 8.0, whereas the fluorescence method gave a value of 5.5 to 6.0. These data indicate that the fluorescence assay is of limited value in certain situations.

Biological Transport↗
PubMed1983

Analysis of tetracycline resistance encoded by transposon Tn10: deletion mapping of tetracycline-sensitive point mutations and identification of two structural genes.

Deletions in the tet genes derived from Tn10 were formed from different tet::Tn5 insertion mutations by removing DNA sequences located between a HindIII site in Tn5 and a HindIII site adjacent to the tet genes. Tetracycline-sensitive point mutations were mapped in recombination tests with the deletions and were thus aligned with the genetic and physical map of the tet region. Plasmids carrying point mutations were tested for complementation with derivatives of pDU938, a plasmid carrying cloned tet genes derived from Tn10 which had been inactivated by Tn5 insertions. Complementation occurred between promoter-proximal tet point mutations and distal tet::Tn5 insertions, suggesting the existence of two structural genes, tetA and tetB. These results, together with the analysis of polypeptides in minicells harboring pDU938tet::Tn5 mutants, suggested that tetA and tetB are expressed coordinately in an operon. The tetB gene encodes the previously characterized 36,000-dalton cytoplasmic membrane TET protein, but the product of tetA was not identified. Point mutations in either tetA or tetB led to the defective expression of the resistance mechanism involving tetracycline efflux. It is suggested that the tetA and tetB products interact cooperatively in the membrane to express resistance.

Biological Transport↗
PubMed1982

Tetracycline resistance determinants from groups A to D vary in their ability to confer decreased accumulation of tetracycline derivatives by Escherichia coli.

The ability of four genetically distinct plasmid-located tetracycline resistance determinants (TetA, B, C and D) to confer decreased accumulation of tetracycline and some of its analogues by Escherichia coli K12 was examined. Accumulation of oxytetracycline, tetracycline, demethylchlorotetracycline, 6-demethyl-6-deoxy-5-hydroxy-6-methylene-tetracycline, chlorotetracycline, doxycycline and 6-demethyl-6-deoxytetracycline was examined by fluorescence spectroscopy. The determinants varied in their ability to promote decreased accumulation of tetracyclines, defined as an R+/R- fluorescence ratio of less than 0.85. Plasmid pIP7 (TetA) caused reduced accumulation of only oxytetracycline, tetracycline and chlorotetracycline, but plasmid pDU301 (TetB) promoted reduced accumulation of all the compounds tested except 6-demethyl-6-deoxytetracycline. The TetC determinant of pBR322 caused decreased uptake of five derivatives, but not doxycycline or 6-demethyl-6-deoxytetracycline. Plasmid RA1 (TetD) encoded reduced accumulation of oxytetracycline, tetracycline, 6-demethyl-6-deoxy-5-hydroxy-6-methylenetetracycline and chlorotetracycline. In general, the resistance determinants were more efficient in promoting decreased accumulation of hydrophilic tetracyclines. These accumulation studies provide a satisfactory method for the phenotypic identification of Tet resistance determinants.

Drug Resistance, Microbial↗