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Biomedical subjects

D B Scott

Publications and source records attributed to D B Scott.

At least 217 records · Page 12Linked to original sources
PubMed1963

EFFECT OF OZONE ON SURVIVAL AND PERMEABILITY OF ESCHERICHIA COLI.

Scott, D. B. McNair (University of Pennsylvania, Philadelphia) and E. C. Lesher. Effect of ozone on survival and permeability of Escherichia coli. J. Bacteriol. 85:567-576. 1963.-Escherichia coli cultures in the logarithmic phase or resting were treated with various concentrations of ozone in saline solution. Approximately 2 x 10(7) molecules of ozone per bacterium killed 50% of the cells. Ozone caused leakage of cell content into the medium, and lysis of some cells. Low concentrations of ozone did not react with the glutathione within the cells, although reaction with glutathione in solution was immediate and stoichiometric. The effect on nucleic acid within the cells was to change the solubility and to cause the release of ultraviolet-absorbing material into the medium. Ozone attacked the ring structure of the base or the carbohydrate only when the substance was in the medium. Nucleic acids released into the medium were reabsorbed by cells which were not lysed. Viable cells resumed growth immediately, and grew at rates determined by the nutrients either added to the medium or which resulted from leakage and lysis of nonviable cells. It is postulated that the primary attack of ozone was on the cell wall or membrane of the bacteria, probably by reaction with the double bonds of lipids, and that leakage or lysis of the cells depended on the extent of that reaction.

Bacteria↗
PubMedDate not supplied

Isolation and characterization of a Rhizobium loti gene required for effective nodulation of Lotus pedunculatus.

A Rhizobium loti gene required for effective invasion of the host Lotus pedunculatus has been identified by transposon Tn5 mutagenesis. Cosmids that complemented a previously isolated mutation (239) at this invasion (inv) locus were identified by in planta complementation and used to construct a physical map of the gene region. The insertion site of Tn5 in PN239 was mapped to a 7.5-kb EcoRI fragment, which complemented the mutation when subcloned into pLAFR1. Further Tn5 mutagenesis of the 7.5-kb fragment was carried out in Escherichia coli using bacteriophage lambda 467, and the mutations homogenotized into R. loti NZP2037. Three additional Fix- mutations were isolated, and these were found to map adjacent to the position of the original mutation in strain PN239. All the other Tn5 insertions isolated in the 7.5-kb fragment gave a Fix+ phenotype on L. pedunculatus. Electron microscopic examination of the L. pedunculatus nodules induced by the isolated Fix- mutants showed that bacteria were either blocked in release from the infection threads or were unable to undergo normal bacteroid development. The inv locus as defined by the Tn5 insertions was sequenced, and a single open-reading frame (ORF) of 576 bp, corresponding to a polypeptide of 21.3 kDa, was identified. The position and orientation of this ORF were consistent with those of the isolated Tn5 Fix- insertions.

Amino Acid Sequence↗
PubMedDate not supplied

Fracture properties of human enamel and dentin.

Fracture in enamel is anisotropic with respect ot the orientation of the enamel rods, with Wf for fracture parallel to the rods being 0.3 X 10(2) J/meter2 but on the order of 2.0 X 10(2) J/meter2 for fracture perpendicular to the rods. Fractographs of enamel showed that the enamel rods behaved as integral units during controlled fracture. A model was proposed to explain the fracture properties of enamel involving the assumption that the hydroxyapatite crystals did not fracture but that their orientation determined the fracture properties. Dentin was also anisotropic, with fracture parallel to the dentinal tubules being the strong direction - Wf was 5.5 X 10(2) J/meter2 for fracture parallel to the tubules, but 2.7 X 10(2) J/meter2 for fracture perpendicular to the tubules. Wf for dentin was of the same magnitude as the high strain rate Wf found for bone (2.0 X 10(2) J/meter2 [ref 3]). The data obtained in this investigation indicated that dentin and enamel are brittle substances and that forces occasionally imposed during mastication or bruxism probably are capable of initiating fracture when the normal anatomy of the tooth has been altered by cavity preparation. As consequence, the design of cavity preparations should include considerations to reduce stress concentrations and thus improve the practical fracture strength of tooth structure.

Adolescent↗